Whether you're reading a foreign weather forecast, following a recipe from another country, or just curious, converting between Celsius and Fahrenheit comes up more than you'd expect. The exact formula is simple, and a couple of shortcuts get you close enough in your head.
This guide covers the precise conversion both ways, a fast mental estimate, and the reference temperatures worth knowing by heart.
What you need
- ✓The temperature you want to convert.
- ✓The formula below (or a converter for exact results).
- ✓A couple of reference points for quick sense-checks.
Step-by-step
- 1
Convert Celsius to Fahrenheit exactly
Multiply the Celsius temperature by 9/5 (which is 1.8), then add 32. So 20°C × 1.8 = 36, plus 32 = 68°F. That's the exact formula: °F = (°C × 1.8) + 32.
- 2
Convert Fahrenheit to Celsius exactly
Reverse it: subtract 32 first, then multiply by 5/9. So (68°F − 32) = 36, × 5/9 = 20°C. The formula is °C = (°F − 32) × 5/9.
- 3
Use the quick mental shortcut
For a fast estimate from Celsius to Fahrenheit: double the Celsius and add 30. 20°C → 40 + 30 = 70°F (the exact answer is 68, close enough for weather). It's not precise, but it's instant.
- 4
Memorise a few anchor points
0°C = 32°F (water freezes), 37°C ≈ 98.6°F (body temperature), 100°C = 212°F (water boils), and 16°C ≈ 61°F, 28°C ≈ 82°F for everyday weather. Anchors let you sanity-check any conversion.
- 5
Use a converter for accuracy
For cooking, science, or anything where a degree matters, use a temperature converter for the exact value rather than the mental shortcut — small errors add up in an oven or a lab.
Examples
- A 25°C summer day: 25 × 1.8 = 45, + 32 = 77°F (the shortcut gives 25×2+30 = 80, close).
- A recipe at 180°C: 180 × 1.8 = 324, + 32 = 356°F — exactly why you'd use the real formula for the oven.
Tips
- →Exact: °F = °C × 1.8 + 32; °C = (°F − 32) × 5/9.
- →Quick weather estimate: double the Celsius and add 30.
- →Memorise 0°C=32°F, 37°C≈98.6°F, 100°C=212°F as anchors.
- →The two scales cross at −40° (−40°C = −40°F) — a handy fact.
- →Use a converter for cooking and science where a degree counts.
Common mistakes
- Forgetting the order for F to C. Subtract 32 first, then multiply by 5/9 — not the other way round.
- Using the shortcut for cooking. The 'double and add 30' trick is for rough weather; use the exact formula for recipes.
- Adding 32 before multiplying (C to F). Multiply by 1.8 first, then add 32.
- Mixing up the fractions. C to F uses 9/5 (1.8); F to C uses 5/9 — they're inverses.
Conclusion
Converting Celsius and Fahrenheit is one formula each way — ×1.8 + 32 out, −32 then ×5/9 back — plus a 'double and add 30' shortcut for quick weather estimates. Learn a few anchor points to sanity-check yourself, and use a converter whenever a degree actually matters.