Celsius to Fahrenheit is the only everyday conversion where you cannot just multiply, because the two scales disagree about where zero is. That one fact explains the formula, the famous −40 coincidence, and why a “double it” shortcut needs a +32 tacked on. Learn three anchor points and one trick and you will never be lost in a foreign weather forecast again; for everything else there is our free temperature converter.
In this guide
The formula and why it has two steps
°F = °C × 9/5 + 32
Most conversions are a single multiplication because both units agree that zero is zero: zero meters is zero feet. Temperature scales do not. Celsius puts 0 at freezing water; Fahrenheit puts freezing at 32. So the conversion needs a stretch (each Celsius degree is 1.8 Fahrenheit degrees) and a shift (+32 to relocate zero). That shift is why temperature intuition breaks: 20°C is not “twice as hot” as 10°C in Fahrenheit terms (68 vs 50), and it is the detail every wrong mental conversion forgets. The deeper rule, factor versus factor-plus-shift, is covered in our unit conversion pillar.
Anchor points worth memorizing
| °C | °F | Worth remembering as |
|---|---|---|
| −40 | −40 | The crossing point: the one temperature both scales agree on |
| 0 | 32 | Water freezes |
| 10 | 50 | Jacket weather |
| 20 | 68 | Room temperature |
| 30 | 86 | Beach weather |
| 37 | 98.6 | Body temperature |
| 100 | 212 | Water boils |
The −40 row is not trivia: it falls straight out of the algebra (set C = F and solve, and the stretch and shift cancel exactly there). The middle rows are the practical ones; with 10/50, 20/68, and 30/86 in memory, every weather forecast on earth becomes readable by interpolation, two Fahrenheit degrees per Celsius degree between anchors.
The exact mental trick (and the lazy one)
The exact one: double the Celsius, subtract 10% of the doubled value, add 32. For 20°C: 40, minus 4 is 36, plus 32 is 68°F, exact. It works because 2 × 0.9 = 1.8, precisely the 9/5 factor; this is a rare shortcut with zero error.
The lazy one: double and add 30. For 20°C: 70°F, two degrees high. It is fine for deciding on a jacket and drifts as temperatures rise (35°C gives 100 instead of the true 95). Use it for weather, never for cooking or medicine.
Fahrenheit to Celsius
°C = (°F − 32) × 5/9: shift first, then shrink. 72°F is (72 − 32) × 5/9 = 22.2°C. The mental version mirrors the exact trick: subtract 32, halve, add 10% of the half. For 72°F: 40, halved is 20, plus 2 is 22, a tenth of a degree off the true 22.2. The order matters in both directions: people who multiply before shifting get answers that are wrong by double-digit degrees.
Ovens, fevers, and weather
- Ovens: 180°C is 356°F, which recipes round to 350. The standard pairs: 160°C/325°F, 180°C/350°F, 200°C/400°F, 220°C/425°F. Recipe conversions round to the nearest 25°F because ovens are not precise instruments anyway; this is sanctioned rounding, not sloppiness. Ray Bradbury fans: paper’s famous 451°F is 233°C.
- Fevers: normal body temperature is around 37°C / 98.6°F, and the common fever threshold of 38°C lands exactly on the memorable 100.4°F. Medical conversions deserve the exact formula, not the lazy shortcut, and any health decision deserves a thermometer and a professional rather than arithmetic.
- Weather: the anchors table covers it. One asymmetry worth knowing: Fahrenheit’s finer degrees mean a one-degree F change is barely feelable, while one degree C is noticeable, which is why F forecasts look jumpier than C ones for the same weather.
Frequently asked questions
Why does Fahrenheit exist at all?
History: Daniel Fahrenheit built his scale in the early 1700s around the coldest brine mixture he could make (his zero) and body temperature, decades before Celsius’s water-based scale. It survives in the US for the same reason all customary units survive there, infrastructure and habit.
Where does Kelvin fit in?
Kelvin is Celsius shifted so that zero sits at absolute zero: K = °C + 273.15. Same degree size as Celsius, no negative values, and it is the scale science uses. Room temperature is about 293 K.
Is 98.6°F really “normal” body temperature?
It is the textbook conversion of 37.0°C, but normal body temperature varies by person, time of day, and measurement site, typically across roughly 36.5 to 37.5°C. Treat 98.6 as an anchor point, not a pass-fail line, and fever questions as a job for a thermometer and a clinician.
Why do US and European recipes disagree about “moderate oven”?
They do not, they just round in their own units: 180°C and 350°F are the same moderate oven within four degrees. Convert exactly, round to the nearest standard setting, and the cake cannot tell the difference.