Fahrenheit to Celsius

Subtract 32, then multiply by 5/9: °C = (°F − 32) × 5/9. The subtraction must come first because it removes the offset between the scales' zero points. Check it against the fixed points: freezing, (32 − 32) × 5/9 = 0 °C, and boiling, (212 − 32) × 5/9 = 100 °C.

The standard threshold is 100.4 °F, which converts to exactly 38 °C: (100.4 − 32) × 5/9 = 38.0. A reading of 102 °F is 38.89 °C, and 104 °F is exactly 40 °C, the level usually classed as a high fever. Normal body temperature sits at 98.6 °F = 37 °C, with healthy variation of roughly half a degree either side.

Recipe pairings are rounded to the settings ovens actually offer. The exact conversion of 350 °F is 176.67 °C, and the nearest standard Celsius oven setting is 180 °C (Gas Mark 4) — which, converted exactly, is 356 °F. The few degrees of difference are smaller than the swing of a typical oven thermostat, so either setting bakes the same.

Subtract 30 and halve: 86 °F gives 28 against the exact 30 °C. The estimate runs low, and the gap grows with temperature — at 100 °F it gives 35 against the exact 37.78 °C. A closer mental route for round numbers: subtract 32, then take half and add a ninth of that half (that is the ×5/9 step split into two easy parts).

−459.67 °F, which is exactly −273.15 °C: (−459.67 − 32) × 5/9 = −273.15. It is the floor of both scales — no temperature converts to anything below it — and the point where the third scale, kelvin, reads 0 K. The Temperature Converter shows all three scales together.