Kelvin to Rankine Converter
Convert K to °R when SI thermodynamic data has to be used in a calculation done in US customary units.
°R = K × 1.8
Why There Is No Offset
Kelvin and Rankine both start at absolute zero, so 0 K and 0°R are the same temperature. The only difference is degree size: a kelvin matches a Celsius degree, and a degree Rankine matches a Fahrenheit degree, which is 5/9 as large. Converting is a single multiplication.
Because there's no offset, ratios carry straight across. A gas at 600 K is twice as hot, in absolute terms, as one at 300 K, and 1,080°R is twice 540°R. That's not true of Celsius or Fahrenheit.
Per-Kelvin Units Go the Other Way
A temperature in kelvin gets multiplied by 1.8. A quantity per kelvin, such as a specific heat or a gas constant, gets divided by 1.8 for the temperature part, because each degree Rankine is smaller than a kelvin. Mixing these up is the most common error when moving thermodynamic data between SI and English units. Here are common constants converted, including the energy and mass parts:
| Quantity | SI value | English units |
|---|---|---|
| Universal gas constant | 8.314 J/(mol·K) | 1.9859 Btu/(lbmol·°R) |
| Specific heat of air (cp) | 1.005 kJ/(kg·K) | 0.24 Btu/(lbm·°R) |
| Specific heat of liquid water | 4.18 kJ/(kg·K) | 0.998 Btu/(lbm·°R) |
The same factor, 0.2388, converts J/(mol·K) to Btu/(lbmol·°R) and kJ/(kg·K) to Btu/(lbm·°R), because the energy and mass ratios cancel the same way.
Worked Example: Specific Heat of Air
Convert the specific heat of air, 1.005 kJ/(kg·K), for use in a Btu-based calculation.
Step 1: Energy (1 kJ = 0.947817 Btu)
Step 2: Mass (1 kg = 2.204623 lbm)
Step 3: Temperature (1 K = 1.8°R, so divide)
0.240 Btu/(lbm·°R), the value printed in English-unit property tables. Multiplying by 1.8 in step 3 would give 0.778, more than three times too high.
Kelvin to Rankine Table
From cryogenic boiling points (helium, hydrogen, nitrogen) to combustion temperatures.
| K | °R |
|---|---|
| 1 K | 1.8°R |
| 4.22 K | 7.596°R |
| 20.28 K | 36.504°R |
| 77.36 K | 139.248°R |
| 100 K | 180°R |
| 200 K | 360°R |
| 273.15 K | 491.67°R |
| 288.15 K | 518.67°R |
| 300 K | 540°R |
| 500 K | 900°R |
| 1,000 K | 1,800°R |
| 1,500 K | 2,700°R |
| 2,000 K | 3,600°R |
| 3,000 K | 5,400°R |
Frequently asked questions
Is a kelvin bigger than a degree Rankine?
Yes, 1.8 times bigger. That's why the Rankine number is always larger: 300 K is 540°R.
Can I convert through Celsius and Fahrenheit instead?
You can, but it adds two offsets that cancel out. Multiplying by 1.8 directly is simpler and avoids rounding errors.
Why is the specific heat of water about 1 Btu/(lbm·°R)?
The Btu was originally defined as the heat needed to raise one pound of water by one degree Fahrenheit, and a degree Rankine is the same size. Converting 4.18 kJ/(kg·K) lands almost exactly on 1.
Sources
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI): kelvin, Rankine, Btu and pound conversion factors.
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.