How to convert temperatures
Choose the unit you have in From, the unit you want in To, type the value and press Convert. The converter handles Celsius, Fahrenheit, kelvin, Rankine, Réaumur and the triple point of water, in any direction, and refuses values colder than absolute zero, which cannot exist.
Unlike length or weight, temperature scales do not share a common zero, so converting is not a single multiplication. Most conversions need an offset as well as a scale factor, which is why the formulas below have two steps.
Temperature conversion formulas
| From | To | Formula |
|---|---|---|
| Celsius | Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit | Celsius | °C = (°F − 32) × 5/9 |
| Celsius | kelvin | K = °C + 273.15 |
| kelvin | Celsius | °C = K − 273.15 |
| Fahrenheit | kelvin | K = (°F + 459.67) × 5/9 |
| kelvin | Rankine | °R = K × 9/5 |
| Fahrenheit | Rankine | °R = °F + 459.67 |
| Celsius | Réaumur | °Ré = °C × 4/5 |
Worked examples
- 25 °C to Fahrenheit: 25 × 9/5 = 45, and 45 + 32 = 77 °F.
- 98.6 °F to Celsius: 98.6 − 32 = 66.6, and 66.6 × 5/9 = 37 °C.
- 300 K to Celsius: 300 − 273.15 = 26.85 °C.
- 350 °F to Celsius (a common oven setting): (350 − 32) × 5/9 ≈ 176.67 °C.
Reference temperatures
| Reference point | °C | °F | K | °R | °Ré |
|---|---|---|---|---|---|
| Absolute zero | -273.15 | -459.67 | 0 | 0 | -218.52 |
| Celsius and Fahrenheit are equal | -40 | -40 | 233.15 | 419.67 | -32 |
| Water freezes (at 1 atm) | 0 | 32 | 273.15 | 491.67 | 0 |
| Typical room temperature | 20 | 68 | 293.15 | 527.67 | 16 |
| Typical human body temperature | 37 | 98.6 | 310.15 | 558.27 | 29.6 |
| Water boils (at 1 atm, approx.) | 100 | 212 | 373.15 | 671.67 | 80 |
| Moderate oven | 180 | 356 | 453.15 | 815.67 | 144 |
The temperature scales
- Celsius (°C) is used for everyday temperatures almost everywhere in the world. Water freezes at 0 °C and boils at about 100 °C at sea-level pressure.
- Fahrenheit (°F) is the everyday scale in the United States. Water freezes at 32 °F and boils at about 212 °F, so a 100-degree span in Celsius is 180 degrees in Fahrenheit.
- kelvin (K) is the SI unit of temperature and starts at absolute zero, the lowest temperature possible. One kelvin is the same size as one degree Celsius. It is written without a degree sign and cannot be negative.
- Rankine (°R) is an absolute scale like kelvin, but with degrees the size of Fahrenheit degrees. It still appears in some American engineering and thermodynamics work.
- Réaumur (°Ré) sets freezing at 0 and boiling at 80. It was once common in parts of Europe and is now mostly found in historical texts and old recipes.
- Triple point of water (T/Ttpw) expresses a temperature as a ratio to 273.16 K, the temperature at which ice, liquid water and water vapour coexist. That point defined the kelvin until the SI was revised in 2019; since then the kelvin has been defined by fixing the Boltzmann constant, and the triple point is very close to 273.16 K.
Temperatures versus temperature differences
The formulas above convert a temperature. A difference between two temperatures converts without the offset. If the weather warms up by 10 °C, it has warmed by 10 × 9/5 = 18 °F, not by 50 °F. A change of 1 °C is exactly a change of 1 K, and a change of 1 °F is exactly a change of 1 °R.
Frequently asked questions
At what temperature are Celsius and Fahrenheit the same?
At −40 degrees: −40 °C is exactly −40 °F, as the reference table shows.
Why does kelvin not use a degree sign?
Since 1967 the unit has been named "kelvin", with the symbol K and no degree sign, because it measures from absolute zero rather than from an arbitrary reference point. So 300 K is read "300 kelvins".
Is 20 °C twice as warm as 10 °C?
No. Ratios only make sense on absolute scales. In kelvin, 20 °C is 293.15 K and 10 °C is 283.15 K, so the warmer temperature is only about 3.5% higher in absolute terms.
Why does the converter reject some negative values?
Nothing can be colder than absolute zero: 0 K, −273.15 °C or −459.67 °F. Negative Celsius and Fahrenheit values above those limits are converted normally.