Temperature Converter
Every other unit conversion is a multiplication. Temperature is not, because the scales do not share a zero — and that has consequences beyond the arithmetic.
Fahrenheit
212.00°F
- Celsius
- 100.00°
- Fahrenheit
- 212.00°
- Kelvin
- 373.15°
- Description
- At or above boiling
- Temperature is the one conversion that is not a multiplication. Every other unit family shares a zero — no length is a different length in another unit — but the temperature scales do not, so each conversion needs an offset as well as a factor.
- The consequence is that temperature differences do not scale. Twenty degrees Celsius is not twice as warm as ten in any meaningful sense, and 68°F is not twice 50°F.
- Minus forty is the same on both Celsius and Fahrenheit, which is the only point where the two scales meet.
About temperature conversion
No length is a different length in another unit — a metre and 3.28 feet are the same distance, and zero is zero in both. Temperature scales are not like that. Zero Celsius is the freezing point of water; zero Fahrenheit is a brine mixture Fahrenheit happened to use; zero Kelvin is the coldest anything can be. Three different zeros means every conversion needs an offset as well as a ratio.
The practical consequence is that temperature differences do not scale. Twenty degrees Celsius is not twice as warm as ten in any meaningful sense, and 68°F is not twice 50°F. Only on the Kelvin scale, which starts at absolute zero, does doubling a number mean doubling anything physical.
Minus forty is the same on both Celsius and Fahrenheit — the only point where the two scales cross. It is a useful check on any conversion you are unsure of, and a genuinely cold day in a few places.
Kelvin has no degree symbol, by convention: it is written 300 K rather than 300°K. That is because it measures absolute thermodynamic temperature rather than a position on an arbitrary scale, and the distinction is deliberate.
What it works out
- Celsius, Fahrenheit and Kelvin
- All three shown at once
- A plain description of what the temperature is like
The formula
°F = °C × 9/5 + 32 °C = (°F − 32) × 5/9 K = °C + 273.15
One hundred Celsius times 9/5 is 180, plus 32 is 212 — the boiling point of water on both scales.
The 9/5 is the ratio of the degree sizes: a Celsius degree is 1.8 times larger than a Fahrenheit one, because 100 Celsius degrees span the same range as 180 Fahrenheit degrees between freezing and boiling. The 32 is the offset between the two zeros.
Kelvin shares its degree size with Celsius and only shifts the zero, so the conversion is a straight addition of 273.15. Absolute zero is −273.15°C, and nothing can be colder.
A quick approximation for weather: double the Celsius and add 30. It gives 50 for 10°C where the true answer is 50, and 70 for 20°C where the true answer is 68 — close enough to know whether to take a coat.
The check worth remembering is that −40 is the same on both scales. If a conversion does not agree there, the offset has been applied wrongly.
- 9/5
- The ratio of degree sizes. A Celsius degree is 1.8 Fahrenheit degrees.
- 32
- The offset between the two zeros.
- 273.15
- The offset between Celsius and Kelvin. Absolute zero is −273.15°C.
A worked example
One hundred degrees Celsius in Fahrenheit.
That works out to 212.00 °F.
- Celsius
- 100.00°
- Fahrenheit
- 212.00°
- Kelvin
- 373.15°
- Description
- At or above boiling
Questions
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 and add 32. One hundred Celsius is 212 Fahrenheit. In your head, doubling and adding 30 gets close enough for weather.
How do I convert Fahrenheit to Celsius?
Subtract 32 first, then multiply by 5/9. The order matters — multiplying before subtracting gives the wrong answer.
At what temperature are Celsius and Fahrenheit the same?
Minus forty. It is the only point where the two scales cross, and a quick check that a conversion is applying the offset correctly.
Why is temperature conversion not a simple multiplication?
Because the scales have different zeros. Every other unit family shares a zero — nothing is zero metres and non-zero feet — so a factor is enough. Temperature needs an offset as well.
Is 20°C twice as warm as 10°C?
No. Because the Celsius zero is arbitrary, doubling the number does not double anything physical. Only on the Kelvin scale, which starts at absolute zero, does that hold.
Why is there no degree symbol on Kelvin?
Because it measures absolute temperature rather than a position on an arbitrary scale. It is written 300 K, not 300°K, and the omission is deliberate.
What is absolute zero?
−273.15°C, or 0 K — the coldest anything can be, where thermal motion is at its minimum. It cannot be reached, only approached.