Temperature Conversion Guide

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Ce guide n'a pas encore été traduit en FR. La version anglaise est affichée ci-dessous.

Temperature is the conversion category that breaks every shortcut. There's no single multiplier — each scale has its own zero point and its own degree size, so converting Celsius to Fahrenheit takes both a multiplication and an addition. There is one famous fixed point where the two scales agree (−40°), and one fixed point where Kelvin makes them both look arbitrary (absolute zero at 0 K = −273.15 °C). This guide covers the four working scales in modern use, three historical scales kept for completeness, and the small set of rules you actually need to remember.

What is temperature?

Temperature is a measure of average kinetic energy per particle in a system. Hot things have particles moving fast; cold things have particles moving slowly. The lower bound is absolute zero — the point at which all classical motion stops — and it is reached at exactly −273.15 °C or 0 K. No system can go colder, which is why the Kelvin scale starts at that point and counts up. The scientific scales (Kelvin, Rankine) follow physics; the everyday scales (Celsius, Fahrenheit) follow water and weather.

Because each scale defines both a zero point and a degree size differently, conversions between them are affine functions (multiply-then-add), not pure ratios. This is the single feature that makes temperature different from every other category in this site. It also means that a temperature difference and an absolute temperature behave differently under conversion — a fact that quietly trips up scientific manuscripts and engineering specifications more often than any other.

History of temperature scales

Daniel Gabriel Fahrenheit designed his mercury-in-glass thermometer and the scale that bears his name around 1724. He fixed three calibration points: a brine ice-bath at 0 °F, water's freezing point at 32 °F, and human body temperature at roughly 96 °F. Later refinement adjusted the upper anchor so that the boiling point of water at standard pressure lands at exactly 212 °F — making the gap from freezing to boiling 180 degrees, a convenient multiple of 60 for navigation tools.

Anders Celsius proposed the centigrade scale in 1742 with 100 degrees between freezing and boiling, though originally he ran it backwards (0 at boiling, 100 at freezing). The convention flipped within his lifetime; the scale was renamed Celsius in 1948. William Thomson (Lord Kelvin) extended Celsius downward in 1848 to absolute zero, defining the thermodynamic temperature scale that bears his name. The Rankine scale does the same trick on top of Fahrenheit: 0 °R is absolute zero, and the degree size matches Fahrenheit.

Three historical scales survive in our converter for completeness: Delisle (degrees count downward from boiling water, popular in 18th-century Russia), Newton (proposed by Isaac Newton around 1701 with 0 at freezing water and 33 at boiling, which was a brave first attempt before precision thermometry), and Réaumur (a 0-to-80 scale used in continental Europe through the 19th century, still occasionally cited in historical chemistry and cheese-making). None of the three is in scientific or everyday use today, but they show up in archival texts and in occasional Wikipedia-driven trivia.

Units we cover

Our temperature category supports seven scales. Working scales: Celsius (°C), Fahrenheit (°F), Kelvin (K), and Rankine (°R). Historical scales: Delisle (°De), Newton (°N), and Réaumur (°Ré). The Kelvin and Rankine scales never take a degree sign in current SI style — 300 K, not 300 °K — and never go negative, because absolute zero is the floor.

Three calibration points are worth memorising. Water freezes at 0 °C = 32 °F = 273.15 K. Water boils (at 1 atm) at 100 °C = 212 °F = 373.15 K. The two everyday scales cross at exactly −40 °C = −40 °F, which is the single fastest sanity-check there is — if your conversion of −40 in one scale doesn't read −40 in the other, the math is wrong somewhere. A second handy anchor: 100 °F is roughly 38 °C, the clinical fever threshold in degree-Celsius medicine. A third: 200 °C is roughly 392 °F, the standard moderate-oven setting in metric and imperial recipe books respectively.

How to convert

Temperature uses explicit formulas instead of multiplication factors. Each scale converts to and from Kelvin through a defined formula rather than a single multiplier. To go Celsius → Fahrenheit, multiply by 9/5 and add 32. To go Fahrenheit → Celsius, subtract 32 and multiply by 5/9. To get Kelvin, add 273.15 to Celsius (or follow the chain Fahrenheit → Celsius → Kelvin). The temperature methodology section lists every formula explicitly, including the historical scales.

Because the transformations are affine rather than linear, you cannot multiply a temperature difference by the same factor as an absolute temperature. A change of 1 °C is a change of 1.8 °F, but 1 °C absolute is 33.8 °F absolute. The two are different operations even though they share a multiplier.

Common conversions you'll need

Weather and cooking dominate temperature searches. Celsius to Fahrenheit and Fahrenheit to Celsius cover thermostats, oven settings, weather apps, and recipe conversions. The same pair handles the body-temperature questions — 37 °C lines up with 98.6 °F, and 38.0 °C is the standard clinical fever threshold (100.4 °F). For scientific or engineering work, Celsius to Kelvin handles thermodynamics and gas-law calculations; the offset is always exactly 273.15. The reverse, Kelvin to Celsius, is the one a student needs the first time they read a paper that quotes room temperature as '293 K' rather than '20 °C'.

The full temperature comparison table lists every scale side by side, with sample anchor points so you can spot a bad conversion at a glance. Use it when you're cross-checking a historical chemistry paper that cites Réaumur, or when you simply want all four working scales lined up for an anchor temperature like the human body, the freezing point of water, or the surface of the Sun.

Pitfalls and gotchas

Difference vs absolute: a temperature change of 5 °C is 9 °F (multiply by 9/5), but 5 °C absolute is 41 °F absolute (multiply by 9/5 then add 32). Confusing the two is the most common temperature error in introductory engineering courses.

Kelvin doesn't take a degree sign: write 'K', not '°K'. Rankine does — '°R'. This is a holdover from the 1968 SI revision and is enforced in scientific publishing.

Body-temperature precision: the textbook 'normal body temperature is 98.6 °F' is the conversion of 37 °C exactly. The original 19th-century measurements that produced 37 °C rounded aggressively, and modern population means come in closer to 36.8 °C. For fever, the more reliable threshold is ≥ 38.0 °C (= 100.4 °F), recommended by the WHO.

Oven temperature 'gas marks' on UK ovens: gas mark 1 = 140 °C = 275 °F, gas mark 4 = 180 °C = 350 °F, gas mark 7 = 220 °C = 425 °F. The non-linearity isn't gas-mark-specific; oven thermometers themselves are typically accurate to ±10 °F, so don't trust the third significant figure.

Newton vs newton: the 18th-century Newton temperature scale shares a name with the SI unit of force. The temperature one is rare; the force one is everywhere. Our converter resolves the right entity per page so structured data stays clean.