Math & Everyday Calculators

Percentages, dates, grades, and the quick arithmetic that comes up daily. All 212 are free and run in your browser. Each one shows the formula behind the result.

Percentage Calculator

Every common percentage question in one place.

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Age Calculator

Exact age in years, months, days, and total units.

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Date Difference Calculator

Days between two dates, with or without weekends.

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GPA Calculator

Weighted GPA from course grades and credit hours.

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Fraction Calculator

Add, subtract, multiply, and divide fractions.

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Average Calculator

Mean, median, mode, range, and standard deviation.

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Z-Score Calculator

Standard deviations from the mean, and the percentile that follows.

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Confidence Interval Calculator

The range around an estimate, for a mean or a proportion.

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Sample Size Calculator

How many responses a target margin of error actually costs.

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Probability Calculator

Two events combined, and the odds over repeated trials.

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Permutation and Combination Calculator

nPr and nCr, with and without repetition.

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Day of the Week Calculator

What day any date falls on, past or future.

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Week Number Calculator

ISO 8601 week numbers, including the year-boundary cases.

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Final Grade Calculator

The score you need on the final, and whether it is reachable.

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Circle Calculator

Area, circumference, radius and diameter from any one of them.

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Circle Sector Calculator

Sector area, arc length and chord from a radius and an angle.

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Arc Length Calculator

Arc length and chord, with the radian conversion shown.

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Ellipse Calculator

Area exactly, perimeter by Ramanujan's approximation.

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Pythagorean Theorem Calculator

The missing side of a right triangle, plus its angles and area.

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Trapezoid Calculator

Area from the two bases and the height, with the midsegment.

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Sphere Calculator

Volume and surface area, and why one is the derivative of the other.

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Cube Calculator

Volume, surface area and the space diagonal.

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Rectangular Prism Calculator

Box volume, surface area and the diagonal that decides what fits.

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Cylinder Calculator

Volume, surface area, and the label that peels off flat.

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Cone Calculator

Volume by vertical height, surface area by slant height.

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Pyramid Calculator

Volume and surface area, with both slant heights.

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Volume Calculator

Six shapes, with the litre conversion built in.

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Surface Area Calculator

Six shapes, and the ratio that explains why small things cool fast.

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Slope Calculator

Slope, intercept and the line's equation from two points.

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Distance Formula Calculator

Distance between two points, which is Pythagoras in disguise.

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Midpoint Calculator

The point exactly halfway between two others.

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GCF Calculator

Greatest common factor, with every common factor listed.

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LCM Calculator

Lowest common multiple, computed without overflowing.

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Prime Factorization Calculator

Prime factors, the divisor count, and where it comes from.

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Factorial Calculator

n!, up to the point where a computer runs out of room.

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Modulo Calculator

Remainder and modulo, which differ for negative numbers.

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Logarithm Calculator

Any base, via the change-of-base identity.

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Exponent Calculator

Powers, including negative and fractional exponents.

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Square Root Calculator

Any root, with the simplified surd where one exists.

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Scientific Notation Calculator

Scientific, E and engineering notation side by side.

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Significant Figures Calculator

Rounding to sig figs, and where the convention breaks down.

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Ratio Calculator

Simplify a ratio, and see it as shares of the whole.

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Proportion Calculator

Solve a : b = c : d for whichever term is missing.

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Inverse Proportion Calculator

When the product stays constant instead of the ratio.

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Weighted Average Calculator

When some values count for more than others.

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Harmonic Mean Calculator

The right average for speeds and rates.

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Geometric Mean Calculator

The right average for growth rates and returns.

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Percentage Change Calculator

Change, and the reverse move needed to undo it.

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Percentage Increase Calculator

How much something went up, and the multiplier behind it.

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Percentage Decrease Calculator

How much something fell, and what it takes to recover.

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Percentage Difference Calculator

A symmetric comparison, where neither value is the reference.

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Exponential Growth Calculator

Growth or decay, with doubling time and half-life.

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Binary Calculator

Binary to decimal, hex and octal, with validation that works.

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Hex Calculator

Hexadecimal to decimal, binary and octal.

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Octal Calculator

Octal to decimal, binary and hexadecimal.

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Base Converter

Decimal into binary, octal and hexadecimal at once.

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Decimal to Fraction Calculator

Exact fractions, recovered by continued fractions.

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Fraction to Decimal Calculator

Decimal, percentage, and whether it terminates.

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Mixed Number Calculator

Mixed numbers to improper fractions and back.

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Absolute Value Calculator

Distance from zero, and distance between any two numbers.

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Correlation Calculator

Pearson's r, and the r² that deflates it.

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Linear Regression Calculator

Line of best fit, with r² and the standard error.

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Normal Distribution Calculator

Probability between bounds, with the full empirical rule.

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Split Bill Calculator

Per person with tip, and what rounding up leaves over.

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Fuel Cost Calculator

Journey cost, one way and return, split per person.

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Gas Mileage Calculator

US mpg, imperial mpg, km/L and L/100km, converted properly.

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Cooking Conversion Calculator

Kitchen volumes, and weight only when you give a density.

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Business Days Calculator

Working days between two dates, holidays excluded properly.

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Date Add and Subtract Calculator

What date is 90 days from now — with month-end handled.

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Time Duration Calculator

Hours between two times, including shifts past midnight.

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Time Card Calculator

A week of shifts totalled, with overtime split out.

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Military Time Converter

24-hour to 12-hour, with midnight and noon right.

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Median Calculator

Median, quartiles and why it beats the mean.

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Triangle Area Calculator

Area from three sides, using Heron's formula.

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Right Triangle Calculator

Any two sides give the third, the area and both angles.

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Law of Cosines Calculator

The third side from two sides and the angle between them.

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Law of Sines Calculator

Including the ambiguous case, with both answers.

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Trapezoid Area Calculator

Mean of the parallel sides, times the height.

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Parallelogram Area Calculator

Base times height — or side and angle, if that is what you have.

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Rhombus Area Calculator

Half the product of the diagonals — the half matters.

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Regular Polygon Calculator

Area, angles, apothem and diagonals for any side count.

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Prism Calculator

Base area times length, for any cross-section.

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Torus Calculator

Volume and surface area of a ring, by Pappus.

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Combination Calculator

nCr, computed so 52 choose 5 is still exact.

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Permutation Calculator

nPr — arrangements, where order changes the answer.

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Quadratic Equation Calculator

Roots, discriminant and vertex — computed stably.

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Cubic Equation Calculator

All real roots, including the case Cardano cannot reach directly.

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System of Equations Calculator

Two equations, two unknowns — and the two ways it fails.

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Matrix Determinant Calculator

Any square size, by elimination rather than cofactors.

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Matrix Multiplication Calculator

With the dimension check that decides whether it exists at all.

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Arithmetic Sequence Calculator

nth term and sum, with the off-by-one that catches everyone.

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Geometric Sequence Calculator

nth term, sum, and whether the infinite series converges.

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Binomial Probability Calculator

Exactly k, at most k, at least k — plus the mean.

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Poisson Distribution Calculator

Rare events at a steady rate — where variance equals the mean.

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T-Test Calculator

One-sample t-test, with what a p value does not mean.

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Chi-Square Calculator

Goodness of fit, with the expected-count rule checked.

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Margin of Error Calculator

Why polls stop at a thousand people.

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Oven Temperature Converter

°C, °F, gas mark and fan — all four at once.

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Recipe Scaling Calculator

Ingredients scale. Temperature never does.

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Wind Chill Calculator

NWS 2001 formula, with frostbite timings.

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Heat Index Calculator

What the air actually feels like, and when it is dangerous.

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Dew Point Calculator

The humidity figure that actually predicts comfort.

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Fuel Efficiency Converter

mpg, L/100km, and the 20% gallon problem.

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Road Trip Calculator

Fuel, cost per person and time including breaks.

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Wallpaper Calculator

Counted in drops, which is how it is actually hung.

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Roof Pitch Calculator

Angle, rafter length, and the area you actually need.

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Decking Calculator

Boards, joists and fasteners — with the gap counted.

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Firewood Calculator

Cords, weight and the heat moisture costs you.

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Weighted Grade Calculator

Ungraded categories are left out, not counted as zeros.

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Final Exam Mark Calculator

The mark you need — even when it is over 100.

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Semester GPA Calculator

Credit-weighted, because a four-credit course is not one vote.

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Attendance Percentage Calculator

How many more you can miss before it costs you.

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Class Rank Percentile Calculator

Both definitions, because they disagree by a whole rank.

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Study Time Calculator

Your pace, not a number the calculator made up.

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Reading Time Calculator

Silent reading and read-aloud, because half of you are timing a talk.

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Words Per Page Calculator

Double spacing halves it. That is the whole problem.

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Typing Speed Calculator

Net WPM, where every error costs you a word.

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Reverse Percentage Calculator

Undoing a percentage means dividing, not subtracting.

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Odds Converter

Decimal, fractional, American and the probability behind all three.

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Dice Probability Calculator

Exact counts, not a normal approximation that fails at the tails.

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Birthday Paradox Calculator

23 people. Better than even. Yes, really.

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Collatz Conjecture Calculator

Halve if even, triple and add one if odd. Nobody knows why it works.

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Fibonacci Calculator

The sequence, the ratio, and where the arithmetic stops being exact.

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Number Base Converter

Validates the digits instead of silently truncating them.

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Digital Root Calculator

Sum the digits until one is left. It is always n mod 9.

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Soil & Compost Volume Calculator

Litres, bags and the weight nobody checks.

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Fertiliser Calculator

N-P-K are percentages. That is the whole problem.

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Grass Seed Calculator

Rates vary by species more than by area.

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Plant Spacing Calculator

Offset rows fit 13% more — in a bed big enough to notice.

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Compost C:N Ratio Calculator

30:1 or it either stinks or sits there.

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Cat Age Calculator

Fifteen in the first year, nine in the second, four thereafter.

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Pet Food Portion Calculator

Metabolism scales to the 3/4 power, not per kilogram.

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Aquarium Volume Calculator

Litres, gallons, and the 250 kg your desk cannot hold.

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Excess Baggage Calculator

Past about five kilos over, a second bag is usually cheaper.

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Derivative Calculator

Symbolic, exact, and right about 2^x.

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Definite Integral Calculator

Adaptive Simpson, with the error it actually has.

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Limit Calculator

Both sides, kept separate, so a jump reads as a jump.

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Taylor Series Calculator

Exact coefficients, and the error you actually get.

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Inverse Matrix Calculator

With the condition number, so you know what it is worth.

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Eigenvalue Calculator

Complex eigenvalues named, not refused.

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Dot Product Calculator

The angle between two vectors, in any dimension.

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Cross Product Calculator

A perpendicular vector, and the area it measures.

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Complex Number Calculator

All four operations, plus polar form.

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Polynomial Roots Calculator

Complex roots named, and no cancellation error.

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Simultaneous Equations Solver

Solved, then checked by substitution.

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P-Value Calculator

What it is, and pointedly what it is not.

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Correlation Coefficient Calculator

Pearson's r, r², and the line — with the caveats.

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Binomial Distribution Calculator

Exact in log space, so large n still works.

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Standard Error Calculator

Not the standard deviation. The other one.

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Effect Size Calculator

Cohen's d, Hedges' g, and what a 'large' effect looks like.

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Quartile & IQR Calculator

Names the definition, because there are nine of them.

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Covariance Calculator

With the correlation that makes it interpretable.

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Coefficient of Variation Calculator

Refuses the cases where it means nothing.

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Paint Coverage Calculator

The tin's coverage assumes a wall you probably do not have.

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Stud Wall Calculator

Spacing is set by the board, not the structure.

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Binary & Bitwise Calculator

AND, OR, XOR and NOT, with the overflow width.

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Bit Shift Calculator

Shifting left multiplies. Shifting right truncates.

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Dice Pool Calculator

The whole distribution, not just the average.

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Drop Rate Calculator

At the expected number of attempts, you are only 63% there.

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XP to Level Calculator

The curve matters more than the numbers.

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Holiday Accrual Calculator

Entitlement accrues with service, not all at once.

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Notice Period Calculator

The greater of statutory and contractual. Always.

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Redundancy Pay Calculator

Statutory pay usually uses a capped weekly wage.

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Golf Handicap Calculator

Best eight of twenty, but only once you have twenty.

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Bowling Score Calculator

A frame's value is not known when it is bowled.

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Batting Average Calculator

The famous statistic and the least useful one.

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Basketball Efficiency Calculator

True shooting counts the free throws.

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Elo Rating Calculator

Zero-sum: what one gains, the other loses.

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Chess Rating Calculator

FIDE's K-factor slides from 40 to 10.

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Tennis Score Calculator

Six games does not win a set at 6-5.

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Dart Checkout Calculator

Seven numbers cannot be checked out at all.

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Betting Odds Converter

Add the implied probabilities and the margin appears.

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Parlay Calculator

The margin compounds along with the odds.

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Lottery Odds Calculator

Weekly play would take longer than recorded history.

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Cube Root Calculator

Odd roots of negatives are real; even roots are not.

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Rounding Calculator

Half-up and half-even disagree at exactly the half.

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Quadratic Formula Calculator

The discriminant decides everything before you solve.

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Slope Intercept Calculator

A vertical line has undefined slope, not infinite.

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Circle Area Calculator

A circle fills 78.54% of its square, always.

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Rectangle Area Calculator

Same perimeter, wildly different areas.

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Perimeter Calculator

Three lengths only make a triangle if two beat the third.

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Polygon Area Calculator

Exterior angles always sum to 360°, whatever the shape.

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Cylinder Volume Calculator

Widening beats heightening — radius is squared.

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Cone Volume Calculator

Exactly one third of its cylinder, always.

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Sphere Volume Calculator

Two thirds of its cylinder — Archimedes' favourite result.

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Mean Median Mode Calculator

The mean is dragged by outliers; the median is not.

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Variance Calculator

Sample divides by n−1, and it matters.

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Percentile Calculator

There is more than one definition, and they disagree.

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Interquartile Range Calculator

Robust because it ignores how extreme the extremes are.

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Percentage of a Number Calculator

Percentages commute — 4% of 75 is 75% of 4.

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Dovetail Layout Calculator

Half pins stop the end tail splitting off.

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Mitre Angle Calculator

Compound cuts move both angles at once.

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Cut List Calculator

Three millimetres a cut costs a whole board.

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Crosswind Component Calculator

The clock rule is a knot or two optimistic at 45°.

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Weight and Balance Calculator

Legal at takeoff is not legal at landing.

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Anchor Scope Calculator

Not the sounder reading. Add the tide and the bow.

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Tide Height Calculator

The middle two hours carry half the range.

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Sail Area to Displacement Calculator

Written to exclude boats, not to certify them.

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Great Circle Distance Calculator

On a Mercator chart the straight line is the long one.

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Dead Reckoning Calculator

Departure is not longitude until you divide by cos.

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Compass Course Calculator

West is best going one way. Not the other.

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Julian Date Calculator

No months, no leap years, no calendar reforms.

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Slope Distance Calculator

A tape along the ground over-measures. Sometimes a lot.

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Time Zone Conversion Calculator

The world spans 26 hours, not 24.

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About math & everyday calculations

The calculations here come up constantly and are just fiddly enough to be worth checking. Percentages have four distinct forms that people routinely mix up. Date arithmetic has to handle uneven month lengths and leap years. Fractions need a common denominator before anything can happen.

Each tool shows its working rather than only its answer, because for this kind of arithmetic the method is usually what you actually wanted, once you have seen why a 25% rise followed by a 25% fall does not return you to where you started, you do not need the calculator again.

These are the tools with the broadest audience on the site: students checking homework, people working out an age or a deadline, anyone who needs a percentage and would rather not risk the mental arithmetic.

Which one do you need?

I need a percentage

The percentage calculator handles all four forms separately: a share of a number, one number as a share of another, the change between two numbers, and reversing a percentage back to the original. Mixing these up is the usual source of error, so they are kept apart deliberately.

I need to work with dates

The age calculator gives exact years, months and days plus totals in every unit. The date difference calculator counts the gap between two dates and separately counts business days, which is the figure you need for notice periods and delivery windows.

I need to average some numbers

The average calculator gives mean, median, mode, range and standard deviation from a pasted list. When the mean and median disagree noticeably, outliers are pulling the average and the median is the more honest summary.

I need help with fractions or grades

The fraction calculator handles all four operations with mixed numbers and shows each step. The GPA calculator weights by credit hours, which is what your transcript does, a 4-credit course moves your average twice as far as a 2-credit one.