How to use it
- Type a temperature. Negative numbers are fine for Celsius and Fahrenheit.
- Choose the scale you have and the one you want, or tap swap to reverse them.
- See the answer in all three scales, plus the nearest familiar reference point.
Three scales, one temperature
Celsius puts the freezing and boiling points of water (at standard sea-level pressure) at 0° and 100°. Fahrenheit, still the everyday scale in the United States, puts them at 32° and 212°, so its degree is 5/9 the size of a Celsius degree. Kelvin is the SI unit. It uses the same size of degree as Celsius but starts at absolute zero, so it is never negative. It is written “K”, not “°K”.
The formulas
°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9
K = °C + 273.15
°C = K − 273.15
K = (°F − 32) × 5/9 + 273.15
Why it isn’t a simple multiplication
Length or weight conversions just multiply by a factor. Temperature scales also have different zero points, so the formula needs an offset (the + 32 or + 273.15). A change in temperature has no offset, though: a rise of 10 °C is a rise of 18 °F, not 50 °F.
Worked examples
20 °C in Fahrenheit
20 × 9/5 + 32
68 °F
98.6 °F in Celsius
(98.6 − 32) × 5/9
37 °C
350 °F oven in Celsius
(350 − 32) × 5/9
176.67 °C
−40 °C in Fahrenheit
−40 × 9/5 + 32
−40 °F
300 K in Celsius
300 − 273.15
26.85 °C
0 °F in Celsius
(0 − 32) × 5/9
−17.78 °C
Accuracy and limits
Results are calculated at full precision and rounded only for display. Nothing can be colder than absolute zero (0 K = −273.15 °C = −459.67 °F), so the converter rejects anything lower instead of returning an impossible number. Boiling and freezing points shown are for pure water at standard sea-level pressure; they shift with altitude.
Questions
At what temperature are Celsius and Fahrenheit equal?
At −40°. −40 °C and −40 °F are the same temperature, which is why the two scales line up there on the thermometer.
Can kelvin be negative?
No. 0 K is absolute zero, the lowest possible temperature, so kelvin values are zero or positive.
What is 37 °C in Fahrenheit?
98.6 °F. It is often quoted as typical human body temperature, though normal readings vary from person to person.
What is a quick mental shortcut from °C to °F?
Double it and add 30. 20 °C gives about 70 °F (exact: 68 °F). It gets less accurate at extreme temperatures.