Kelvin to Rankine Converter

Multiply by 1.8 and the job is done

Rankine temperature is kelvin temperature times 1.8. Written as a formula, °R = K x 1.8, and the reverse is K = °R / 1.8. There is no second constant to remember, because the two scales share the same zero: 0 K and 0 °R are both absolute zero. Only the size of the degree differs, and one kelvin spans 1.8 Rankine degrees, so a Rankine degree is 5/9 of a kelvin.

The NIST Guide to the SI, Appendix B.9 (2008 edition) tabulates the relation as T/K = (T/°R)/1.8, which is the same statement solved for kelvin. The BIPM definition of the kelvin (SI Brochure, 9th edition, 2019) fixes the kelvin through the Boltzmann constant, and the Rankine degree inherits its size from the Fahrenheit degree, which is 5/9 K.

Work one example. 350 K times 1.8 is 630 °R. Check it by dividing back: 630 / 1.8 = 350 K. Every conversion on this page runs on that one pair of operations, so a wrong answer almost always means the two operations got swapped.

The label on the scale matters when you read a number off a printed chart. Some tables put a capital R with no degree mark, some write deg R, and some write R without the degree sign at all. The NIST Guide for the Use of the International System of Units (SP 811, 2008 edition) writes the unit as °R, and it treats the Rankine degree as a unit outside the SI whose factor to the kelvin is 5.555556 x 10^-1. Wherever the symbol comes from, the meaning is the same: Fahrenheit sized steps counted up from absolute zero.

Temperature differences: 1 K is a 1.8 °R step

The 1.8 factor applies to differences as well as to readings. A temperature that rises by 1 K has risen by 1.8 °R, and a gap of 10 K between two readings is a gap of 18 °R. NIST's Appendix B lists the interval conversion as 1 °R = 5.555556 x 10^-1 K, which is 5/9 rounded for display; the exact statement is one kelvin per 1.8 Rankine degrees.

A Rankine difference and a Fahrenheit difference are the same size, just as a kelvin difference and a Celsius difference are. Differences never involve an offset, so the only number you ever apply to a temperature change is 1.8.

Four conversions to try with a pencil

Four conversions cover the patterns you will meet:

• 520 K to Rankine: 520 x 1.8 = 936 °R.
• 1,200 K to Rankine: 1,200 x 1.8 = 2,160 °R.
• 720 °R back to kelvin: 720 / 1.8 = 400 K.
• 630 °R to Fahrenheit: 630 - 459.67 = 170.33 °F.

The last one is worth a second route. Divide first: 630 / 1.8 = 350 K, then subtract the Celsius offset, 350 - 273.15 = 76.85 °C, and convert up: 76.85 x 1.8 + 32 = 170.33 °F. Both routes land on the same number, which is the cheapest kind of check there is. If they ever disagree, the error is in the arithmetic, because the conversion factors themselves are exact.

The fixed points, side by side

Four anchors line up neatly across the scales, and each row is exact by definition:

The middle rows tie together: 32 + 459.67 = 491.67, which is the freezing row read across. The gap between freezing and boiling is 100 K and also 180 °R, the same 9 to 5 ratio that produces the 1.8 factor. The Fahrenheit zero row comes from the definition of the Fahrenheit scale, 0 °F = 459.67 °R, and dividing 459.67 / 1.8 = 255.372 K confirms the kelvin column.

American engineering and the Rankine degree

The Scottish engineer and physicist William John Macquorn Rankine set out the scale in 1859, and it stuck wherever thermodynamics is written in US customary units. Steam tables, gas turbine performance work, rocket nozzle calculations, HVAC psychrometric charts and cryogenic references printed in the United States have traditionally used °R, so an English textbook equation that mixes pounds, feet and temperature usually expects Rankine on the temperature slot.

Rankine is not an SI unit. NIST SP 811 treats it as a unit outside the SI and provides the conversion factors in Appendix B, and international journals ask for kelvin. The practical rule is to match the units already on the page in front of you: a heat balance written in Btu and pounds works cleanly in °R, while the same balance written in joules and kilograms works in K.

One more habit pays off when you work through an old book. Many engineering texts of the mid 20th century printed a conversion table on the inside cover listing 459.67 next to 0 °F and 491.67 next to 32 °F, and they used the Rankine scale in every steam table. Checking one worked example from the text against the 1.8 factor tells you quickly whether the book means absolute Rankine or a Rankine reading that has been offset, and it takes about a minute with a calculator.

Energy per degree and the exact constants

Since the 2019 revision of the SI, the kelvin is defined by fixing the Boltzmann constant at 1.380649 x 10^-23 J/K, a value the NIST CODATA tables mark as exact. Entropy and heat capacity values are quotients by a temperature interval, so their unit follows the degree size.

A Rankine degree is 5/9 of a kelvin, so a constant per kelvin divides by 1.8 to convert: 1.380649 / 1.8 = 0.767027, which puts the Boltzmann constant at 7.670272 x 10^-24 J/°R. Going the other way, a value quoted per °R is 1.8 times larger when restated per K, because a kelvin is the bigger degree. The same move converts a heat capacity in Btu per °R into J/K and back, using the factors NIST publishes in Appendix B.

Kelvin to Rankine values worth memorizing

Round kelvin values and their Rankine partners, each row the product of a single multiplication by 1.8:

To read the chart in reverse, divide the Rankine column by 1.8. Midpoints work too: halfway between 900 °R and 1,350 °R is 1,125 °R, and 1,125 / 1.8 = 625 K, which is the midpoint between 500 K and 750 K.

For quick mental arithmetic, split the kelvin value into hundreds and leftovers. 450 K is 400 K plus 50 K, which is 720 °R plus 90 °R, so 810 °R. The same split works in reverse: 810 °R is 720 plus 90, which is 400 K plus 50 K, so 450 K. Multiplying by 1.8 feels like a chore because the brain wants to double the number and then shave 10 percent off: 520 doubled is 1,040, and a tenth of 1,040 is 104, which leaves 936 °R in two steps that need no long multiplication.

Where the 1.8 goes wrong

• Adding 459.67 to a kelvin reading. That offset belongs to Fahrenheit; kelvin and Rankine share their zero, so the kelvin to Rankine direction is a pure multiplication.
• Dividing by 1.8 when converting K to °R. Division belongs to the return trip. 350 K is 630 °R, and 630 / 1.8 = 350 K is the way home.
• Quoting a Rankine temperature as a Fahrenheit one. The degrees are the same size but the zeros differ, and °R = °F + 459.67 is the bridge between them.

On rounding: two decimals fit lab tables, one decimal fits HVAC work, whole degrees fit engineering checks. Rounding 491.67 °R to 492 °R costs 0.33 °R, about 0.18 K, which matters in a heat balance and vanishes in a weather note. One habit catches most slips: convert by 1.8, then convert back, and confirm the original value came back unchanged.