Celsius and Fahrenheit use different zero points and step sizes. Exact conversion is straightforward; mental shortcuts are useful only when their approximation is understood.
Exact formulas
Fahrenheit = Celsius × 9 ÷ 5 + 32. Celsius = (Fahrenheit − 32) × 5 ÷ 9. The 32 offset means this is not a simple scale multiplication.
Worked example
For 20°C, calculate 20 × 9 ÷ 5 + 32 = 68°F. For 68°F, calculate (68 − 32) × 5 ÷ 9 = 20°C.
A mental approximation
For a quick estimate, double Celsius and add 30: 20°C becomes about 70°F. It is close for everyday conversation but should not replace the exact formula for safety, cooking, medical, or technical decisions.
Watch the negative crossover
The scales meet at −40°, so both −40°C and −40°F represent the same temperature. Negative values are a useful test case for any conversion implementation.
Sources
The figures and rules in this guide are checked against:
- SI Units: Temperature — NIST Office of Weights and Measures
- Temperature Conversion Calculator — NOAA National Weather Service
- SI base unit: kelvin — BIPM
- Fahrenheit temperature scale — Encyclopaedia Britannica