Ohm's Law Calculator
Any two of volts, amps, ohms and watts give the other two.
Work out Ohm's Law. Any two of volts, amps, ohms and watts give the other two. Built to be checked against a rating plate and a bill.
Power
1,440.00 W
120.00 V · 12.000 A · 10.000 Ω
Two relations give all four quantities: V = IR and P = VI. Everything else — P = I²R, P = V²/R, R = V²/P — is one substituted into the other, which is why any two knowns are enough. Note the square on current in P = I²R: doubling the current quadruples the heat, and that is why undersized wiring overheats disproportionately.
How the Ohm's Law Calculator works
Two relations cover the whole of basic circuit analysis: V = IR, and P = VI. Everything else — P = I²R, P = V²/R, R = V²/P — is one substituted into the other. Which means any two of the four quantities determine the remaining two, and this solves from whichever pair you happen to have.
Also known as: voltage current resistance calculator · V=IR calculator · electrical power formula calculator
Two relations, twelve formulas
V = IR and P = VI. Everything else follows by substitution.
Put V = IR into P = VI and the voltage cancels to give P = I²R. Put I = V/R in instead and you get P = V²/R. Rearranging each of those for its other terms produces the full set of twelve, and all twelve are the same two facts.
Which is why any two of the four quantities determine the other two — there is never a case needing three.
In practice
120 V across 10 ohms draws 12 A and dissipates 1,440 W.
Start from any pair of those four and you land on the same quartet: volts and amps, volts and ohms, amps and watts, ohms and watts — all six pairings reconstruct the identical circuit. That is a useful property to check a calculator against, and this one is tested on exactly that.
The squared term is the important one
P = I²R means power rises with the square of current. Double the current and you quadruple the heat dissipated in the conductor.
That is why undersized wiring fails disproportionately as load rises, and why a small increase in load on a marginal circuit can produce a large increase in temperature.
It is also the reason transmission runs at high voltage. For a given power, higher voltage means lower current, and losses fall with the square of that reduction.
Where the figure deceives
Ohm's law holds for ohmic conductors at constant temperature. Filaments, semiconductors and gas discharges are not ohmic — a tungsten lamp's cold resistance is roughly a tenth of its hot resistance, which is why bulbs almost always fail at switch-on rather than during use.
For AC with reactance, resistance becomes impedance and the power relation needs a power factor. Motors, transformers and anything with a capacitor do not obey the simple form.
And resistance itself moves with temperature, so a circuit that measures one way cold behaves differently under load.
Acting on it
Use RMS values for AC, and only for resistive loads.
When sizing anything thermal, remember the square: the heat is not proportional to the load.
For a lamp or semiconductor, measure at operating temperature rather than calculating from a cold resistance.
Frequently asked questions
What is Ohm's law?
Voltage equals current times resistance. Combined with power equals voltage times current, it gives every relation between the four quantities — twelve formulas in total, all derived from those two.
How do I calculate power from current and resistance?
P = I²R. Substitute V = IR into P = VI and the voltage cancels out. Current is squared, which is why doubling the current quadruples the heat dissipated.
Does Ohm's law always hold?
For ohmic conductors at constant temperature, yes. Semiconductors, filaments and gas discharges are non-ohmic — a bulb's cold resistance is a fraction of its hot resistance, which is why bulbs fail at switch-on.
Does this work for AC?
For resistive AC loads, yes, using RMS values. Reactive loads need impedance rather than resistance, and the power relation then needs a power factor — that is the watts-to-amps tool rather than this one.
Why does the current square in the power formula?
Because power is voltage times current and voltage is itself proportional to current through a resistance. That squaring is why undersized wiring overheats disproportionately as load rises.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
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