Kelvins are required by physics and chemistry: the ideal gas law, Stefan–Boltzmann radiation, Arrhenius reaction rates, thermal noise in electronics. If your temperatures come from a US weather station, a lab instrument set to Fahrenheit or a homework problem written in °F, you need to get them onto the absolute scale before plugging them in. This converter does it in one pass and shows the intermediate Celsius value.
How to use the Fahrenheit to Kelvin converter
- Enter a value in Temperature in Fahrenheit (°F), as low as −459.67 °F.
- Read the result in kelvins. Celsius and Rankine values appear underneath.
- The steps show the subtraction, the 5/9 scaling and the 273.15 offset, so you can check each stage.
Fahrenheit to Kelvin formula
There are two equivalent ways to write it:
The first passes through Celsius. The second passes through Rankine, the absolute scale that uses Fahrenheit-sized degrees: adding 459.67 moves the zero to absolute zero, and the 5/9 shrinks Fahrenheit degrees into kelvins.
Worked example: room temperature at 68 °F
Route 1. 68 − 32 = 36; 36 × 5/9 = 20 °C; 20 + 273.15 = 293.15 K.
Route 2. 68 + 459.67 = 527.67 °R; 527.67 × 5/9 = 293.15 K.
Why absolute temperature matters: a radiation example
A surface’s thermal radiation is proportional to the fourth power of its absolute temperature. Suppose a dark roof warms from 50 °F in the morning to 100 °F in the afternoon. In Fahrenheit, the temperature “doubled”. In kelvins it went from 283.15 K to 310.93 K — only 9.8% higher. The radiated power rises by (310.93 ÷ 283.15)⁴ = 1.454, so about 45% more energy is emitted. Plugging Fahrenheit values into the ratio would have predicted 16 times more. Any formula that multiplies, divides or raises temperature to a power needs kelvins (or Rankine).
Everyday Fahrenheit temperatures in kelvins
| °F | K | Context |
|---|---|---|
| −40 | 233.15 | Where °F and °C meet |
| 0 | 255.37 | Fahrenheit’s zero |
| 32 | 273.15 | Water freezes |
| 50 | 283.15 | Cool spring day |
| 70 | 294.26 | Comfortable room |
| 98.6 | 310.15 | Body temperature |
| 212 | 373.15 | Water boils (nominal, sea level) |
| 350 | 449.82 | Moderate oven |
Common mistakes
- Adding 273.15 to Fahrenheit. That mixes scales. 68 + 273.15 = 341.15 is meaningless; convert to Celsius first.
- Dividing before subtracting. (68 × 5/9) − 32 is not the same as (68 − 32) × 5/9.
- Writing °K. The kelvin takes no degree sign: 293.15 K.
- Converting a temperature change with the full formula. A rise of 18 °F is a rise of 10 K; only the 5/9 applies to differences, as the As a temperature change row shows.
US engineering references sometimes stay in Fahrenheit-sized absolute units instead — see Fahrenheit to Rankine. To go back, use Kelvin to Fahrenheit.
Frequently asked questions
What is the formula for Fahrenheit to Kelvin?
K = (°F − 32) × 5/9 + 273.15. An equivalent form is K = (°F + 459.67) × 5/9. Both give 293.15 K for 68 °F.
What is absolute zero in Fahrenheit?
−459.67 °F, which is 0 K. The converter rejects anything colder.
Can I just convert to Celsius first?
Yes. Convert °F to °C with (°F − 32) × 5/9, then add 273.15. That two-step route is exactly what the one-line formula does.
What is 0 °F in kelvin?
0 °F is 255.37 K (−17.78 °C). Daniel Fahrenheit set his zero using a freezing mixture of ice, water and salt, not at any fundamental physical point.