The basic formula: subtract 32, then multiply by 5/9
To convert a Fahrenheit temperature to Celsius, subtract 32 from the Fahrenheit number, then multiply the result by 5/9 (or divide by 1.8, which gives the same answer). That is the only calculation you need.
The formula written out: Celsius = (Fahrenheit − 32) × 5/9
For example, if the temperature is 68°F, you would subtract 32 to get 36, then multiply 36 by 5/9 to get 20°C. If the temperature is 32°F (the freezing point of water), you subtract 32 to get 0, then multiply by 5/9 to get 0°C, which is also the freezing point of water in Celsius.
Key Takeaways
- Subtract 32 from the Fahrenheit temperature first, then multiply the result by 5/9 to get Celsius.
- You can also divide by 1.8 instead of multiplying by 5/9 — both methods produce the same result.
- Water freezes at 32°F (0°C) and boils at 212°F (100°C), which are useful reference points to check your math.
- Rounding to the nearest whole number is usually fine for everyday temperatures unless you need precision for cooking or scientific work.
Why the formula works this way
Fahrenheit and Celsius use different starting points and different sized degrees. On the Fahrenheit scale, water freezes at 32 degrees and boils at 212 degrees — a range of 180 degrees. On the Celsius scale, water freezes at 0 degrees and boils at 100 degrees — a range of 100 degrees.
Because Celsius degrees are larger (100 degrees covers the same temperature range as 180 Fahrenheit degrees), you have to multiply by the ratio 5/9. The subtraction of 32 accounts for the fact that the two scales start at different points.
Quick reference for common temperatures
| Fahrenheit | Celsius | What it means |
|---|---|---|
| 32°F | 0°C | Water freezes |
| 50°F | 10°C | Cool day |
| 68°F | 20°C | Room temperature |
| 86°F | 30°C | Warm day |
| 104°F | 40°C | Very hot |
| 212°F | 100°C | Water boils |
How to do the math by hand
If you do not have a calculator, you can work through the conversion in steps. First, subtract 32 from the Fahrenheit temperature. Write down that number.
Next, multiply that number by 5. Then divide the result by 9. If the answer does not come out even, round to the nearest whole number for everyday use.
For example, with 77°F: subtract 32 to get 45. Multiply 45 by 5 to get 225. Divide 225 by 9 to get 25°C.
Using a calculator or phone
Most phones have a built-in calculator app that can handle this conversion. Type the Fahrenheit number, subtract 32, then multiply by 5 and divide by 9 in that order.
Many weather apps and search engines also convert temperature for you automatically. If you search "77 fahrenheit to celsius" in any search engine, the conversion appears at the top of the results without you having to do any math.
When you might need this conversion
You may need to convert temperatures when reading weather forecasts from other countries, following recipes from international sources, or understanding medical information. Many countries outside the United States use Celsius as their standard temperature scale.
If you are cooking, note that oven temperatures in recipes from the UK, Canada, Australia, or Europe will be in Celsius. A recipe calling for 180°C is roughly 350°F — a common baking temperature in the US.
Frequently Asked Questions
Is there a quick way to estimate the conversion without a calculator?
A rough estimate: subtract 30 instead of 32, then divide by 2. This is not exact, but it gets you close enough for everyday purposes. For 68°F, subtract 30 to get 38, divide by 2 to get 19°C — the real answer is 20°C. The error grows larger at extreme temperatures, so use the full formula if you need accuracy.
What is the difference between Celsius and Centigrade?
Celsius and Centigrade are the same scale. "Centigrade" was the older name (meaning 100 steps), and "Celsius" is the modern name used internationally. You will see both terms, but they refer to the same temperature measurement.
Why does the US still use Fahrenheit?
The United States adopted Fahrenheit early in its history and has kept it for everyday use, even though most of the world switched to Celsius. Scientists and doctors in the US use Celsius in their work, but weather forecasts and thermostats remain in Fahrenheit for the general public.
Can I convert Celsius back to Fahrenheit with the same formula?
No — the reverse formula is different. To convert Celsius to Fahrenheit, multiply by 9/5 (or 1.8), then add 32. For example, 20°C becomes (20 × 1.8) + 32 = 68°F.