Temperature Converter

The converter handles positive and negative temperatures on all four scales.

1. Type a temperature in the Value box. Use a leading minus sign for readings below zero, such as -15.
2. Choose the scale you have in From and the scale you want in To, for example Celsius (°C) and Fahrenheit (°F). The result appears straight away.
3. Read the Formula line under the result, which shows the equation used for that pair of scales.
4. Press Swap units to reverse the two scales without retyping.
5. Check the In every unit list for the same temperature in Celsius, Fahrenheit, Kelvin and Rankine together.

Typical uses include reading an oven setting from a recipe written for another country, checking a forecast while travelling, converting laboratory readings to kelvin and understanding a thermostat. The maths runs in your browser and nothing you type is sent to a server. Results use up to four decimal places for values between 1 and 1,000, so 350 °F shows as 176.6667 °C rather than a long string of sixes.

Temperature scales do not share a zero point, so one multiplication is not enough as it is for length or weight. Each conversion needs a scale factor and an offset.

• Celsius to Fahrenheit: F = C × 9/5 + 32
• Fahrenheit to Celsius: C = (F - 32) × 5/9
• Celsius to Kelvin: K = C + 273.15
• Fahrenheit to Kelvin: K = (F - 32) × 5/9 + 273.15
• Fahrenheit to Rankine: R = F + 459.67
• Kelvin to Rankine: R = K × 9/5

The converter works through Celsius for every pair. The six pairs between Celsius, Fahrenheit and Kelvin display their formula under the result, while pairs that involve Rankine show only the note Formula: via Celsius. A kelvin is the same size as a Celsius degree, and a Rankine degree is the same size as a Fahrenheit degree, so a temperature difference of 1 °C equals a difference of 1.8 °F. The converter applies the equations in full precision and rounds only the display.

These values can be typed into the converter to check it.

• 25 °C is 25 × 9/5 + 32 = 77 °F
• 350 °F is (350 - 32) × 5/9 = 176.6667 °C
• 0 °C is 273.15 K and 491.67 °R
• 300 K is 26.85 °C or 80.33 °F
• -15 °C is 5 °F
• 180 °C is 356 °F, a common oven setting

Anchor points make a result easy to sanity-check. Water freezes at 0 °C (32 °F, 273.15 K) and boils at 100 °C (212 °F, 373.15 K) at sea level pressure. Room temperature is about 20 °C or 68 °F. Absolute zero is -273.15 °C, -459.67 °F, 0 K and 0 °R. The scales cross at -40, where -40 °C and -40 °F are the same temperature. Solving C = 1.8C + 32 gives that answer.

For a quick mental estimate of Celsius to Fahrenheit, double the figure and add 30. That gives 70 °F for 20 °C against the exact 68 °F, close enough for weather but not for a laboratory protocol or a sous-vide bath.

The commonest error is converting a change as if it were a reading. A rise of 10 °C is a rise of 18 °F, not 50 °F, because the offset of 32 applies only to a point on the scale. Another is saying degrees kelvin; the unit is simply the kelvin, written K without a degree sign. This converter labels Rankine as °R.

The tool does not stop you typing a value below absolute zero, so check any result that falls below 0 K. It also cannot allow for fan ovens, gas marks or altitude. Recipes often call for a lower setting in a fan oven, so follow the method written for your appliance. For body temperature, 37 °C is 98.6 °F, and 38 °C is 100.4 °F. These figures are general information and not medical advice.

Related tools cover other quantities in recipes and science. The volume converter handles cups and millilitres, the pressure converter covers pascals and psi, and the energy converter converts joules, kilocalories and kilowatt-hours. The weight converter and scientific calculator are useful when a formula combines several quantities.