How this works
Pick a unit, type a temperature, see the equivalent in the other two scales. Unlike length or weight, temperature scales don't share a zero — Celsius freezes at 0, Fahrenheit at 32, and Kelvin starts at absolute zero (−273.15°C). That's why every conversion involves an offset, not just a multiplication, and why "twice as hot" doesn't mean what you'd think (20°C → 40°C is not "twice the heat" — only Kelvin gives that property).
The formula
All conversions route through Celsius as the canonical scale: F or K → C → target. The 5/9 factor in F→C is the ratio of degree-sizes (a Fahrenheit degree is smaller — 100 of them between freezing and boiling vs 100 Celsius from 0 to 100). The 273.15 offset on Kelvin is the agreed temperature of the freezing point of water under the 1990 international scale.
Example calculation
- Enter 20 °C — a comfortable room temperature in the UK or continental Europe.
- °F: 20 × 9/5 + 32 = 68 °F. K: 20 + 273.15 = 293.15 K.
- Quick mental check: 20 °C ≈ 68 °F is a useful anchor — body temp ≈ 37 °C ≈ 98.6 °F is another. Doubling C and adding 30 gives a rough F estimate.
Frequently asked questions
What's the easy mental conversion between °C and °F?
"Double and add 30" gives a rough °F from °C. 20 °C → ~70 °F (actual 68). 30 °C → ~90 °F (actual 86). Going the other way, "subtract 30 and halve" gives an approximate °C. The error grows above ~40 °C — for cooking or scientific use, use the calculator for the exact answer.
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 temperature changes — thermostats and weather feel more "tunable" — but for science the US uses Celsius and Kelvin like everyone else.
When should I use Kelvin instead of Celsius?
Whenever you're multiplying or dividing temperatures — the gas laws (PV = nRT), Stefan-Boltzmann radiation, anything in thermodynamics. Celsius has a non-physical zero so ratios are meaningless ("0 °C × 2" doesn't mean anything). For everyday weather, cooking, or body-temperature talk, Celsius is fine; for physics or chemistry, switch to Kelvin.