What this calculator does
Temperature conversion is the one that needs an offset as well as a factor, which is what makes it different from every other converter on the site. Celsius and Fahrenheit have different zero points and different degree sizes, so converting requires both a multiplication and an addition, and getting the order wrong is the usual mistake.
Kelvin and Rankine are absolute scales, starting at absolute zero rather than at a property of water. Kelvin uses the Celsius degree size and Rankine the Fahrenheit one, so a Kelvin figure is Celsius plus 273.15 and Rankine is Fahrenheit plus 459.67. Neither takes a degree sign, since they measure from an absolute floor rather than relative to anything.
The formula
Whatever unit is entered is converted to Celsius first, then out to all four scales at once. Celsius to Fahrenheit multiplies by 9/5 and adds 32; the reverse subtracts 32 first and then multiplies by 5/9, and doing those two steps in the wrong order is where most conversion errors come from. Kelvin adds 273.15 to Celsius, and Rankine is Kelvin multiplied by 9/5.
| Term | Meaning |
|---|---|
| Celsius (°C) | Zero at the freezing point of water, 100 at its boiling point at standard pressure. |
| Fahrenheit (°F) | Freezing at 32 and boiling at 212, so a Fahrenheit degree is 5/9 the size of a Celsius one. |
| Kelvin (K) | Absolute scale using Celsius-sized degrees, starting at absolute zero, which is −273.15 °C. |
| Rankine (°R) | Absolute scale using Fahrenheit-sized degrees, starting at the same absolute zero. |
The inputs explained
| Field | What to enter |
|---|---|
| Temperature | The temperature value. |
| Unit | The scale it is measured on. All four scales are reported regardless of which you choose. |
When to use it
Converting an oven temperature
Recipes from the United States give Fahrenheit and most others Celsius, and the gap is wide enough that guessing produces either a raw or a burnt result.
Reading a weather forecast from elsewhere
The two scales cross at −40 and diverge from there, so an intuition built on one gives no useful reading of the other without converting.
Working in absolute temperature
Gas laws, thermodynamics and any calculation involving a ratio of temperatures need Kelvin, because ratios are meaningless on a scale whose zero is arbitrary.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What are common Celsius temperatures on the other scales?
A range of reference temperatures in Celsius, converted to the other three scales.
| Celsius | Fahrenheit | Kelvin | Rankine |
|---|---|---|---|
| -40 °C | -40.00 °F | 233.15 K | 419.67 °R |
| 0 °C | 32.00 °F | 273.15 K | 491.67 °R |
| 25 °C | 77.00 °F | 298.15 K | 536.67 °R |
| 37 °C | 98.60 °F | 310.15 K | 558.27 °R |
| 100 °C | 212.00 °F | 373.15 K | 671.67 °R |
Questions
How do I convert Fahrenheit to Celsius?
Subtract 32 first, then multiply by 5/9. The order matters: multiplying first gives a wrong answer every time. Going the other way, multiply by 9/5 and then add 32. A rough mental version is to subtract 30 and halve, which is close enough for weather.
At what temperature are Celsius and Fahrenheit the same?
At −40. It is the single point where the two scales cross, and it falls out of solving the conversion for the value that maps to itself. It is why −40 needs no unit when quoting a very cold temperature.
Why does Kelvin not use a degree symbol?
Because it is an absolute unit rather than a degree on a relative scale. The convention is 300 K, not 300 °K, and it was formalised in 1967. Rankine keeps the degree symbol despite also being absolute, which is inconsistent but established.
When do I need Kelvin rather than Celsius?
Whenever the calculation involves a ratio or a product of temperatures, which includes the gas laws, radiation calculations and most of thermodynamics. Doubling a temperature is only meaningful from an absolute zero, so doubling 10 °C means nothing while doubling 283.15 K means something definite.
For a direct Fahrenheit to Celsius conversion, see the Fahrenheit to Celsius calculator. For energy rather than temperature, see the energy converter.