Fahrenheit to Celsius
Subtract 32 first, then multiply by 5/9. Doing those two steps in the other order is the single most common mistake in this conversion, by about 18 degrees.
Subtract 32 before multiplying by 5/9, not after. Doing it the other way round is the most common mistake in this conversion and gives an answer that is wrong by about 18 degrees at room temperature.
With temperature 72, temperature converter comes to 22.22 — converted value. It is reached in 4 steps, the last of which is 22.222222 * 1 + 72 * 0 + 295.37222 * 0, and each one is printed on the page with its numbers filled in. The formula is the one published by NIST, not an approximation fitted to it.
Formula and sources checked · How we check
- In Celsius
72 * 0 + (72 - 32) * 5 / 9 * 1 + (72 - 273.15) * 022.222 °C- In Fahrenheit
22.222222 * 9 / 5 + 3272 °F- In Kelvin
22.222222 + 273.15295.372 K- Converted value
22.222222 * 1 + 72 * 0 + 295.37222 * 022.222
Is there a mental shortcut?
Subtract 30 and halve, which is the reverse of the trick in the other direction: 70°F gives 20 against a true 21.1. It stays within about two degrees over ordinary weather and drifts wider at oven temperatures, where the exact arithmetic is worth the extra few seconds.
Why does the order matter this much?
Because 32 is an offset rather than a factor. Multiplying first scales the offset along with the temperature and inflates it by 9/5. At 70°F the wrong order gives 71 rather than 21 — an error so large it announces itself, which is the only good thing to be said for it.
The formula
- In Celsius
value * from_c + (value - 32) * 5 / 9 * from_f + (value - 273.15) * from_k - In Fahrenheit
celsius * 9 / 5 + 32 - In Kelvin
celsius + 273.15 - Converted value
celsius * to_c + fahrenheit * to_f + kelvin * to_k
Formula from NIST — SI units and temperature scales, BIPM — the kelvin and the International System of Units.
The same calculator, other cases
Undecided? The Temperature Converter carries every case behind one control.