Celsius, Fahrenheit and Kelvin Converter

Why the formula needs two steps

Celsius and Fahrenheit disagree about two separate things. They disagree about where zero sits, and they disagree about how big one degree is. A Fahrenheit degree is five ninths of a Celsius degree, and Fahrenheit puts its zero 32 degrees below the freezing point of water.

That is why the conversion is F = C × 9/5 + 32 rather than a single multiplication. Every other unit on this site converts with one factor. Temperature cannot, which is the whole reason it has a tool of its own instead of living in the general unit converter.

The mistake nearly everyone makes once

Converting a temperature and converting a temperature difference are not the same operation. If a forecast says tomorrow will be 10 degrees Celsius warmer, that is 18 degrees Fahrenheit warmer, not 50.

For a difference you use the scale factor only and drop the offset: multiply by 9/5 to go from Celsius to Fahrenheit, by 5/9 to come back. The offset exists to line up the zero points, and a difference has no zero point to line up. This tool converts temperatures, not intervals, so if you are working with a rise, a drop or a tolerance, do the multiplication yourself.

Where the scales cross

Minus 40 is the same number on both: -40 C is -40 F. It falls out of the algebra, since setting C = C × 9/5 + 32 gives exactly one solution. It is also the one conversion you never have to look up.

Kelvin and Rankine

Kelvin starts at absolute zero and uses Celsius-sized degrees, so it is just Celsius shifted by 273.15. Two conventions that people get wrong: there is no degree sign on Kelvin and you do not say "degrees Kelvin". It is 300 K, read as three hundred kelvin. The unit was redefined in 2019 in terms of the Boltzmann constant, so it no longer depends on the properties of water at all.

Rankine is the same idea with Fahrenheit-sized degrees: absolute zero is 0 R, water freezes at 491.67 R. It survives in parts of US engineering, particularly thermodynamics and aerospace, and almost nowhere else.

Oven temperatures and gas marks

Gas marks look arbitrary in Celsius — 140, 150, 170, 180, skipping 160 entirely — because they were never defined in Celsius. Gas mark n is 250 + 25n degrees Fahrenheit. Mark 1 is 275 F, mark 4 is 350 F, mark 9 is 475 F. Perfectly regular, in the other scale.

The Celsius column in every published chart, including the one in this tool, is rounded to a number an oven dial can actually show. Gas mark 3 is 325 F, which is really 162.8 C, but the charts say 170 C. The published values drift up to about seven degrees from the exact conversion. That is fine for baking and worth knowing if you are converting a precise recipe.

For a fan or convection oven, the standard advice is to set it 20 C lower than a conventional recipe asks for, because moving air transfers heat faster. Some manufacturers say 25 C. Neither is a law of physics, and the safe move on an unfamiliar oven is to check early.

What this tool does not tell you

• What the oven is actually doing. Domestic thermostats are frequently off their set point, and the rack position matters as much as the dial. If precision matters, an oven thermometer costs very little and settles the question.
• Feels-like temperatures. Wind chill and heat index are not conversions. They are models that combine temperature with wind speed or humidity, and each weather agency uses its own formula.
• Body temperature norms. 98.6 F is exactly 37 C converted, and 37 C was itself an average taken in the nineteenth century. It was never a fixed value for an individual person, and it varies through the day.
• Endless precision. Each field rounds to four decimal places, so typing a very long value into one box and reading it back from another can shift the last digit.

Last updated September 19, 2026