How to convert Fahrenheit to Celsius
Use the exact temperature formula or a fast mental estimate to convert Fahrenheit and Celsius.
The exact formulas
Fahrenheit and Celsius use different zero points and different degree sizes, so you need both subtraction and multiplication for an exact conversion.
Work inside the parentheses first. For 77°F, subtract 32 to get 45. Multiply 45 by 5/9 to get 25°C.
Convert Celsius back to Fahrenheit
Reverse the operations by multiplying Celsius by 9/5 and then adding 32. For 30°C, multiply by 1.8 to get 54, then add 32. The result is 86°F.
A fast mental estimate
For a quick Fahrenheit-to-Celsius estimate, subtract 30 and divide by two. This is easy but not exact. At 70°F, the shortcut gives 20°C while the exact answer is about 21.1°C.
For Celsius to Fahrenheit, double the Celsius temperature and add 30. At 20°C, the shortcut gives 70°F instead of the exact 68°F. The estimate is most useful for ordinary weather temperatures.
Common reference temperatures
A few memorized reference points can be quicker than calculating when you only need a practical sense of the temperature.
- 32°F = 0°C: water freezes
- 50°F = 10°C: cool weather
- 68°F = 20°C: mild room temperature
- 86°F = 30°C: hot weather
- 104°F = 40°C: very hot
Frequently asked questions
What is the easiest Fahrenheit-to-Celsius shortcut?
Subtract 30 and divide by two. It is an estimate, not an exact conversion.
At what temperature are Fahrenheit and Celsius equal?
The two scales are equal at −40 degrees.
Is 20°C hot or cold?
20°C equals 68°F, which most people experience as mild.
Published August 3, 2026 · Reviewed for clarity and source accuracy.