How this works
Going from Fahrenheit to Celsius is the inverse of the F = C × 9/5 + 32 formula: subtract 32 first to remove the offset, then scale by 5/9 to switch from the smaller Fahrenheit degree to the larger Celsius degree. Useful when reading US weather while travelling, or interpreting an American medical chart, recipe, or HVAC specification in metric units.
The formula
For mental conversion, "subtract 30 and halve" gives an estimate within ~2 °C across normal weather: 70 °F → ~20 °C (actual 21.1), 90 °F → ~30 °C (actual 32.2). Anchors worth memorising: 32 °F = 0 °C, 50 °F = 10 °C, 68 °F = 20 °C (room temp), 100 °F ≈ 38 °C (heatwave / mild fever).
Example calculation
- 32 °F = 0 °C
- 70 °F = 21.1 °C (a comfortable indoor temperature)
- 98.6 °F = 37 °C (classic body temperature)
- 212 °F = 100 °C (water boils)
Frequently asked questions
Why does the US still use Fahrenheit?
Mostly inertia. The US, along with Belize and the Cayman Islands, never made the metric switch for everyday use. Proponents argue Fahrenheit's finer granularity (1 °F is roughly half a °C) maps better to how humans perceive small weather changes — thermostats and hourly forecasts feel more "tunable" — but for science and medicine the US uses Celsius and Kelvin like everyone else.
How can I tell if a US fever reading is serious?
Rough rule: anything above 100.4 °F (38 °C) is generally considered a fever; above 103 °F (39.4 °C) warrants medical attention; above 104 °F (40 °C) is treated urgently. Convert with (°F − 32) × 5/9 to get the °C equivalent. Always check the device documentation — different thermometers have slightly different reference ranges (oral vs ear vs forehead).