Kelvin Converter: Kelvin, Celsius, Fahrenheit and Rankine

Kelvin conversion formulas

Kelvin is the SI base unit of temperature, used throughout physics, chemistry and engineering. It starts at absolute zero — the coldest possible temperature — and its degrees are the same size as Celsius degrees. That makes the Kelvin–Celsius conversion a simple shift:

K = °C + 273.15  ·  °C = K − 273.15

Fahrenheit needs two steps, through Celsius:

K = (°F − 32) × 5/9 + 273.15  ·  °F = (K − 273.15) × 9/5 + 32

Rankine is to Fahrenheit what Kelvin is to Celsius: an absolute scale with Fahrenheit-sized degrees.

°R = °F + 459.67  ·  °R = K × 9/5

Note that Kelvin is written without a degree sign: it is “300 K”, read as “300 kelvins”, not “300 degrees Kelvin”. Rankine keeps the degree sign (°R).

Source: NIST temperature conversion formulas for Celsius, Fahrenheit and Kelvin. Rankine uses Fahrenheit-sized degrees starting at absolute zero; °R = K × 1.8.

Temperature scale comparison table

The same physical temperatures expressed on all four scales. Notice that Kelvin and Rankine are both zero at absolute zero, and that there are no negative values on either absolute scale.

Worked examples

300 K to Celsius and Fahrenheit

300 − 273.15 = 26.85 °C. Then 26.85 × 9/5 + 32 = 80.33 °F. Physicists often use 300 K as a round figure for “room temperature”, although it is a fairly warm room.

25 °C to Kelvin

25 + 273.15 = 298.15 K. This is the “standard ambient temperature” used in many chemistry data tables.

50 °F to Kelvin

First to Celsius: (50 − 32) × 5/9 = 10 °C. Then add 273.15: 283.15 K.

0 K to Fahrenheit

0 − 273.15 = −273.15 °C, then −273.15 × 9/5 + 32 = −459.67 °F, which is also 0 °R.

Why scientists use Kelvin

Many physical laws depend on the absolute amount of thermal energy, not on an arbitrary zero point. In the ideal gas law, for example, pressure is proportional to temperature only when temperature is measured from absolute zero. Doubling a temperature from 10 °C to 20 °C does not double the energy of a gas, but doubling from 283.15 K to 566.3 K does. Using Kelvin keeps those calculations honest and avoids negative numbers.

Kelvin also appears outside the lab. Light bulbs and camera white-balance settings describe colour temperature in kelvins: a warm-white bulb is around 2700 K, while daylight is about 5500–6500 K. That is a description of colour, not of how hot the bulb feels.

Common mistakes

• Adding 273 instead of 273.15 is fine for rough work, but exam answers and lab reports usually expect 273.15.
• A temperature difference is the same in kelvins and Celsius degrees: a rise of 10 °C is a rise of 10 K.
• Any value below 0 K, −273.15 °C or −459.67 °F is impossible — the converter flags these.

Kelvin in the classroom

School science questions often give gas temperatures in Celsius and expect you to switch to Kelvin before using a formula. A quick habit helps: write the Kelvin value next to every Celsius number as soon as you read the question, and only switch back at the end if the answer is asked for in Celsius. Keep the 0.15 unless the question tells you to use 273.

For everyday Celsius–Fahrenheit conversion, the °C to °F converter is quicker, and the temperature conversion worksheet has Kelvin practice questions with answers.