Temperature Converter
Convert a temperature between Celsius, Fahrenheit and Kelvin, with a note on how it feels and a check against absolute zero.
Quick Presets
Temperature Conversion Formulas
• Celsius to Fahrenheit: (°C × 9/5) + 32
• Fahrenheit to Celsius: (°F - 32) × 5/9
• Celsius to Kelvin: °C + 273.15
• Kelvin to Celsius: K - 273.15
Updated Formula covered by automated tests
Enter a reading in degrees Celsius, degrees Fahrenheit or kelvins and see it on the other two scales, with a plain-language note on how that temperature feels. Cooks use it for oven settings in foreign recipes, travelers for weather forecasts, parents for a child’s fever reading, and students for chemistry and physics problems that require kelvins.
Temperature behaves differently from every other unit on this site. Meters, liters and pounds all start at the same zero, so converting is a single multiplication. Celsius and Fahrenheit put their zeros in different places, so a conversion needs a multiplication and an offset, and the order of those two steps matters. The converter also refuses values below absolute zero, the point where no colder temperature is physically possible.
The formula
- °C
- Degrees Celsius, where water freezes at 0 and boils at about 100 at sea-level pressure
- °F
- Degrees Fahrenheit, where water freezes at 32 and boils at about 212
- K
- Kelvin, the SI unit, which starts at absolute zero and uses steps the same size as Celsius degrees
- 9 ÷ 5
- The ratio of degree sizes: 180 Fahrenheit degrees span the same range as 100 Celsius degrees
Between freezing and boiling water there are 100 Celsius degrees and 180 Fahrenheit degrees, so one Celsius degree equals 1.8 Fahrenheit degrees. The 32 shifts the zero point, because Fahrenheit placed freezing water at 32 rather than 0. To go from Fahrenheit to Celsius you remove the offset first and then rescale; doing it in the other order is the most common mistake.
Kelvin uses Celsius-sized steps but counts from absolute zero, which is −273.15 °C or −459.67 °F. The Celsius scale is now defined from the kelvin rather than from boiling water, and since 2019 the kelvin is fixed through the Boltzmann constant. One side effect is that pure water at standard pressure boils at about 99.97 °C, a hair under the round 100 that the scale was originally built on. Kelvin values are written without a degree sign: 300 K, not 300 °K.
Worked example
Aiko is following an American cookie recipe that says to bake at 350 °F, but her oven in Osaka is marked in Celsius. Her chemistry homework also asks for the same temperature in kelvins.
- 1 Remove the Fahrenheit offset: 350 − 32 = 318.
- 2 Rescale to Celsius-sized degrees: 318 × 5 ÷ 9 = 176.67 °C.
- 3 Add the Kelvin offset: 176.67 + 273.15 = 449.82 K.
Result: 350 °F is 176.67 °C, so Aiko sets her oven to 175 or 180 °C, which is what European and Japanese recipes usually print. In kelvins it is 449.82 K.
Before you rely on the result
- Results are rounded to two decimal places, which is finer than kitchen thermometers or most weather stations measure.
- The boiling point of water drops with altitude (to about 95 °C in Denver), so the "water boils" preset assumes sea-level pressure.
- Rankine and other historical scales are not included.
Frequently asked questions
If the forecast says it will be 10 degrees warmer tomorrow, how much is that in Fahrenheit?
A change is not a reading, so the 32 offset does not apply. A rise of 10 °C is a rise of 18 °F (10 × 1.8). Plugging 10 into the reading formula would give 50 °F, which is wrong for a difference. The same rule applies to tolerances such as "±2 °C", which equals ±3.6 °F.
At what temperature are Celsius and Fahrenheit equal?
At −40. Setting °F = °C in °F = °C × 1.8 + 32 gives −0.8 × °C = 32, so °C = −40. It is a useful check that you have the formula the right way round.
Is 100.4 °F a fever?
100.4 °F is 38.0 °C, the threshold many health services use for fever in adults and children. Readings vary by measuring site (mouth, ear, forehead, armpit), so follow your doctor’s or local health service’s guidance, and seek medical advice promptly for a fever in a baby under three months.
Why does doubling a temperature not make it twice as hot?
Celsius and Fahrenheit have arbitrary zeros, so 20 °C is not twice 10 °C in any physical sense. Ratios only make sense in kelvins: 293.15 K versus 283.15 K is only about 3.5 percent higher on the absolute scale, not double.
Sources
- NIST: SI units, temperature (nist.gov)
- BIPM: The International System of Units (SI Brochure) (bipm.org)