Temperature Converter
Convert between Celsius, Fahrenheit, Kelvin and Rankine instantly
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Temperature Scales Explained
Temperature measures the average kinetic energy of particles in a substance. The four main scales differ in their zero points and degree sizes.
The Four Scales
Absolute Zero
Absolute zero (0 K / −273.15°C / −459.67°F) is the theoretical lowest possible temperature — the point at which all molecular motion stops. It has never been achieved in practice, though scientists have reached within billionths of a degree of it in laboratory conditions.
Temperature Scales: Why the Conversion Has a "+32" In It
Built and verified by Andrius R. · Updated June 2026
Temperature is the odd one out among conversions: every other everyday unit converts by multiplying, but temperature scales also disagree about where zero is. That one fact explains the formula, the famous −40 coincidence, and the most common conversion mistake in science homework.
The formulas and the anchors
°F = °C × 9/5 + 32 · °C = (°F − 32) × 5/9 · K = °C + 273.15
Worth memorizing as landmarks: water freezes at 0 °C = 32 °F = 273.15 K; body temperature ≈ 37 °C = 98.6 °F; water boils at 100 °C = 212 °F (at sea level); a 20 °C room is 68 °F. And the party fact: −40 °C = −40 °F exactly — the one point where the scales cross, because their different zero-points and step-sizes happen to cancel there.
The mistake everyone makes once: differences vs temperatures
A temperature rise of 10 °C is a rise of 18 °F — multiply by 9/5 only, no +32. The offset locates zero; it plays no role in how big a degree is. Same logic in reverse: an oven recipe saying "reduce by 25 °F" means reduce by ~14 °C, not by −4. Whenever the word is "warmer," "cooler," "difference" or "range," drop the 32.
Where the scales came from
- Fahrenheit (1724): Daniel Fahrenheit set 0° at the coldest brine mixture he could reliably make and ~96° near body temperature. Arbitrary-sounding, but his mercury thermometers were the precision instruments of the age, so the scale stuck.
- Celsius (1742): Anders Celsius pinned 0 and 100 to water's freezing and boiling — famously in the reverse order at first (0 = boiling); colleagues flipped it after his death.
- Kelvin (1848): same step size as Celsius, but zero placed at absolute zero (−273.15 °C), the floor where molecular motion is minimal. No degree symbol, no negative values — and it's the only scale where "twice the temperature" means twice the thermal energy, which is why physics equations demand it.
- Rankine: Fahrenheit's step size with Kelvin's zero — surviving mainly in some US engineering fields.
Why "twice as hot" usually isn't
20 °C is not twice as hot as 10 °C — in Kelvin that's 293 vs 283, a 3.5% difference. Celsius and Fahrenheit are interval scales (differences are meaningful, ratios aren't) because their zeros are arbitrary; only Kelvin's absolute zero makes ratios honest. The same trap explains why a "100% increase" in °C weather is meaningless marketing, and why gas-law calculations done in Celsius give nonsense answers.
Practical corners
Quick mental approximation for weather: °F ≈ °C × 2 + 30 (exact at 10 °C, within a couple of degrees across the everyday range). Fevers: 38 °C = 100.4 °F is the standard threshold — a number worth knowing in both scales if you travel. Cooking: most ovens worldwide are Celsius, US recipes Fahrenheit, and "gas mark" is its own British ladder (mark 4 ≈ 180 °C ≈ 350 °F). For everything beyond temperature, the unit converter covers the multiplicative world.
From the Blog
// Quick °C → °F
Double the Celsius, subtract 10%, then add 32. For 20°C: 40 − 4 + 32 = 68°F (exact). Works best for common temperatures.
// The −40 Point
−40°C = −40°F exactly. This is the only temperature where the two scales meet. Useful as a mental checkpoint.
// Kelvin in Science
Scientists almost always use Kelvin because it's an absolute scale — no negatives. Gas law calculations (PV = nRT) require Kelvin.
// Body Temperature
Normal body temperature is 37°C (98.6°F). A fever is typically considered > 38°C (100.4°F). Hypothermia below 35°C (95°F).