Temperature Converter

What this temperature converter does

This tool converts a temperature between the three scales in everyday and scientific use: Celsius (°C), used by most of the world and by science teaching; Fahrenheit (°F), used mainly in the United States and a handful of its territories; and kelvin (K), the SI base unit of thermodynamic temperature. All three fields are live inputs, so you can start from whichever scale your source uses — a recipe in Fahrenheit, a weather forecast in Celsius, or a physics problem in kelvin — and read the equivalents immediately.

Everything is calculated in your browser with plain JavaScript. There is no server call, no stored history, and no tracking of what you convert.

How to use it

Click into the Celsius, Fahrenheit, or Kelvin box and type a value. The other two fields recalculate as you type, and the panel on the right repeats all three rounded to one decimal place, which is usually the sensible precision for weather, cooking, and health readings. Negative numbers and decimals both work: -12.5 is a valid Celsius entry. If you clear a field or leave it partially typed (just a minus sign, say), the other fields blank out rather than showing NaN. Enter a temperature colder than absolute zero and the tool tells you so instead of pretending the number is physical.

The formulas

Celsius and Fahrenheit have different zero points and different degree sizes: one Celsius degree equals exactly 1.8 Fahrenheit degrees. Kelvin shares Celsius's degree size but starts at absolute zero, so converting between them is a simple offset of 273.15.

• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Celsius to kelvin: K = °C + 273.15
• Kelvin to Celsius: °C = K − 273.15
• Fahrenheit to kelvin: K = ((°F − 32) × 5/9) + 273.15
• Kelvin to Fahrenheit: °F = ((K − 273.15) × 9/5) + 32

Internally the converter always routes through Celsius: it turns your input into a Celsius value first, then produces the other two from it. That keeps the three fields mathematically consistent no matter which one you typed in, and avoids the rounding drift you get from chaining Fahrenheit straight into kelvin.

Note that kelvin values are written without a degree symbol and without the word "degrees" — 300 K is read as "three hundred kelvin". Kelvin is also never negative, because 0 K is the bottom of the scale.

A worked example

Suppose a US recipe calls for an oven at 350 °F and your oven dial is in Celsius. Subtract 32 to remove the offset: 350 − 32 = 318. Multiply by 5/9 to rescale the degrees: 318 × 5/9 = 176.666… So 350 °F is about 176.7 °C, which in practice means setting the dial to 175 or 180 °C. To express the same temperature in kelvin, add 273.15: 176.67 + 273.15 ≈ 449.8 K.

Going the other way: normal human body temperature of 37 °C becomes (37 × 9/5) + 32 = 66.6 + 32 = 98.6 °F, or 37 + 273.15 = 310.15 K.

Reference points worth memorising

• Absolute zero: 0 K = −273.15 °C = −459.67 °F
• Water freezes: 273.15 K = 0 °C = 32 °F
• Comfortable room temperature: ~293 K = 20 °C = 68 °F
• Body temperature: ~310.15 K = 37 °C = 98.6 °F
• Water boils at sea level: 373.15 K = 100 °C = 212 °F
• The crossover point: −40 °C = −40 °F, the one temperature where both scales agree

For rough mental arithmetic in the field, doubling Celsius and adding 30 gets you within a couple of degrees of the Fahrenheit value across normal weather temperatures (20 °C → about 70 °F, actual 68 °F). Use the exact formula above, or this tool, whenever the difference matters — cooking, medicine, and lab work all care about that couple of degrees.

References

• Celsius — Wikipedia
• Kelvin — Wikipedia
• SI Units — Temperature (NIST)