Temperature converter

Unlike every other category on this site, temperature scales don't share a common zero point, so you can't convert between them by multiplying by a single factor. Celsius sets 0° at the freezing point of water and 100° at boiling; Fahrenheit sets those same two physical events at 32° and 212°; Kelvin starts at absolute zero, the coldest temperature physically possible, and uses the same degree size as Celsius. The rest — Rankine, Réaumur, Delisle, the Newton scale, and Rømer — are historical scales built by different scientists with their own zero points and degree sizes.

The formula for the most common pair, Celsius to Fahrenheit, is:

Fahrenheit = Celsius × 9/5 + 32

To reverse it, subtract 32 first, then multiply by 5/9: Celsius = (Fahrenheit − 32) × 5/9. Kelvin slots in alongside Celsius with a simple offset: Kelvin = Celsius + 273.15, and the reverse is just Celsius = Kelvin − 273.15.

A couple of worked examples make the pattern clear. Normal human body temperature, 37°C, converts to 37 × 9/5 + 32 = 98.6°F. Water boils at 100°C, which is 100 × 9/5 + 32 = 212°F and also 100 + 273.15 = 373.15 K. Notice that Kelvin readings are always 273.15 higher than the matching Celsius reading — there's no multiplication involved, just addition.

Celsius dominates everyday life and science outside the US; Fahrenheit persists in US weather reports and home thermostats; and Kelvin is the one scientists reach for whenever a measurement needs to be absolute — thermodynamics, astrophysics, and any calculation where a negative temperature would make the physics nonsensical, since Kelvin has no negative values at all. Rankine plays the same “absolute” role for Fahrenheit-based engineering calculations in some US industries. The other scales — Réaumur, Delisle, Newton, and Rømer — are mostly of historical interest now, occasionally turning up in older European scientific texts or antique thermometers.

The most common mistake is trying to convert temperatures the same way you'd convert length or weight — by multiplying a value by a single ratio. Because Celsius and Fahrenheit don't share a zero point, that shortcut only works for a true ratio-to-ratio relationship like Celsius to Kelvin (which is addition only) or a size-only relationship; Fahrenheit needs both a multiplication and an addition, in the right order. Reversing the order of operations — adding 32 before multiplying, say — is the single easiest way to get a wrong answer.