Temperature Converter

Why temperature is harder than other conversions

Converting metres to feet is multiplication. Converting Celsius to Fahrenheit is not, because the two scales disagree about two separate things: how big a degree is, and where zero sits.

• A Celsius degree is 1.8 times larger than a Fahrenheit degree.
• Celsius puts zero at the freezing point of water; Fahrenheit puts it 32 degrees lower.

So every conversion needs a multiplication and an addition, and getting them in the wrong order is the classic mistake.

The formulas

Conversion	Formula
Celsius → Fahrenheit	(°C × 9 ÷ 5) + 32
Fahrenheit → Celsius	(°F − 32) × 5 ÷ 9
Celsius → Kelvin	°C + 273.15
Kelvin → Celsius	K − 273.15
Fahrenheit → Kelvin	(°F + 459.67) × 5 ÷ 9

Worked example. 20 °C to Fahrenheit: 20 × 9 = 180, ÷ 5 = 36, + 32 = 68 °F.

And back. 68 °F to Celsius: 68 − 32 = 36, × 5 = 180, ÷ 9 = 20 °C.

Doing it in your head

Celsius to Fahrenheit: double it and add 30.

Actual	Shortcut	True value
10 °C	50 °F	50 °F
20 °C	70 °F	68 °F
30 °C	90 °F	86 °F
40 °C	110 °F	104 °F

Accurate at 10 °C and drifting by about 6 degrees by 40 °C — fine for deciding on a jacket, not for cooking.

Fahrenheit to Celsius: subtract 30 and halve. 70 °F → 40 ÷ 2 = 20 °C, true value 21.1.

The one to memorise: −40. Celsius and Fahrenheit meet at minus forty. It is the only point where the two scales agree.

Reference points

	°C	°F	K
Absolute zero	−273.15	−459.67	0
Water freezes	0	32	273.15
A cold room	15	59	288.15
Room temperature	20	68	293.15
Body temperature	37	98.6	310.15
A hot day in Karachi	40	104	313.15
Water boils (at sea level)	100	212	373.15
Oven, moderate	180	356	453.15
Oven, hot	220	428	493.15

Absolute zero, and why it is a floor

At −273.15 °C a system has the least energy it can physically have. There is nothing below it — not because it is very cold, but because “less than the minimum” is not a thing that exists.

This is why the Kelvin scale starts there. Kelvin has no negative numbers and no degree symbol: 300 K, never 300 °K. A change of one kelvin is exactly the same size as a change of one degree Celsius, which is why converting between them is just an addition.

If you type a temperature below absolute zero into the converter above, it will still do the arithmetic — and tell you the result cannot exist.

Oven temperatures

Recipes cross borders and ovens do not. Gas marks, in particular, are a British convention that convert awkwardly:

Gas mark	°C	°F (exact)	°F (as usually printed)
1	140	284	275
2	150	302	300
3	170	338	325
4	180	356	350
5	190	374	375
6	200	392	400
7	220	428	425
8	230	446	450

Those last two columns rarely match, and that is not an error. Gas marks, °C and °F oven settings each evolved as their own set of round numbers, so published charts pair them by convention rather than by conversion. The exact column is what the arithmetic gives; the printed column is what a cookbook will say. An oven’s thermostat is not accurate to 10 degrees anyway.

One further wrinkle: a fan oven runs about 20 °C hotter than the same setting on a conventional one, so recipes usually say to drop the temperature by 20 °C or shorten the time.