What next?
Temperature rarely travels alone in a recipe or a spec.
How to use this converter
- Type a value into any of the Celsius, Fahrenheit or Kelvin boxes.
- The other two update instantly.
- Read all three scales at once.
The one temperature where Celsius and Fahrenheit agree
At exactly −40°, Celsius and Fahrenheit read the same number — a neat quirk of how the two scales were built. Everywhere else, the mix of scales causes endless confusion between ovens, weather apps and science class.
Kelvin adds a third layer for science, starting at absolute zero. Convert cleanly between all three and no thermostat, recipe or physics problem can catch you out.
Formulas
K = °C + 273.15
Example conversion
Converting 20°C:
K = 20 + 273.15 = 293.15 K
Reference points
What your result means
Temperature is the one conversion on this site that is not a simple multiplication. Celsius and Fahrenheit have different zero points as well as different degree sizes, so every conversion needs an offset as well as a scale factor. That is why you cannot convert a temperature difference the same way you convert a temperature: a rise of 10 °C is a rise of 18 °F, not 50 °F.
Kelvin behaves differently again. It shares the degree size of Celsius but starts at absolute zero, so it is a pure offset of 273.15 with no scaling — and a Kelvin value can never be negative. If your result comes out below 0 K, the input was not a real temperature.
Why this one is different
Both conversions are written out as steps rather than as a factor, because temperature scales start at different zeros — ×9/5 then +32 for Fahrenheit, +273.15 for Kelvin. The reference table is annotated with what each temperature actually is, from a fridge at 4°C to a fan oven at 180°C.
Converting a temperature and converting a temperature difference are different sums
This catches out professionals, not just students. To convert a reading, you need both the scale factor and the offset: 20 °C is 68 °F. To convert a change, you need only the scale factor: a rise of 20 °C is a rise of 36 °F, not 68. Applying the plus-32 to a difference is how a specification for a 10-degree tolerance quietly becomes a 50-degree one.
The offset exists because the two scales were anchored to different reference points. Celsius pinned 0 and 100 to the freezing and boiling points of water at standard pressure; Fahrenheit's zero was set by a freezing brine mixture, with body temperature intended to land near 96. Kelvin removed the problem by starting at absolute zero, which is why every thermodynamic formula uses it — you cannot multiply or divide a temperature meaningfully on a scale whose zero is arbitrary.
A practical consequence: gas laws, radiation calculations and efficiency ratios all require kelvin. Doubling a Celsius value does not double the thermal energy, but doubling a kelvin value does. If a formula involves a temperature in a multiplication or division rather than an addition, convert to kelvin first and convert back only at the end.
Frequently asked questions
Why is Kelvin used in science?＋
Kelvin starts at absolute zero, so it has no negative values. That makes physics equations involving temperature much simpler.
At what temperature do °C and °F meet?＋
At −40°, the two scales read exactly the same: −40°C = −40°F.
What is the formula both ways?＋
F equals C times 9/5 plus 32, and C equals F minus 32 times 5/9. Kelvin is simply C plus 273.15.
How do I convert a temperature difference?＋
Drop the offset. A change of 10 C is a change of 18 F, because only the 9/5 scale factor applies to differences, not the plus 32.
Is there a quick mental approximation?＋
Double the Celsius value and add 30. It is within a few degrees across everyday weather temperatures, so 20 C reads as about 70 F.
Why can nothing be below absolute zero?＋
Absolute zero, 0 K or -273.15 C, is the point at which a system has no thermal energy left to remove. It is a floor on the scale, not an arbitrary marker.
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Assumptions & limitations
The scales differ in both step size and zero point, which has some odd consequences:
- A Celsius degree and a kelvin are the same size, so a temperature difference of 5 °C is a difference of 5 K — but 5 °C and 5 K are wildly different temperatures.
- A Fahrenheit degree is 5/9 the size of a Celsius degree, so temperature differences do not convert with the +32 offset.
- Kelvin is written without a degree symbol, and cannot be negative — it starts at absolute zero.
- Boiling point depends on pressure: water boils at about 93 °C at 2,000 m altitude, which is why high-altitude cooking times differ.