Of all the temperature conversions, this is the cleanest: the Kelvin and Rankine scales share the same zero — absolute zero — and differ only in how big a degree is. That makes the conversion a single multiplication by 1.8, with no offset and no special handling for temperature differences. It’s the step you need when moving SI data into calculations done in Btu, pounds and feet.
How to use the Kelvin to Rankine converter
- Enter a value in Temperature in kelvins (K). Kelvins cannot be negative.
- Read the result in °R, along with Celsius and Fahrenheit.
- The chart lists values from absolute zero to 5,000 K, with landmarks such as helium and nitrogen boiling points and standard ambient temperature.
Kelvin to Rankine formula
The reverse is °R × 5/9. Since both scales are absolute, ratios of temperatures are the same in either one, which is why thermodynamic efficiencies and gas-law ratios don’t change when you switch.
Worked example: 300 K
Step 1. 300 × 1.8 = 540 °R.
Cross-check. 300 K is 26.85 °C, which is 80.33 °F, and 80.33 + 459.67 = 540 °R.
Converting constants and property data
Because a Rankine degree is 5/9 of a kelvin, any quantity expressed “per kelvin” gets divided by 1.8 when expressed “per degree Rankine”. This is where the conversion is used most:
| Quantity | SI value | Per °R |
|---|---|---|
| Universal gas constant | 8.314462618 J/(mol·K) | 4.6191 J/(mol·°R) |
| Same, in US units | — | 1.9859 Btu/(lb-mol·°R) |
| Specific heat of liquid water (about) | 4.1868 kJ/(kg·K) | 1.0 Btu/(lb·°R) |
The last line is not a coincidence. The International Table Btu was defined so that one Btu per pound per degree Fahrenheit equals exactly 4.1868 kJ per kilogram per kelvin, which is why water’s specific heat comes out close to 1 in both the calorie-based and Btu-based systems.
Reference points on both absolute scales
| Point | K | °R |
|---|---|---|
| Absolute zero | 0 | 0 |
| Helium boils | 4.22 | 7.60 |
| Nitrogen boils | 77.36 | 139.25 |
| Water freezes | 273.15 | 491.67 |
| Standard sea-level air | 288.15 | 518.67 |
| Standard ambient (SATP) | 298.15 | 536.67 |
| Water boils (nominal) | 373.15 | 671.67 |
| 1,000 K furnace | 1,000 | 1,800 |
Which scale should you work in?
If the rest of your inputs are SI — pascals, cubic meters, joules — stay in kelvins and convert only the final answer. If your gas constant is in psia·ft³ or Btu, or your steam table is in English units, convert temperatures to Rankine first and keep everything consistent. Mixing kelvins with English-unit constants is a classic source of answers that are off by exactly 1.8.
About the Rankine scale
William J. M. Rankine, a Scottish engineer and physicist, proposed the scale in 1859 as the Fahrenheit counterpart of Kelvin’s absolute scale. It is written °R (sometimes °Ra). For Fahrenheit inputs, use Fahrenheit to Rankine; to put a kelvin value into everyday terms, see Kelvin to Fahrenheit or Kelvin to Celsius.
Frequently asked questions
What is the formula for kelvin to Rankine?
°R = K × 9/5, or K × 1.8. For 300 K: 300 × 1.8 = 540 °R. There is no offset to add or subtract.
Why is there no offset between Kelvin and Rankine?
Both scales put zero at absolute zero. They differ only in the size of their degrees — a kelvin equals a Celsius degree, and a Rankine degree equals a Fahrenheit degree — so the conversion is a single ratio.
Is 1 K exactly 1.8 °R?
Yes. The Fahrenheit degree, and therefore the Rankine degree, is defined as exactly 5/9 of a kelvin, so 1 K = 1.8 °R with no rounding.
Do temperature differences convert the same way?
Yes. Because there is no offset, a temperature of 10 K and a temperature change of 10 K both convert to 18 °R. That is not true for Celsius-to-Fahrenheit conversions.