The US uses Fahrenheit for weather, ovens and thermometers, while nearly everything else uses Celsius. The two scales disagree wildly in feel: a comfortable room is 25°C but 77°F. Enter any temperature on either side and the other updates instantly — Kelvin is included for science homework.
The conversion formula
Fahrenheit = Celsius × 9 ÷ 5 + 32. Going the other way: Celsius = (Fahrenheit − 32) × 5 ÷ 9. Example: 25°C = 25 × 1.8 + 32 = 77°F, and 98.6°F = (98.6 − 32) × 5/9 = 37°C. Mental shortcut for weather: subtract 30 and halve — 77°F ≈ (77−30)/2 ≈ 24°C.
The −40° coincidence
−40 is the one temperature where both scales read exactly the same, because the conversion lines cross there. It is a handy anchor: below −40°, Fahrenheit numbers are actually smaller than Celsius.
Reference points worth memorizing
Water freezes: 0°C = 32°F · Comfortable room: 25°C = 77°F · Normal body: 37°C = 98.6°F · Fever: 38.5°C ≈ 101.3°F · Water boils: 100°C = 212°F · Moderate oven: 180°C = 356°F. When American news says "in the 90s", that is 32–37°C heat.