The basic formula for converting Fahrenheit to Celsius
To convert a temperature from Fahrenheit to Celsius, subtract 32 from the Fahrenheit number, then multiply the result by 5/9. The formula is: Celsius = (Fahrenheit − 32) × 5/9.
This works because the two scales have different starting points and different sized degrees. Water freezes at 32°F but 0°C, and boils at 212°F but 100°C. That 180-degree span on the Fahrenheit scale equals 100 degrees on the Celsius scale, which is why you multiply by the fraction 5/9.
You can also express this as multiplying by 0.556 if you prefer decimals, though the fraction 5/9 is more precise.
Key Takeaways
- Subtract 32 from the Fahrenheit temperature, then multiply the answer by 5/9 to get Celsius.
- The reverse conversion multiplies by 9/5 and then adds 32 to go from Celsius to Fahrenheit.
- Common reference points like freezing (32°F = 0°C) and boiling (212°F = 100°C) can help you check your math.
- A rough mental conversion divides the Fahrenheit number by 2 and subtracts 15, which gives an approximate answer without a calculator.
Step-by-step example with a real temperature
Say you see a weather report showing 68°F and want to know what that is in Celsius.
Step 1: Subtract 32 from 68. You get 36.
Step 2: Multiply 36 by 5/9. You can do this by multiplying 36 by 5 (which gives 180), then dividing by 9 (which gives 20). The answer is 20°C.
So 68°F equals 20°C. This is a comfortable room temperature in most places.
Converting negative Fahrenheit temperatures
The formula works the same way for temperatures below freezing. If it is −4°F outside, subtract 32 to get −36, then multiply by 5/9 to get −20°C.
The only difference is that you are working with negative numbers. The subtraction step still comes first, and the result will be negative if the Fahrenheit temperature is below −40°F. (At exactly −40, both scales show the same number.)
Using a calculator or conversion tool
If you do not want to do the math by hand, a basic calculator makes the conversion when ready. Type the Fahrenheit number, subtract 32, then multiply by 5, then divide by 9. Most phones have a calculator app built in.
Online conversion tools and weather websites often show both scales at once, so you can look up a temperature without calculating it yourself. Many thermometers sold in the United States show both Fahrenheit and Celsius, so you can read the answer directly.
The reverse: converting Celsius to Fahrenheit
If you need to go the other direction, the formula reverses: Fahrenheit = (Celsius × 9/5) + 32.
For example, if a recipe calls for 180°C, multiply 180 by 9/5 (or 1.8) to get 324, then add 32 to get 356°F. This is a common oven temperature.
Quick mental math without a calculator
For a rough estimate, divide the Fahrenheit number by 2 and subtract 15. This does not give an exact answer, but it is close enough for everyday purposes. Using 68°F: divide by 2 to get 34, subtract 15 to get 19. The real answer is 20°C, so you are within one degree.
This shortcut works best for temperatures between 50°F and 100°F. Outside that range, the error grows larger, so use the full formula if you need accuracy.
Common reference 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 |
Memorizing a few anchor points makes it easier to sense-check your conversions. If you know that 32°F is freezing and 212°F is boiling, you can estimate where any temperature falls between those extremes.
Frequently Asked Questions
What is the easiest way to remember the formula?
Think of it as two steps: first, shift the scale by subtracting 32 (because the two scales start at different numbers). Second, resize it by multiplying by 5/9 (because Celsius degrees are larger). Writing it down a few times helps it stick.
Why is the conversion factor 5/9 and not something simpler?
The Fahrenheit scale divides the gap between freezing and boiling into 180 degrees, while Celsius uses 100 degrees for the same gap. The ratio 100 to 180 simplifies to 5 to 9, which is why that fraction appears in the formula.
Is −40°F the same as −40°C?
Yes. At −40, both scales show the same number. This is the only temperature where they match. It is a useful checkpoint if you are testing your understanding of how the scales relate.
Can I round the answer, or do I need to be exact?
It depends on what you are doing. For weather or cooking, rounding to the nearest whole number is fine. For science or medical purposes, you may need more decimal places. The formula itself is exact; rounding is your choice based on how precise you need to be.