Fahrenheit to Kelvin

Fahrenheit and kelvin differ in both their zero point and their degree size, so the conversion has two parts. A Fahrenheit degree is 5/9 the size of a kelvin, and the Fahrenheit zero sits 459.67 degrees above absolute zero. The cleanest formula is therefore K = (°F + 459.67) × 5/9. Going through Celsius gives the same answer: 98.6 °F is 37 °C, which is 310.15 K.

Some reference points to check against: 0 °F is 255.37 K, the freezing point of water at 32 °F is 273.15 K, a 70 °F room is 294.26 K and boiling water at 212 °F is 373.15 K. At −40 °F the Fahrenheit and Celsius scales meet, which is 233.15 K.

Students and engineers in the US often have Fahrenheit data and formulas that need absolute temperature, such as gas laws, radiation or heat-engine efficiency. Physics textbooks and most scientific software expect kelvin, so the conversion is a routine step in homework and lab reports.

American thermodynamics also has its own absolute scale, the Rankine, named after the Scottish engineer William Rankine. It uses Fahrenheit-sized degrees starting from absolute zero, so °R = °F + 459.67. Converting Rankine to kelvin is then just a multiplication by 5/9, and that is exactly the shortcut formula above. Older steam tables and some aerospace work still use degrees Rankine.

The usual mistake is multiplying before subtracting. 212 × 5/9 − 32 + 273.15 gives 359 K, which is wrong; you must remove the 32-degree offset first, then scale, then shift to kelvin. Another slip is stopping at Celsius and labeling the result K. If your answer is a small number for a normal weather temperature, you are still in Celsius.

For Celsius readings use Celsius to kelvin, and for everyday weather use Fahrenheit to Celsius.