How to Convert Between Fahrenheit and Celsius Quickly
If you travel, cook from international recipes, or read science papers, you will eventually need to convert between Fahrenheit and Celsius. The formulas are simple but easy to forget under pressure — especially the one involving 5/9 and 32.
This guide nails down both formulas, provides three mental shortcuts for instant estimates, and includes a reference table of the temperatures you are most likely to encounter in real life.
✨Key takeaways
- °C = (°F − 32) × 5/9. °F = (°C × 9/5) + 32.
- Mental shortcut: double °C and add 30 to get a rough °F.
- Key anchors: 0°C = 32°F (freezing), 100°C = 212°F (boiling), 37°C = 98.6°F (body temp).
- The two scales cross at −40: −40°C = −40°F.
The exact formulas
Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Example: 72°F → (72 − 32) × 5/9 = 40 × 0.5556 = 22.2°C.
Celsius to Fahrenheit: °F = (°C × 9/5) + 32. Example: 25°C → (25 × 1.8) + 32 = 45 + 32 = 77°F.
The key difference from most unit conversions: temperature requires both a multiplier (9/5 or 5/9) and an offset (+32 or −32) because the two scales do not share a zero point.
Three mental shortcuts
Shortcut 1 (°C → °F): Double it and add 30. 20°C → 40 + 30 = 70°F (actual: 68°F). Accurate within ±3°F for everyday temperatures.
Shortcut 2 (°F → °C): Subtract 30, then halve. 80°F → 50 ÷ 2 = 25°C (actual: 26.7°C). Same accuracy range.
Shortcut 3: Memorise three anchors — 0/32, 10/50, 20/68, 30/86, 37/98.6, 100/212 — and interpolate. If you know 20°C = 68°F and 30°C = 86°F, then 25°C is halfway: 77°F.
Quick reference table
−40°C = −40°F (where the scales cross). −18°C = 0°F. 0°C = 32°F. 10°C = 50°F. 20°C = 68°F. 25°C = 77°F. 30°C = 86°F. 37°C = 98.6°F. 40°C = 104°F. 100°C = 212°F.
Print this table or bookmark it. For anything between these values, interpolate linearly — the error is tiny.
When precision matters
Cooking: oven temperatures matter. 180°C = 356°F, 200°C = 392°F, 220°C = 428°F. A 20°C difference in oven temp changes baking outcomes significantly, so use the exact formula or the Unit Converter rather than the mental shortcut.
Medical: a body temperature of 38°C is 100.4°F — that 0.4 can be the difference between "normal" and "low-grade fever." Always use the precise formula for medical contexts.
Science: laboratory work uses Celsius (or Kelvin). Converting experimental data to Fahrenheit for a report requires exact values, not estimates.
Try the calculators referenced in this guide
Put the maths into practice — every calculator is free and runs entirely in your browser.
Frequently Asked Questions
Why does the US still use Fahrenheit?
Historical inertia. The US adopted Fahrenheit before the metric system became the global standard. Congress authorised metric adoption in 1975 but made it voluntary, so the switch never fully happened for everyday use.
What is the difference between Celsius and Centigrade?
Nothing — they are the same scale. "Centigrade" was the original name (from Latin "centum" = 100 and "gradus" = step). It was officially renamed to "Celsius" in 1948 to honour Anders Celsius.
How do I convert Celsius to Kelvin?
Add 273.15. So 25°C = 298.15 K. Kelvin uses the same degree size as Celsius but starts at absolute zero (−273.15°C).
The Precision Calculator Editorial Team
The editorial team at Get Precision Calculator writes practical, formula-driven guides that explain the maths behind every calculator on this site. All content is reviewed for accuracy before publishing.
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