Temperature Converter

How do you convert temperature units?

To convert a temperature, multiply to adjust the size of the degree, then add or subtract to move the zero point. Temperature is different from length or weight: two scales can disagree on where zero sits as well as on how big one degree is, so a conversion needs both steps. Doing it in the wrong order is the most common mistake.

The Celsius and Fahrenheit formulas

• °C to °F: multiply by 9/5 (1.8), then add 32.
• °F to °C: subtract 32 first, then multiply by 5/9.
• °C to K: add 273.15. K to °C: subtract 273.15.

For example, to convert 350 °F to Celsius, a common oven setting: 350 − 32 = 318, and 318 × 5/9 = 176.67 °C. Going the other way, 25 °C × 1.8 = 45, and 45 + 32 = 77 °F. The Celsius to Fahrenheit converter and the Fahrenheit to Celsius converter do this for any value.

Temperatures versus temperature differences

The offset only applies to actual readings. A difference of 10 °C, such as a rise from 15 °C to 25 °C, is a difference of 18 °F, not 50 °F. When a forecast says “5 degrees warmer” or a spec sheet gives a tolerance of ±2 °C, multiply by 1.8 and leave out the 32.

Where −40 °C equals −40 °F

Because the scales have different slopes, they cross at exactly one point: −40 °C × 1.8 + 32 = −40 °F. Above that point the Fahrenheit number is always larger; below it, the Celsius number is.

What is normal body temperature in Celsius and Fahrenheit?

Normal body temperature is conventionally 37 °C, which is exactly 98.6 °F, although healthy readings vary from person to person, through the day and with where the reading is taken. Readings under the arm tend to be lower than oral readings, and rectal or ear readings tend to be higher.

Because a change of 1 °C equals a change of 1.8 °F, a half-degree rise in Celsius is a 0.9 °F rise in Fahrenheit. The conversions here are exact, but the labels are general guidance, not a diagnosis: follow the advice of a doctor or your local health service, especially for infants. The Celsius to Fahrenheit converter and the Fahrenheit to Celsius converter give any other reading to two decimals.

What is absolute zero, and how did the temperature scales begin?

Absolute zero is the lowest temperature possible, the point at which a system has the least thermal energy it can have. It is 0 K, which is exactly −273.15 °C and −459.67 °F. Nothing can be colder, which is why the kelvin and Rankine scales start there and never go negative.

Four scales, three centuries

• Fahrenheit (1724): Daniel Gabriel Fahrenheit's scale puts water's freezing point at 32 °F and its boiling point at 212 °F at sea-level pressure, 180 degrees apart.
• Réaumur (1730): René de Réaumur split the same interval into 80 degrees. It was common in parts of Europe in the eighteenth and nineteenth centuries but is now historical.
• Celsius (1742): Anders Celsius proposed a 100-degree scale between freezing and boiling water, originally inverted; the modern direction was adopted soon after.
• Kelvin (1848) and Rankine (1859): William Thomson (Lord Kelvin) and William Rankine proposed absolute scales that begin at absolute zero. Kelvin uses Celsius-sized degrees, Rankine uses Fahrenheit-sized ones.

Since the 2019 revision of the SI, the kelvin is defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ J/K, and the degree Celsius is defined as the kelvin minus 273.15. Water's freezing and boiling points are no longer part of any definition, although they remain close to 0 °C and 100 °C at standard pressure. Kelvin is written without a degree sign: 300 K, not 300 °K.

What is ITS-90, and how are thermometers calibrated?

ITS-90, the International Temperature Scale of 1990, is the practical scale laboratories use to calibrate thermometers, built on fixed points such as the triple point of water, assigned 273.16 K (0.01 °C). The kelvin itself is defined through the Boltzmann constant, but that definition is hard to use on a workbench, so national metrology institutes such as NIST realize temperature with ITS-90 instead. It assigns exact values to the melting, freezing and triple points of pure substances and specifies how to interpolate between them with standard platinum resistance thermometers.

One side effect surprises people: on ITS-90, pure water at standard atmospheric pressure boils at about 99.97 °C rather than exactly 100 °C, because the scale is no longer built on water. The boiling point also falls as air pressure drops, so water boils noticeably cooler at altitude; the pressure converter covers the units involved. Keep temperature and heat apart, too: temperature says how hot something is, while heat is energy, measured in joules or calories with the energy converter. For lab readings, the Celsius to kelvin converter adds the exact 273.15 offset.

Where is each temperature scale used?

Celsius is the everyday scale in almost every country, Fahrenheit remains standard in the United States, and kelvin is the scale of science. Rankine and Réaumur survive only in specialist and historical work.

• Celsius: weather, cooking, medicine and everyday life in almost every country, and in most of science alongside kelvin.
• Fahrenheit: weather, thermostats, ovens and body temperature in the United States and a handful of other countries.
• Kelvin: physics, chemistry, astronomy and cryogenics, and the color temperature of light bulbs and camera white balance (for example 2,700 K warm white). The kelvin to Celsius converter brings these back to familiar numbers.
• Rankine: thermodynamics and engineering calculations in US customary units, such as gas-turbine and refrigeration work.
• Réaumur: historical texts and antique thermometers, mostly from continental Europe.