Temperature Converter
Celsius and Fahrenheit cross at −40.
Convert Temperature. Celsius and Fahrenheit cross at −40. Exact where an exact definition exists.
Celsius
20 °C
68 °F · 293.15 K
Temperature scales differ in both degree size and zero point, which is why the conversion needs an offset as well as a factor. Celsius and Fahrenheit cross at −40, the one temperature where both scales read the same. Kelvin and Rankine are absolute scales with no negative values, sharing their zero with each other and their degree size with Celsius and Fahrenheit respectively.
How the Temperature Converter works
Temperature across Celsius, Fahrenheit, Kelvin and Rankine. The scales differ in both degree size and zero point, which is why the conversion needs an offset as well as a factor.
Also known as: celsius to fahrenheit converter · fahrenheit to celsius · kelvin to celsius converter · what temperature do the scales meet
Different zeros and different degrees
Celsius and Fahrenheit differ in both the size of a degree and where zero sits, which is why the conversion needs a multiplication and an addition rather than a single factor.
A Celsius degree is 1.8 Fahrenheit degrees, because 100 Celsius degrees between water's freezing and boiling points span 180 Fahrenheit degrees. The 32 accounts for the different zero.
The two scales cross at −40, where both read the same. It is the single solution to the equation, and it falls out of two lines with different slopes and different intercepts meeting exactly once.
Absolute scales and absolute zero
Kelvin shares Celsius degree size and puts zero at absolute zero, −273.15 °C. Rankine does the same with Fahrenheit-sized degrees, putting absolute zero at 0 °R and water's freezing point at 491.67.
Absolute zero is where thermal motion reaches its minimum. The third law of thermodynamics says it cannot be reached in a finite number of steps — it is approached asymptotically, and laboratories have got within billionths of a degree.
Kelvin takes no degree symbol, because it is an absolute scale rather than degrees relative to a reference. The convention changed in 1967, and 300 K rather than 300 °K has been correct ever since.
What Fahrenheit was thinking
Fahrenheit set zero at the temperature of a brine mixture, the coldest he could reproduce reliably, and intended human body temperature to sit near 96 — a number chosen because it divides conveniently by twelve.
The scale was later adjusted so water froze at exactly 32 and boiled at exactly 212, a tidy 180 degrees apart. That adjustment moved body temperature to about 98.6, which is why it is such an odd figure.
The finer degree is a genuine advantage for weather reporting — Fahrenheit resolves ambient temperature about twice as finely as Celsius without decimals. Whether that outweighs the convenience of 0 and 100 for water is a matter of what you use it for.
Where to go next
The Temperature Converter question rarely arrives on its own. These are the ones that usually come with it:
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Frequently asked questions
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. Twenty degrees Celsius is 68 Fahrenheit. The reverse is subtract 32 then multiply by 5/9.
At what temperature do Celsius and Fahrenheit agree?
At −40, which reads the same on both scales. It is the single crossing point, and it falls out of the two scales having different degree sizes and different zeros.
What is absolute zero?
−273.15 °C, or 0 K. It is the temperature at which thermal motion is minimal, and the third law of thermodynamics says it cannot be reached in a finite number of steps.
Why does Kelvin have no degree symbol?
Because it is an absolute scale rather than a scale of degrees relative to a reference — it is written as 300 K rather than 300 °K. The convention changed in 1967.
What is Rankine?
An absolute scale using Fahrenheit-sized degrees, so absolute zero is 0 °R and water freezes at 491.67 °R. It appears in some American engineering thermodynamics.
Why is Fahrenheit's zero where it is?
Fahrenheit set it at the temperature of a brine mixture, which was the coldest he could reliably reproduce. Human body temperature was meant to sit near 96, and the scale was later adjusted.
Why does Fahrenheit use 32 and 212?
Fahrenheit set zero at a brine freezing point and body temperature near 96, then the scale was adjusted so water froze at exactly 32 and boiled at 212 — a convenient 180 degrees apart.
What is wind chill?
An estimate of how cold moving air feels, based on heat loss from exposed skin. It is not an air temperature, which is why it does not affect a thermometer or a car radiator.
What is the heat index?
The equivalent temperature accounting for humidity, since sweat evaporates poorly in humid air. It is why 30 °C at high humidity is far more dangerous than 30 °C in dry heat.
How cold is space?
The cosmic microwave background is about 2.7 K. An object in space heats or cools mainly by radiation, so a sunlit surface can be very hot while a shaded one is extremely cold.
What is the highest temperature possible?
There is no established upper limit in the way absolute zero is a lower one. The Planck temperature, around 10³² K, is where current physics stops describing things usefully.
Why does water boil at a lower temperature on a mountain?
Because boiling happens when vapour pressure equals ambient pressure, and ambient pressure falls with altitude. It is why high-altitude cooking takes longer despite the water boiling sooner.
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