Fahrenheit to Celsius Without a Calculator (2 Shortcuts)

The exact formula is (°F − 32) × 5/9. But there are two mental shortcuts that get you within a degree in a couple of seconds.

The exact formula

°C = (°F − 32) × 5 ÷ 9. Reverse: °F = °C × 9 ÷ 5 + 32. Both scales meet at −40°, which is the same temperature in each.

Shortcut 1: subtract 30, halve

70 °F → 70 − 30 = 40 → 20 °C (exact 21.1). Fast and good to about 1–2 degrees across normal weather temperatures.

Shortcut 2: the doubling rule for °C → °F

Double the Celsius value and add 30. 25 °C → 50 + 30 = 80 °F (exact 77). Accurate enough for weather, not for cooking.

Anchor points worth memorising

0 °C = 32 °F (freezing) · 10 °C = 50 °F · 20 °C = 68 °F (room) · 30 °C = 86 °F · 37 °C = 98.6 °F (body) · 100 °C = 212 °F (boiling).

Oven temperatures

160 °C = 325 °F · 180 °C = 350 °F · 190 °C = 375 °F · 200 °C = 400 °F · 220 °C = 425 °F. Fan ovens run about 20 °C hotter than the dial suggests — reduce accordingly.

Kelvin, briefly

K = °C + 273.15. Kelvin has no degree sign and no negatives; 0 K is absolute zero, −273.15 °C.

Sources & further reading

BIPM — The International System of Units (SI Brochure)
NIST — Guide for the Use of the International System of Units
WHO — Body weight & health guidance

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