Temperature Converter

Four temperature scales

• Kelvin (K) is the SI unit of temperature. It starts at absolute zero, the point where nothing can be colder, so it has no negative values. Since 2019 the kelvin has been defined by fixing the Boltzmann constant at exactly 1.380649 × 10⁻²³ joules per kelvin. It is written without a degree sign: “300 K”, read as “300 kelvins”.
• Celsius (°C) uses degrees the same size as kelvins, shifted so that 0 °C is 273.15 K. That puts the freezing point of water at about 0 °C and its boiling point at about 100 °C at standard atmospheric pressure, the two points the scale was originally built on.
• Fahrenheit (°F), used for weather and cooking in the United States, puts water’s freezing point at 32 °F and its boiling point at 212 °F. That is 180 degrees for the span Celsius covers in 100, so a Fahrenheit degree is 100/180 = 5/9 of a Celsius degree.
• Rankine (°R) is to Fahrenheit what Kelvin is to Celsius: Fahrenheit-sized degrees counted from absolute zero. It appears mainly in US engineering thermodynamics.

Temperature conversion formulas

Unlike other units, temperature scales do not share a zero point, so converting takes a multiplication and an addition. The factor 1.8 is 9/5, the ratio of a Celsius degree to a Fahrenheit degree.

Order matters. Going to Fahrenheit you multiply first and then add 32; coming back you subtract 32 first and then divide. The calculator uses whichever form keeps every constant exact, which is why its working shows “÷ 1.8” rather than “× 0.5555…”.

Worked examples

A European recipe says 180 °C. 180 × 1.8 = 324, and 324 + 32 = 356 °F. That is why recipes treat 180 °C and 350 °F as the same oven setting: they are 6 °F apart.

An American recipe says 350 °F. 350 − 32 = 318, and 318 ÷ 1.8 = 176.6666667 °C. European recipes round this to 175 °C or 180 °C. For the cups and spoons in the same recipe, use the volume converter; for ounces and pounds, the weight converter.

Room temperature in kelvins. 25 °C + 273.15 = 298.15 K, the standard reference temperature used in chemistry tables.

Reference temperatures

* At standard atmospheric pressure (101.325 kPa). Water boils at a lower temperature at altitude, because the air pressure is lower, which is why cooking times change in the mountains.

About body temperature: 37 °C (98.6 °F) is a 19th-century average, not a fixed normal. Healthy temperatures vary from person to person, over the day and with where they are measured (mouth, ear, forehead or armpit), and a large 2020 analysis of US records found that average body temperature has fallen since that figure was set. Ask a clinician what counts as a fever for you or your child.

Things that trip people up

• A change is not a temperature. A rise of 10 °C is a rise of 18 °F, not 50 °F: differences convert with the factor alone (× 1.8), without the + 32. A change of 1 °C is exactly a change of 1 K.
• Ratios only work on absolute scales. 20 °C is not “twice as warm” as 10 °C. In kelvins it is 293.15 K against 283.15 K, about 3.5% warmer. The same applies to any calculation that multiplies or divides temperatures, such as the gas laws.
• “Degrees kelvin” is outdated. The unit has been called simply the kelvin, symbol K, since 1967.
• Nothing is colder than absolute zero. The calculator rejects anything below −273.15 °C, −459.67 °F, 0 K or 0 °R.