Fahrenheit is fine for reading a thermometer, but it can’t go into the ideal gas law: its zero point is arbitrary, so doubling a Fahrenheit number doesn’t double anything physical. US engineers solve this with the Rankine scale — the same degree size as Fahrenheit, but counted up from absolute zero. Converting is one addition.
How to use the Fahrenheit to Rankine converter
- Enter a value in Temperature in Fahrenheit (°F), down to −459.67 °F.
- Read the result in °R, with Celsius and kelvin for the same temperature.
- The table lists Fahrenheit values from absolute zero up to 3,000 °F, the range of furnace and combustion work.
Fahrenheit to Rankine formula
Because the degree sizes match, temperature differences carry over unchanged: a rise of 50 °F is a rise of 50 °R.
Worked example: a 70 °F room
Step 1. 70 + 459.67 = 529.67 °R.
Check. In kelvins this is 529.67 × 5/9 = 294.26 K, the same as converting 70 °F through Celsius (21.11 °C + 273.15).
Example: density of air with the ideal gas law
In US customary units, the ideal gas law is often written as ρ = P × M ÷ (R × T), with P in psia, M in lb per lb-mol, R = 10.7316 psia·ft³/(lb-mol·°R) and T in °R. For dry air (M ≈ 28.97) at sea-level pressure (14.696 psia):
| Air temperature | Rankine | Density (lb/ft³) |
|---|---|---|
| 60 °F | 519.67 °R | 0.0763 |
| 68 °F | 527.67 °R | 0.0752 |
| 70 °F | 529.67 °R | 0.0749 |
The 70 °F result is the familiar “standard air” density of 0.075 lb/ft³ used in HVAC fan and duct calculations. Had you used 70 °F directly, the answer would be off by a factor of about 7.6.
Standard temperatures in US practice
| Standard | °F | °R |
|---|---|---|
| Water freezes | 32 | 491.67 |
| US natural-gas measurement base | 60 | 519.67 |
| Standard atmosphere at sea level | 59 | 518.67 |
| HVAC standard air | 70 | 529.67 |
| Water boils (nominal, sea level) | 212 | 671.67 |
Natural gas in the US is metered in standard cubic feet, commonly referenced to 60 °F and 14.73 psia, so 519.67 °R appears in countless gas-volume corrections.
Rankine and Kelvin side by side
Both scales start at absolute zero; they differ only in degree size. One kelvin equals exactly 1.8 °R. If you have to hand a result to a colleague who works in SI, multiply by 5/9, or use Fahrenheit to Kelvin directly. For metric source data, see Celsius to Rankine, and for SI property tables use Kelvin to Rankine.
A note on notation
Write the unit as °R. Older and some European references use °Ra to avoid confusion with the historical Réaumur scale, which also used R. Unlike kelvin, Rankine keeps its degree sign.
Frequently asked questions
How do you convert Fahrenheit to Rankine?
Add 459.67. For 70 °F: 70 + 459.67 = 529.67 °R. No scaling is needed, because a Rankine degree and a Fahrenheit degree are the same size.
Why 459.67?
Absolute zero is −459.67 °F. Adding 459.67 moves the zero of the scale down to absolute zero, exactly as adding 273.15 turns Celsius into kelvins.
Is 0 °R absolute zero?
Yes. 0 °R = 0 K = −459.67 °F = −273.15 °C. Rankine values can never be negative.
When do I need Rankine instead of Fahrenheit?
Whenever a formula multiplies or divides by temperature: the ideal gas law, Charles's law, Carnot efficiency, speed of sound, radiation. Using °F in those formulas gives wrong answers because its zero is arbitrary.