Temperature converter
Convert Celsius to Fahrenheit, Kelvin, Rankine and Réaumur with the offset maths shown, an absolute-zero guard and a full lookup chart.
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1 °C = 33.8 °F
Automatic formatting keeps up to 12 significant digits and switches to scientific notation only for numbers past 10¹⁸ or below 10⁻⁹. You can paste values with thousands separators, and fractions such as 3/4 or 1 1/2 are understood too.
°F = °C × 1.8 + 32
Your value: 1 × 1.8 + 32 = 33.8 °F
Celsius: Water freezes at 0 °C and boils at 100 °C at sea level.
Fahrenheit: Water freezes at 32 °F and boils at 212 °F.
| °C | °F |
|---|---|
| -40 | -40 |
| -20 | -4 |
| -10 | 14 |
| 0 | 32 |
| 5 | 41 |
| 10 | 50 |
| 15 | 59 |
| 20 | 68 |
| 25 | 77 |
| 30 | 86 |
| 37 | 98.6 |
| 40 | 104 |
| 50 | 122 |
| 100 | 212 |
| °F | °C |
|---|---|
| -40 | -40 |
| -20 | -28.8888888889 |
| -10 | -23.3333333333 |
| 0 | -17.7777777778 |
| 5 | -15 |
| 10 | -12.2222222222 |
| 15 | -9.44444444444 |
| 20 | -6.66666666667 |
| 25 | -3.88888888889 |
| 30 | -1.11111111111 |
| 37 | 2.77777777778 |
| 40 | 4.44444444444 |
| 50 | 10 |
| 100 | 37.7777777778 |
| Unit | Symbol | Relation to K | Notes |
|---|---|---|---|
| CelsiusEveryday | °C | K = °C + 273.15 | Water freezes at 0 °C and boils at 100 °C at sea level. |
| FahrenheitEveryday | °F | K = (°F + 459.67) × 5/9 | Water freezes at 32 °F and boils at 212 °F. |
| KelvinAbsolute | K | K = K | SI base unit. Starts at absolute zero; no degree sign. |
| RankineAbsolute | °R | K = °R ÷ 1.8 | Absolute scale with Fahrenheit-sized degrees. |
| RéaumurHistorical | °Ré | K = °Ré × 1.25 + 273.15 | Water freezes at 0 °Ré and boils at 80 °Ré. Still seen in some cheese and syrup recipes. |
How to convert a temperature
- Type the temperature into either field. Negative values are fine — use a normal minus sign, and the tool also understands the typographic minus you get when pasting from a document.
- Pick the two scales. Celsius and Fahrenheit are at the top, the absolute scales (Kelvin and Rankine) next, and Réaumur last.
- The formula panel shows the exact expression with your own number substituted into it, which is usually what you actually need for homework or a report.
Temperature is the one dimension where conversion is not a simple multiplication. Celsius and Fahrenheit have different zero points as well as different degree sizes, so every conversion involves both a scaling step and a shift. That is why "multiply by 1.8" alone gives the wrong answer and why the tool always shows the offset.
The formulas, written out
| Conversion | Formula | Check |
|---|---|---|
| Celsius to Fahrenheit | °F = °C × 9/5 + 32 | 100 °C = 212 °F |
| Fahrenheit to Celsius | °C = (°F − 32) × 5/9 | 98.6 °F = 37 °C |
| Celsius to Kelvin | K = °C + 273.15 | 0 °C = 273.15 K |
| Kelvin to Celsius | °C = K − 273.15 | 310.15 K = 37 °C |
| Fahrenheit to Rankine | °R = °F + 459.67 | 32 °F = 491.67 °R |
| Celsius to Réaumur | °Ré = °C × 4/5 | 100 °C = 80 °Ré |
Why the tool refuses temperatures below absolute zero
Absolute zero is −273.15 °C, −459.67 °F, or 0 K. It is the point at which a system has the least possible thermal energy, and no lower temperature exists. Enter something colder and this converter tells you so instead of quietly returning a negative Kelvin value, because a negative Kelvin figure in a spreadsheet is the kind of error that survives all the way to a printed report.
Everything is converted internally through Kelvin, which is why the guard catches the mistake no matter which pair of scales you picked.
Reference: temperatures worth recognising
| What | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Where the scales cross | −40 | −40 | 233.15 |
| Freezing point of water | 0 | 32 | 273.15 |
| Comfortable room | 21 | 69.8 | 294.15 |
| Human body | 37 | 98.6 | 310.15 |
| Fever threshold | 38 | 100.4 | 311.15 |
| Boiling water at sea level | 100 | 212 | 373.15 |
| Moderate oven | 180 | 356 | 453.15 |
Oven temperatures and gas marks
Recipes are the most common reason people convert temperatures, and ovens are marked in inconvenient steps. British gas marks step in 25 °F increments from gas mark 1 at 275 °F (140 °C). The usual baking range maps as follows: gas 3 is 160 °C or 325 °F, gas 4 is 180 °C or 350 °F, gas 6 is 200 °C or 400 °F, and gas 7 is 220 °C or 425 °F.
Fan or convection ovens run hotter for the same setting, so most recipes tell you to drop a conventional temperature by 20 °C, or about 35 °F, when using fan. That is a rule of thumb about ovens rather than a unit conversion, so the converter above deliberately leaves it to you.
Questions
What is the formula for Celsius to Fahrenheit?
°F = °C × 1.8 + 32. Going the other way, °C = (°F − 32) × 5⁄9. Both are shown live above with your own number substituted in.
At what temperature are Celsius and Fahrenheit the same?
−40°. It is the one point where the two scales cross, which makes it a handy sanity check on any converter.
Why can I not enter a temperature below −273.15 °C?
That is absolute zero, the point where a substance has no thermal energy left. Nothing can be colder, so the tool flags it rather than returning a nonsense number.
Do I need Kelvin for everyday use?
Rarely — but it is the SI unit, so scientific work, colour temperature and gas-law calculations all use it. Kelvin is Celsius shifted by 273.15 with no scaling.
What is Réaumur?
An 18th-century scale where water freezes at 0 °Ré and boils at 80 °Ré. It survives in a few traditional cheese, syrup and confectionery recipes.