Fahrenheit to Kelvin Converter

The formula: K = (°F - 32) × 5/9 + 273.15

Kelvin is an absolute scale with Celsius-sized degrees, so a Fahrenheit reading has to be resized and then shifted. Two steps do it:

K = (°F - 32) × 5/9 + 273.15

The first step is the Fahrenheit to Celsius conversion, and the second adds the 273.15 offset that puts zero at absolute zero. Nothing else is involved, which makes this one of the shorter conversions in the temperature family. The 273.15 is fixed by the definition of the Celsius scale, as the BIPM SI Brochure describes, and the 459.67 offset that appears in the one-step route is printed in NIST SP 811, Appendix B.9.

The one-step version arrives at the same place: K = (°F + 459.67) × 5/9. For 32 °F that is 491.67 × 5/9 = 273.15 K, and for 212 °F it is 671.67 × 5/9 = 373.15 K. The two forms agree because 459.67 × 5/9 is exactly 255.372, the number you get from -32 × 5/9 + 273.15.

The 0.15 matters. Round the offset to 273 and 98.6 °F becomes 309.999 K instead of 310.15 K, a 0.15 K error that is small on a weather app and unacceptable in a lab notebook.

Two ways to do it, and when each one helps

The two-step route is the one to use when you also want the Celsius answer, which is most of the time. A US thermometer reads 68 °F, the first step gives 20 °C, and the second gives 293.15 K. One subtraction, one division and one addition, and you have all three numbers.

The one-step route is faster when kelvin is the only answer you need: add 459.67, multiply by 5/9, done. For 72 °F the sum is 531.67 and 531.67 × 5/9 = 295.37 K. This is the form to use in a spreadsheet, because it is a single expression with no intermediate rounding.

Going backwards is a mirror image. °F = K × 9/5 - 459.67. A 300 K reading is 300 × 1.8 = 540, minus 459.67 = 80.33 °F. The two offsets, 459.67 and 273.15, are related by the same 1.8 factor: 273.15 × 1.8 = 491.67, and 491.67 - 32 = 459.67.

Whichever route you pick, the degree size ratio stays 5/9. A Fahrenheit degree is only 5/9 as large as a kelvin, so a 9 °F change is a 5 K change. That ratio trips people who expect a 1-to-1 conversion, which is why the arithmetic looks asymmetric even though the physics is simple.

Worked examples: freezing, fever, oven and freezer

Four readings from daily life, each computed with the two-step formula.

• Freezing: 32 °F is (32 - 32) × 5/9 = 0, plus 273.15 = 273.15 K.
• Fever: 100.4 °F is (100.4 - 32) × 5/9 = 38.0, plus 273.15 = 311.15 K.
• Oven: 350 °F is (350 - 32) × 5/9 = 176.67, plus 273.15 = 449.82 K.
• Freezer: 0 °F is (0 - 32) × 5/9 = -17.78, plus 273.15 = 255.37 K.

The freezer example is worth a second look, because the Celsius value is negative while the kelvin value is not. That is the point of an absolute scale: -17.78 °C is still 255.37 K above absolute zero, and any gas law calculation needs that distance rather than the signed Celsius reading.

A quick cross-check on the oven: 449.82 K × 9/5 = 809.67 °R, and 809.67 - 459.67 = 350 °F, which returns the starting number. A conversion that does not round-trip has an arithmetic slip somewhere, and the round trip is faster than re-reading the formula.

Fahrenheit to kelvin chart for common readings

The table covers the range a US household thermometer, oven dial and weather app produce, with the Celsius value beside each row.

Each row moves by 5/9 of the Fahrenheit step. From 70 °F to 98.6 °F is 28.6 Fahrenheit degrees, which is 15.9 kelvin, and 310.15 - 294.26 = 15.89 K. The ratio holds everywhere on the table.

Notice that no kelvin value is negative, not even the -459.67 °F row, which lands exactly on 0 K. Below that line there is nothing to convert.

Why kelvin exists, and why Fahrenheit needs converting

Kelvin starts at absolute zero, the temperature at which molecular motion reaches its minimum. That property makes it the scale where ratios mean something: a gas at 600 K holds twice the thermal energy of the same gas at 300 K, and its volume doubles at constant pressure. In Fahrenheit that same doubling runs from 80.33 °F to 620.33 °F, a change that looks enormous and tells you nothing about the ratio.

This is why chemistry problems insist on kelvin before a gas law. NIST's SI temperature page describes the kelvin as the base unit and Celsius as a derived scale offset from it, and every gas constant in the SI system, including 8.314 J/(mol·K), assumes kelvin. Feed in a Fahrenheit number and the answer is wrong by a factor that depends on the temperature itself, which is the worst kind of error to catch later.

Kelvin also explains a few everyday numbers that look strange. A 6500 K light bulb is not hot enough to burn you, because that figure describes the color of an ideal radiator, not the glass temperature. A 2.7 K cosmic background and a 5,772 K solar surface sit at opposite ends of the same scale, and both are quoted in kelvin because Fahrenheit would need a minus sign and a very large number to say the same thing.

In 2019 the kelvin was redefined through the Boltzmann constant, but the size of the unit did not change, so every conversion on this page still holds to the digit.

How the four scales line up

Fahrenheit and kelvin sit at opposite corners of the temperature family: one is a relative scale with small degrees, the other an absolute scale with large ones. The table places all four side by side at the two water anchors.

Two offsets and one ratio cover everything. Fahrenheit to Celsius subtracts 32 and multiplies by 5/9; Celsius to kelvin adds 273.15; kelvin to Rankine multiplies by 9/5. Chain them in the right order and any pair converts.

A useful check is that the ice point and steam point stay 100 kelvin apart on the absolute scales and 180 Fahrenheit degrees apart on the relative ones, so the ratio between the two degree sizes is 100 to 180, or 5 to 9. That single ratio is where the 5/9 in this page's formula comes from.

Mistakes that produce a wrong kelvin value

Adding 273.15 to the Fahrenheit reading. That step belongs to Celsius only. 98.6 + 273.15 = 371.75 is not body temperature in kelvin, which is 310.15 K. Resize the degree first, then add the offset.

Multiplying by 9/5 instead of 5/9. That is the kelvin to Fahrenheit direction. Going from Fahrenheit to kelvin makes the number bigger only because of the 459.67 offset, not because of the factor.

Forgetting the 32 in the two-step form. K = °F × 5/9 + 273.15 turns 32 °F into 290.93 K instead of 273.15 K, a 17.8 K error that comes straight from the missing ice point shift.

Rounding 273.15 to 273 in a gas law problem. The 0.15 K is 0.05 percent at room temperature, which is small, but it compounds once a problem divides by temperature. Keep the two decimals until the final answer.

Reporting a negative kelvin value. Below -459.67 °F there is nothing to convert. A negative result means the Fahrenheit input was below absolute zero, so check the sign before publishing the number.

One check catches all five: 32 °F must give 273.15 K and 212 °F must give 373.15 K. Run those two anchors whenever a formula is new to you.