How to Convert Celsius to Fahrenheit (and Back) Quickly
Simple formulas, mental math shortcuts, and the exact conversions you'll actually use.
Whether you're reading a weather forecast from another country, following a recipe from a foreign cookbook, or just trying to understand what 25°C actually feels like — knowing how to convert between Celsius and Fahrenheit is genuinely useful.
Here's everything you need, from the exact formulas to a mental math trick that gets you close in two seconds.
The exact formulas
Celsius → Fahrenheit:
°F = (°C × 9/5) + 32
Fahrenheit → Celsius:
°C = (°F − 32) × 5/9
That's it. Two formulas, and you can convert in either direction.
The mental math shortcut
When you're standing in a supermarket reading a foreign weather app, you don't want to pull out a calculator. Use this two-step approximation:
- Double the Celsius number
- Add 30
That gives you a Fahrenheit number that's close enough for most daily purposes. It's not exact, but it's fast.
Common conversions you'll actually use
| Celsius | Fahrenheit | What it feels like |
|---|---|---|
| 0°C | 32°F | Water freezes |
| 10°C | 50°F | Cold jacket weather |
| 20°C | 68°F | Room temperature |
| 25°C | 77°F | Warm, pleasant |
| 30°C | 86°F | Hot summer day |
| 37°C | 98.6°F | Body temperature |
| 100°C | 212°F | Water boils |
Where the two scales intersect
Here's a fun fact: −40°C equals −40°F. It's the only temperature where both scales read the same number. You can verify this yourself with the formula:
(−40 × 9/5) + 32 = −72 + 32 = −40
Need to convert right now?
Use our free online temperature converter — it handles Celsius, Fahrenheit, Kelvin and Rankine, updates as you type, and works on any device.
The bottom line
- Use °F = (°C × 9/5) + 32 for exact Celsius to Fahrenheit
- Use °C = (°F − 32) × 5/9 for exact Fahrenheit to Celsius
- For mental math: double, then add 30
- For everything else, use our free temperature converter
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