Rankine to Kelvin Converter
Convert °R to K to move temperatures from US customary engineering data into SI physics formulas.
K = °R × 5/9
Two Absolute Scales, One Factor
Lord Kelvin proposed an absolute temperature scale in 1848, using Celsius-sized degrees. In 1859 the Scottish engineer William Rankine proposed the same idea with Fahrenheit-sized degrees. Both start at absolute zero, so converting between them only resizes the degrees, with no offset.
A degree Rankine is 5/9 of a kelvin, so multiply by 5/9 (or divide by 1.8). Zero stays zero, and doubling a Rankine temperature doubles the kelvin temperature too.
Worked Example: Speed of Sound From a Rankine Temperature
A US test report gives the air temperature as 518.67°R. To find the speed of sound with the SI formula a = √(γ·R·T), where γ = 1.4 and R = 287.05 J/(kg·K) for air, T must be in kelvin.
Step 1: Convert to kelvin
Step 2: Apply the formula
340.3 m/s, the standard sea-level speed of sound. Leaving T in Rankine would overstate it by a factor of √1.8, giving about 456.5 m/s.
Use 5/9, Not 0.5556
5/9 is 0.5555…, repeating forever. Rounding it to 0.5556 makes every result slightly high, and the error grows with temperature. That matters for combustion and turbine data, which can run to thousands of degrees Rankine:
| °R | Exact (× 5/9) | Rounded (× 0.5556) | Error |
|---|---|---|---|
| 500°R | 277.778 K | 277.8 K | +0.022 K |
| 1,000°R | 555.556 K | 555.6 K | +0.044 K |
| 2,000°R | 1,111.111 K | 1,111.2 K | +0.089 K |
| 3,600°R | 2,000 K | 2,000.16 K | +0.16 K |
Rankine to Kelvin Table
| °R | K |
|---|---|
| 0°R | 0 K |
| 100°R | 55.556 K |
| 200°R | 111.111 K |
| 300°R | 166.667 K |
| 491.67°R | 273.15 K |
| 518.67°R | 288.15 K |
| 536.67°R | 298.15 K |
| 671.67°R | 373.15 K |
| 1,000°R | 555.556 K |
| 1,500°R | 833.333 K |
| 2,000°R | 1,111.111 K |
| 2,500°R | 1,388.889 K |
| 3,000°R | 1,666.667 K |
| 3,600°R | 2,000 K |
Frequently asked questions
Why is the kelvin number always smaller?
A kelvin is 1.8 times larger than a degree Rankine, so fewer of them fit in the same temperature. 540°R is 300 K.
Do I need to add or subtract anything?
No. Both scales put zero at absolute zero, so the conversion is a pure multiplication. If you're adding 273.15 or 459.67, you're converting to Celsius or Fahrenheit instead.
Can I put a Rankine value straight into an SI formula?
No. SI constants such as 287.05 J/(kg·K) for air assume kelvin. A Rankine value is 1.8 times too large, so anything proportional to temperature comes out 80% too high.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI): Rankine and kelvin conversion factors.
- BIPM, The International System of Units (SI Brochure): definition of the kelvin.
Related Tools
Convert Celsius to Fahrenheit.
Convert Celsius to Kelvin.
Convert Celsius to Rankine.
Convert Fahrenheit to Celsius.
Convert Fahrenheit to Kelvin.
Convert Fahrenheit to Rankine.