Understanding common temperature reference points
The table below lists familiar reference points across all three scales, useful for sanity-checking a conversion result.
| Reference point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15 °C | −459.67 °F | 0 K |
| Water freezes (1 atm) | 0 °C | 32 °F | 273.15 K |
| Typical human body temperature | ≈37 °C | ≈98.6 °F | ≈310.15 K |
| Water boils (1 atm) | 100 °C | 212 °F | 373.15 K |
- The Celsius-to-Kelvin offset (273.15) is an exact defined value under the SI, not a rounded or measured figure.
- This calculator will not return a result for values below absolute zero (−273.15 °C / −459.67 °F / 0 K), since no physical system can be colder.
- Everyday "normal" body temperature is often quoted as 37 °C / 98.6 °F, but healthy individual readings vary by roughly ±0.5 °C depending on measurement site and time of day.
What are Celsius, Fahrenheit and Kelvin?
Celsius, Fahrenheit and Kelvin are three scales for measuring the same physical quantity — thermodynamic temperature — anchored to different zero points and step sizes. The Celsius scale sets 0 °C at the freezing point of water and 100 °C at its boiling point under standard atmospheric pressure, a convention dating to Anders Celsius in 1742 (originally reversed) and still the everyday scale in most of the world. The Fahrenheit scale, devised by Daniel Gabriel Fahrenheit in the early 1720s, places the same two reference points at 32 °F and 212 °F, a 180-degree span, and remains in everyday use in the United States.
Kelvin is the SI base unit of thermodynamic temperature and the scale used throughout science. It shares the same size of degree as Celsius but starts at absolute zero — the theoretical point at which a system has minimum thermal energy — so the two scales are related by a fixed 273.15-unit offset rather than a scaling factor. Since the 13th General Conference on Weights and Measures (1967/68), the unit is written simply as "kelvin" (symbol K), without the word "degree" or the ° symbol, unlike Celsius and Fahrenheit. The 2019 revision of the International System of Units (SI) fixed the kelvin's magnitude by defining the Boltzmann constant exactly, making the numeric relationship K = °C + 273.15 an exact definition rather than an experimentally measured value.
How to use this temperature converter
- Enter the temperature value you want to convert.
- Select the scale that value is currently in — Celsius, Fahrenheit or Kelvin.
- Read the converted values in the other two scales, shown instantly alongside the original.
- If you enter a Celsius (or equivalent) value below −273.15 °C, the calculator will not return a result, because temperatures below absolute zero are not physically possible.
The formula behind temperature conversion
All three scales convert through Celsius as the common reference. Fahrenheit values are first converted to Celsius (or vice versa) using the exact linear relationship fixed by the two scales' reference points, and Kelvin is obtained by adding the fixed 273.15 offset to the Celsius value — an exact conversion, not an approximation.
Common mistakes
- Applying the formula in the wrong direction — using °C × 9⁄5 + 32 when converting from Fahrenheit to Celsius, instead of subtracting 32 first and then multiplying by 5⁄9.
- Writing "degrees Kelvin" or a ° symbol before K — the SI unit is simply "kelvin," without the word "degree," a convention fixed since 1967.
- Forgetting the offset entirely and assuming the scales are proportional — for example treating 0 °F as equivalent to 0 °C, when 0 °F is actually about −17.8 °C.
- Entering a value below −273.15 °C (or the Fahrenheit/Kelvin equivalent) and expecting a result — such temperatures are physically impossible.
Questions fréquentes
What is the formula to convert Celsius to Fahrenheit?
Multiply the Celsius value by 9⁄5 (1.8) and add 32: °F = °C × 9⁄5 + 32. For example, 20 °C converts to 20 × 1.8 + 32 = 68 °F. This is an exact linear relationship fixed by the two scales' defined reference points, not an approximation.
How do you convert Fahrenheit to Celsius?
Subtract 32 from the Fahrenheit value first, then multiply by 5⁄9: °C = (°F − 32) × 5⁄9. For example, 68 °F converts to (68 − 32) × 5⁄9 = 20 °C. Doing the operations in the wrong order is the most common source of errors when converting by hand.
What is absolute zero?
Absolute zero is the theoretical lower limit of temperature, at which a system holds minimum thermal energy. It equals exactly 0 K, −273.15 °C and −459.67 °F by definition under the International System of Units (SI). No physical process can reach or go below this temperature, which is why this calculator rejects inputs colder than −273.15 °C.
Why does science use Kelvin instead of Celsius or Fahrenheit?
Kelvin is the SI base unit of thermodynamic temperature and starts at absolute zero, so it has no negative values for physically real temperatures and its ratios are physically meaningful (200 K genuinely holds twice the thermal energy scale of 100 K, unlike 200 °C versus 100 °C). Since the 2019 SI revision, the kelvin's size is fixed by an exact definition of the Boltzmann constant.
Is normal body temperature 37 °C or 98.6 °F?
Both figures describe the same traditionally cited average adult body temperature — 37 °C converts exactly to 98.6 °F. Individual healthy readings commonly vary by roughly half a degree Celsius depending on the time of day, measurement site and the person, so a single reading slightly above or below 37 °C is not automatically abnormal.
Why doesn't Kelvin use a degree symbol?
The 13th General Conference on Weights and Measures (1967/68) redefined the unit as simply "kelvin" (symbol K), removing the word "degree" and the ° symbol that Celsius and Fahrenheit retain. This reflects Kelvin's status as an absolute scale tied to a fixed physical zero point rather than an arbitrary reference.
Références
- National Institute of Standards and Technology (NIST). Special Publication 811: Guide for the Use of the International System of Units (SI), 2008 Edition.
- International Bureau of Weights and Measures (BIPM). The International System of Units (SI Brochure), 9th edition, 2019.
- Bureau International des Poids et Mesures. Resolution 3 of the 13th General Conference on Weights and Measures (CGPM), 1967/68 — definition of the unit kelvin.
- NIST. CODATA recommended values — Boltzmann constant and the 2019 redefinition of the kelvin.