Temperature Converter

Celsius, Fahrenheit, Kelvin and Rankine, in any combination. Below the answer sits a short table of landmark temperatures rendered in whichever scale you picked, which is the fastest way to tell whether a number is plausible before you act on it.

Everything pivots through Celsius

Rather than carrying twelve separate formulas, the tool converts your input to Celsius and then out again. Two conversions instead of one, and far fewer places for a sign error to hide.

to Celsius       from Celsius
C = F            F = C × 9/5 + 32
C = (F − 32) × 5/9   K = C + 273.15
C = K − 273.15       R = (C + 273.15) × 9/5
C = (R − 491.67) × 5/9

Rankine is the one people meet least often: Fahrenheit-sized degrees counted from absolute zero, the way Kelvin uses Celsius-sized ones. It still turns up in American thermodynamics and combustion work.

The pivot has a small cost worth stating. Kelvin to Rankine passes through a subtraction and an addition of 273.15, and binary floating point does not always hand those back cleanly, so a raw value can land a fraction of a unit in the last binary place off the textbook answer. At the precision shown here you will never see it.

A degree of difference is not a temperature

Here is the thing most converters never mention, and it causes real errors in specifications and datasheets.

The scales differ in two ways at once: their degrees are different sizes, *and* their zero points are in different places. Converting a reading uses both facts. Converting a difference uses only the first.

A room that warms from 20 °C to 30 °C has risen by 10 °C. That rise expressed in Fahrenheit is 18 °F, because a Celsius degree is 9/5 of a Fahrenheit degree. It is emphatically not 50 °F, which is what you get by running 10 through the absolute-temperature formula — and 50 °F is what this tool will hand you, because it converts readings and has no way of knowing you typed an interval.

The rule is simple once you have it: for a temperature difference, tolerance band or rate of change, multiply by 9/5 or 5/9 and leave the 32 out. Kelvin and Celsius degrees are the same size, so a difference in one is the same number in the other.

Reference points that catch a wrong answer

The panel lists absolute zero, the freezing point of water, room temperature, body temperature and the boiling point of water, all converted into your target scale. A quick glance is usually enough: if you converted an oven setting and the result sits below the freezing row, you converted in the wrong direction.

Two landmarks deserve an asterisk. Water boils at 100 °C only at standard sea-level pressure — in Denver it is closer to 94 °C, and higher inside a pressure cooker. And 98.6 °F is not a biological constant; it is the arithmetic conversion of a rounded 37 °C from a nineteenth-century survey. Both are there to check your arithmetic against, nothing more.

Below absolute zero

Type a value colder than −273.15 °C, −459.67 °F or 0 K and the tool still converts it — the arithmetic is well defined — but adds a line saying the result is not a physically reachable temperature. It is almost always a typo or a sign error, and the note is there so you spot it rather than passing the figure on.

Worth checking your arithmetic against

Two conversions to memorise. −40 is the same number on both the Celsius and Fahrenheit scales, the single point where the two lines cross, and it makes an excellent sanity check on any formula. And 20 °C is 68 °F, 293.15 K and 527.67 °R at once — if a converter disagrees with any of those, it is wrong.

Your scale selection and precision are remembered; the temperature you typed is not.

Companion unit converters

• Weight Converter — mass across metric and customary units.
• Length Converter — nine length units from one defined inch.
• Colour Code Converter — HEX, RGB, HSL and CMYK with contrast figures.