How to Convert Temperature Units

Temperature doesn't work like every other unit

Every other converter multiplies. A metre is 3.28 feet, a kilogram is 2.2 pounds, and one factor does the whole job — because zero means the same thing in both units. Zero metres is zero feet.

Temperature scales don't share a zero. Zero Celsius is 32 Fahrenheit. So converting a temperature needs a multiplication and a shift:

• °F → °C: (F − 32) × 5/9
• °C → °F: (C × 9/5) + 32
• °C → K: C + 273.15

These are definitions, not approximations. Since 1954 the Celsius scale has been defined against the kelvin with 0°C at exactly 273.15 K.

Enter a Temperature, say what it's Measured in, and Celsius, Fahrenheit, Kelvin and the oven gas mark fill in:

A difference is not a temperature

This is the error worth reading the whole article for.

"The oven rose by 10°C" converts to a rise of 18°F. Not 50°F.

A change has no zero point, so the offset doesn't apply — you use the 9/5 ratio alone. The same goes for a range, a tolerance or a margin: ±5°C is ±9°F.

Both answers look plausible, which is exactly why this survives in recipes, weather reports and technical datasheets. The tool above converts temperatures, so if you hand it a difference it will add the 32 and be wrong by that much.

The test: ask whether the number is a position on the scale or a distance along it. Positions get the offset. Distances don't.

Gas mark is a real scale, and it stops being linear

From mark 1 upward, gas marks are perfectly regular. Mark 1 is 275°F, and every further mark adds 25°F:

Gas mark	°F	°C (as books print it)
1	275	140
4	350	180
6	400	200
9	475	240

Below mark 1 it changes character entirely — mark ½ is 250°F and mark ¼ is 225°F, which don't follow the same 25-degree spacing. And above mark 10 (500°F) there's simply nothing there: the arithmetic keeps producing numbers, but no domestic oven has such a position.

So the converter above leaves the gas mark blank at both ends rather than extending a straight line into territory where there's nothing to extend it to. Below 275°F the rule genuinely fails; above 500°F the answer would be a number with nothing behind it. A blank field is an honest answer; "gas mark 32" is not. The fractional marks are on the printable reference sheet.

Why your recipe book disagrees with the arithmetic

A cookbook says 350°F is 180°C. The exact conversion is 176.7°C.

Neither is wrong. Cookbook tables round to the numbers that actually exist on an oven dial, which is the right call for cooking and the wrong one for arithmetic. If a converted figure looks slightly off against a recipe table, this is nearly always why.

For setting an oven, use the book's rounded number. For anything where the arithmetic matters, use the exact one.

−40 is the same in both

Celsius and Fahrenheit cross at −40. That's the one temperature where the two scales agree: −40°C is −40°F.

It makes a fast sanity check on any conversion you've written down. Run −40 through your formula in both directions; if you don't get −40 back, you've got the offset or the ratio the wrong way round.

What Kelvin is for

Kelvin starts at absolute zero — the lowest temperature that can exist, −273.15°C or −459.67°F. Because its zero is a real physical floor rather than an arbitrary reference point, Kelvin is the only one of the three where ratios mean something. 20 K genuinely is twice as much thermal energy as 10 K. 20°C is not twice 10°C in any meaningful sense.

The converter deliberately doesn't block colder-than-possible inputs. Enter one and the Kelvin figure comes out negative, which says "this temperature cannot exist" more clearly than an error message would. Blanking every field instead would hide the reason.

The rough versions

For weather and everyday conversation:

• °C → °F: double it and add 30
• °F → °C: subtract 30 and halve

At 20°C the shortcut gives 70°F against a true 68°F — close enough to decide what to wear, and not close enough for an oven, a fridge, a meat probe or a dose. For those, use the exact formula, because the exact formula is a definition and there's no reason to accept an approximation of one.

One more place it bites

Fan and convection ovens run cooler than a conventional oven at the same dial setting. The tool doesn't adjust for it, because how much cooler depends on the oven — your manufacturer's manual states the offset for your model. A single generic figure applied to every oven is a guess wearing the costume of a conversion, and that's the sort of number this site would rather leave out than invent.