Fahrenheit to Kelvin Converter

1. Enter the Fahrenheit value. Negative values are fine, down to -459.67 deg F.
2. Choose the decimal places. Most Fahrenheit inputs give a kelvin value with repeating decimals.
3. Read the kelvin value, plus Celsius and Rankine. Paste a column of readings into the bulk box to convert them together.

In this guide, deg F is degrees Fahrenheit, deg C is degrees Celsius and deg R is degrees Rankine. Kelvin values carry the symbol K and no degree sign.

A Fahrenheit to kelvin conversion restates a US customary temperature as an absolute SI temperature. It moves the zero down to absolute zero and swaps the Fahrenheit degree for the larger kelvin.

NIST SP 811 gives the relation as T/K = (t/deg F + 459.67) / 1.8. The kelvin on the left is the SI base unit of thermodynamic temperature. NIST SP 330 defines it by fixing the Boltzmann constant at exactly 1.380 649 x 10^-23 J/K, the definition in force since the 2019 revision of the SI.

The conversion matters most where US data meets physics. Gas suppliers and older US engineering sheets quote very cold liquids in Fahrenheit. Formulas for gases, heat engines and radiation need kelvin. Students in US classes face the same gap when a problem starts with a Fahrenheit reading.

The formula first measures the temperature from absolute zero in Fahrenheit-size degrees, then resizes those degrees into kelvins.

1. Add 459.67 to the Fahrenheit value. Example: 68 + 459.67 = 527.67. This is the Rankine value.
2. Divide by 1.8. Example: 527.67 / 1.8 = 293.15 K.
3. Check in reverse: 293.15 x 1.8 - 459.67 = 68 deg F.

Dividing by 1.8 is the same as multiplying by 5/9. A second route also works: subtract 32, divide by 1.8, then add 273.15. The NIST Office of Weights and Measures lists that form as (deg F - 32) / 1.8 + 273.15. Both routes are exact and agree to every decimal.

A quick mental method: add 460, then divide by 1.8. For 68 deg F that is 528 / 1.8 = 293.33 K. The shortcut always reads about 0.18 K high.

Liquid nitrogen at -320.4 deg F. (-320.4 + 459.67) / 1.8 = 77.37 K. NIST lists the normal boiling point as 77.4 K. The small gap comes from rounding in the Fahrenheit figure.

Liquid hydrogen at -423.2 deg F. (-423.2 + 459.67) / 1.8 = 20.26 K, against a listed 20.3 K.

Liquid helium at -452.1 deg F. (-452.1 + 459.67) / 1.8 = 4.21 K, against a listed 4.2 K.

A room at 68 deg F. (68 + 459.67) / 1.8 = 293.15 K. This is 20 deg C.

The point where Fahrenheit and Celsius meet, -40 deg F. (-40 + 459.67) / 1.8 = 233.15 K.

Zero Fahrenheit. (0 + 459.67) / 1.8 = 255.3722 K. The decimals repeat, so pick a sensible rounding.

To turn a kelvin answer back into a US reading, use the kelvin to Fahrenheit converter.

The formula is exact. The quality of the answer depends on these points.

Rounding in the Fahrenheit Source

A Fahrenheit figure rounded to 0.1 deg F is only good to about 0.06 K. That is why -320.4 deg F converts to 77.37 K and not to the listed 77.4 K exactly.

Repeating Decimals

Dividing by 1.8 often gives decimals that never end. The value 0 deg F is 255.3722... K. Choose the decimals that match the precision of your reading.

Negative Inputs

The sign matters in the first step. For -320.4 deg F, the sum is 459.67 - 320.4 = 139.27, not 780.07.

Temperature or Temperature Change

A change skips the 459.67. A drop of 18 deg F is a drop of 10 K. Only a temperature needs the full formula.

Pressure and Boiling Points

The boiling points quoted for cryogenic liquids are normal boiling points, taken at atmospheric pressure. At another pressure a liquid boils at a different temperature, and the conversion does not adjust for that.

Fahrenheit is a relative scale for daily use, and kelvin is an absolute scale for science.

Need the metric everyday reading first? The Fahrenheit to Celsius converter covers that step.

Reading US Cryogenic Data

NIST describes the cryogenic region as the range below about 120 K, which is -243.67 deg F. US suppliers often state boiling points in Fahrenheit. Physics formulas and most research papers use kelvin. Converting -297.3 deg F gives 90.21 K for liquid oxygen, and -258.6 deg F gives 111.71 K for liquid methane.

Solving Gas Problems from a Fahrenheit Reading

Gas pressure in a sealed, rigid tank follows absolute temperature. A tank cooled from 68 deg F to -40 deg F goes from 293.15 K to 233.15 K. The pressure falls to 79.5 percent of its starting value. Fahrenheit numbers cannot give that ratio. The ideal gas law calculator takes the kelvin values directly.

Moving from US Engineering Units to SI

Some US engineering work uses the Rankine scale, which is Fahrenheit plus 459.67. The Fahrenheit to Rankine converter gives that value. Kelvin is the same absolute temperature in SI units.

Checking Sensor Limits

NIST notes that platinum resistance thermometers are commonly used down to about 20 K. In Fahrenheit that limit is -423.67 deg F. Converting a Fahrenheit process value to kelvin shows whether it sits inside a stated sensor range.

Values from K = (deg F + 459.67) / 1.8, rounded to 2 decimals, with Celsius for reference.

Each 9 deg F step adds exactly 5 K. That is why 32, 50, 59, 68 and 77 deg F all give kelvin values ending in .15.

1. Adding 273.15 to the Fahrenheit Number

That offset belongs to Celsius. For 68 deg F it gives 341.15, which is 48 K too high.

2. Dividing Before Adding

68 / 1.8 + 459.67 gives 497.45. The addition must come first, inside the brackets.

3. Stopping at Celsius

(68 - 32) / 1.8 is 20, which is the Celsius value. Add 273.15 to finish at 293.15 K.

4. Stopping at Rankine

68 + 459.67 is 527.67 deg R. It is an absolute temperature, yet it is not kelvin until you divide by 1.8.

5. Losing the Minus Sign

Cryogenic values are far below zero. Typing 320.4 for -320.4 deg F returns 433.37 K, hotter than boiling water.

6. Writing Degrees Kelvin

The unit is the kelvin, symbol K. NIST states that the unit name is kelvin with a small k, and the symbol is a capital K with no degree sign.

The result is exact to the decimals shown, because 459.67 and 1.8 are defined numbers. Rounding happens only in the display.

32 deg F is 273.15 K. This is the freezing point of water and the zero of the Celsius scale.

-459.67 deg F is 0 K, absolute zero. The converter blocks any Fahrenheit value below it.

NIST lists the normal boiling point of nitrogen as 77.4 K, which is -320.4 deg F or -195.8 deg C.

NIST describes the cryogenic region as below about 120 K. That converts to -243.67 deg F, so cryogenic work starts near -244 deg F.

No. The lowest possible reading is -459.67 deg F, and it converts to exactly 0 K. Kelvin values are never negative.

0 deg F is 255.3722 K, usually rounded to 255.37 K.

Yes. It passes through Celsius and gives the same result as (F + 459.67) / 1.8 for every input.

A change of 1 deg F is a change of 5/9 K, about 0.5556 K. A change of 9 deg F is exactly 5 K.

Use =(A2+459.67)/1.8 with the Fahrenheit value in cell A2. Keep the brackets, or the result will be wrong.

NIST lists the normal boiling point of helium as 4.2 K, which is -452.1 deg F. Below 2.17 K liquid helium becomes a superfluid.

No. Your numbers never leave the page. Anything you press Save on is kept in this browser only.

• Degree Fahrenheit to kelvin, T/K = (t/deg F + 459.67) / 1.8 (NIST SP 811, Appendix B.8).
• Definition of the kelvin through the Boltzmann constant (NIST SP 330, The International System of Units, section 2).
• Cryogenic region below about 120 K and the normal boiling points of cryogenic liquids (NIST, About Cryogenics).
• Exact Fahrenheit to kelvin relation (deg F - 32) / 1.8 + 273.15 (NIST Office of Weights and Measures).