Convert Celsius, Fahrenheit and Kelvin without the mental maths
This temperature converter changes a reading between Celsius (°C), Fahrenheit (°F), Kelvin (K), Rankine (°R) and Réaumur (°Ré) as you type. It is handy for a US recipe that says 350 °F when your oven dial shows Celsius, a weather forecast from another country, a thermometer reading for a child with a temperature, or a physics problem that needs kelvins.
Temperature is the one everyday conversion you cannot do by multiplying alone, because the scales start from different zero points. The converter applies the correct offset and ratio for every pair and shows the formula it used — for example °F = °C × 9/5 + 32 — so you can check the result or learn the method.
How to use the Temperature Converter
- Type the temperature in the Value box. Use the ± button (or type a minus sign) for readings below zero.
- Choose the scale you have in From — Celsius, Fahrenheit, Kelvin, Rankine or Réaumur. You can search by name or symbol.
- Choose the scale you want in To, or press the round swap button to flip the direction.
- Read the answer and the formula below it. A warning appears if the value is below absolute zero, which no real temperature can be.
- The list underneath shows the same temperature on every scale; tap a common conversion such as “350 °F → °C”.
Temperature Converter features
All five scales
Celsius, Fahrenheit, Kelvin, Rankine and the historical Réaumur scale, converted in any direction.
Exact fractions
Ratios are kept as fractions (9/5, 5/9, 4/5), so 98.6 °F gives exactly 37 °C rather than 36.99999.
Formula for each pair
See whether the conversion needs an offset, a ratio or both — for example K = °C + 273.15.
Absolute zero check
Values colder than 0 K (−273.15 °C, −459.67 °F) are flagged instead of silently converted.
Negative numbers on phones
A ± button flips the sign, because many phone number pads have no minus key.
Temperature conversion formulas
Celsius and Fahrenheit differ in both size of degree and starting point. Water freezes at 0 °C and 32 °F and boils at 100 °C and 212 °F at sea-level pressure, so 100 Celsius degrees span the same gap as 180 Fahrenheit degrees. That gives the ratio 9/5 (1.8) and the offset 32.
- Celsius to Fahrenheit: °F = °C × 9/5 + 32. Example: 25 °C × 1.8 + 32 = 77 °F.
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Example: (350 − 32) × 5/9 = 176.67 °C, which recipes round to 180 °C.
- Celsius to Kelvin: K = °C + 273.15. Kelvin uses the same size of degree as Celsius but starts at absolute zero, and it is written without a degree sign.
- Fahrenheit to Kelvin: K = (°F − 32) × 5/9 + 273.15.
- Rankine: °R = °F + 459.67. It is the absolute scale with Fahrenheit-sized degrees, still met in some US engineering tables.
- Réaumur: °Ré = °C × 4/5. Water boils at 80 °Ré, a scale once common in Europe and still seen in old recipes.
Reference temperatures
Rounded where marked. Oven recipes usually round to the nearest 10 °C or 25 °F, so 180 °C and 350 °F are treated as the same setting even though they differ by about 3 degrees.
| What | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Celsius and Fahrenheit are equal | −40 | −40 | 233.15 |
| Home freezer | −18 | −0.4 | 255.15 |
| Water freezes | 0 | 32 | 273.15 |
| Fridge | 4 | 39.2 | 277.15 |
| Comfortable room | 20–22 | 68–71.6 | 293.15–295.15 |
| Typical body temperature | 37 | 98.6 | 310.15 |
| Fever (common threshold) | 38 | 100.4 | 311.15 |
| Very hot summer day | 45 | 113 | 318.15 |
| Water boils at sea level | 100 | 212 | 373.15 |
| Moderate oven | 180 | 356 (recipes: 350) | 453.15 |
| Hot oven | 220 | 428 (recipes: 425) | 493.15 |
Worked examples
Fever check: a thermometer shows 101.5 °F. (101.5 − 32) × 5/9 = 38.6 °C. Many health services treat 38 °C (100.4 °F) or above as a fever; the fever temperature checker gives the line for each method and age. Ask a doctor if you are worried.
Food safety: the USDA lists 165 °F as the safe minimum internal temperature for poultry. (165 − 32) × 5/9 = 73.9 °C, usually written as 74 °C on Celsius probes.
Science homework: a gas at 27 °C is 27 + 273.15 = 300.15 K. Gas laws need kelvins, because doubling a Celsius value does not double the energy of the molecules.
For gas marks and fan-oven adjustments, the cooking converter has a full oven chart, and the all-in-one unit converter covers every other kind of measurement.
Where each scale came from
Daniel Gabriel Fahrenheit published his scale in 1724; it is still used for weather and cooking in the United States. Anders Celsius proposed a 100-step scale in 1742 — originally with 0 at boiling and 100 at freezing, reversed soon after — and the name “centigrade” was officially replaced by “Celsius” in 1948. William Thomson, later Lord Kelvin, described an absolute scale in 1848, and since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K. Celsius is now formally defined from the kelvin, so 0 °C is exactly 273.15 K.
A quick mental shortcut for Celsius to Fahrenheit is “double it and add 30”: 20 °C becomes about 70 °F (the exact answer is 68 °F). It drifts at higher values.
Temperature Converter FAQ
How do I convert Celsius to Fahrenheit?
Multiply by 9/5 (or 1.8) and add 32. For example, 30 °C × 1.8 + 32 = 86 °F. The temperature converter does this instantly and shows the formula.
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. For example, (68 − 32) × 5/9 = 20 °C.
What is 37 °C in Fahrenheit?
37 °C is exactly 98.6 °F, the figure traditionally quoted as normal body temperature. Healthy body temperature varies a little from person to person and through the day.
At what temperature are Celsius and Fahrenheit the same?
At −40. Minus forty degrees Celsius equals minus forty degrees Fahrenheit, because −40 × 9/5 + 32 = −40.
Why is Kelvin written without a degree sign?
The kelvin is an SI base unit in its own right, not a degree on an arbitrary scale, so it is written “300 K” rather than “300 °K”. The degree sign was dropped in 1967.
Can a temperature be below 0 K?
No. Zero kelvin (−273.15 °C or −459.67 °F) is absolute zero, the point at which a system has the least possible thermal energy. The converter warns you if you enter anything colder.