Temperature Converter

The formula

5/9 and 9/5

The ratio between the scales. A Celsius degree is 1.8 times the size of a Fahrenheit degree, because 100 Celsius degrees span the same range as 180 Fahrenheit degrees.

32

The Fahrenheit reading at the freezing point of water, which is 0 in Celsius. This is why the conversion needs a shift as well as a scale factor.

273.15

The offset between Celsius and Kelvin. The size of a degree is identical in both, so only the zero point moves.

Worked example

Converting 98.6 °F to Celsius and then to Kelvin.

• 98.6 − 32 = 66.6
• 66.6 × 5 ÷ 9 = 37.00 °C
• 37.00 + 273.15 = 310.15 K
• Back the other way: 37 × 9 ÷ 5 + 32 = 98.6 °F

98.6 °F = 37.00 °C = 310.15 K.

Subtract before you multiply

The order of operations is where hand conversions go wrong. Going from Fahrenheit you subtract 32 first and then scale; going to Fahrenheit you scale first and then add 32. Doing it the other way round on 98.6 gives 22.6 instead of 37 — a large enough error to be obvious, but the same mistake on a temperature near freezing is subtle enough to slip through.

A mental approximation that is close enough

To go from Celsius to Fahrenheit, double it and add 30. For 20 °C that gives 70 against a true 68; for 30 °C it gives 90 against 86. The approximation drifts as temperatures rise because doubling overstates the 1.8 factor, but across ordinary weather it lands within a few degrees, which is usually all you need to decide what to wear.

Kelvin has no degree sign and no negatives

Kelvin is an absolute scale starting at absolute zero, so it is written as 310 K rather than 310 °K, and a negative value is not physically meaningful. The tool will still convert one arithmetically if you enter it, since it simply applies the offset — the check is yours to make.

Reference points worth memorising

Water freezes at 0 °C, 32 °F, 273.15 K and boils at sea level at 100 °C, 212 °F, 373.15 K. The two scales cross at −40, where Celsius and Fahrenheit read the same number. Those three anchors let you sanity-check any conversion in a couple of seconds.

Common questions

How do I convert Fahrenheit to Celsius?

Subtract 32, then multiply by 5 and divide by 9. For 98.6: 98.6 − 32 = 66.6, and 66.6 × 5 ÷ 9 = 37. The subtraction has to come first, because the two scales have different zero points as well as different degree sizes.

What is normal body temperature in Celsius?

The familiar 98.6 °F converts to exactly 37 °C, which is where the round Fahrenheit-sounding figure comes from — it is a conversion of a round Celsius number, not the other way round. Body temperature varies through the day and between people, so treat any single value as a reference point rather than a fixed constant.

Why is Kelvin 273.15 above Celsius?

Because Kelvin starts at absolute zero, the point at which thermal motion is at its minimum, and Celsius starts at the freezing point of water. The degree size is identical in both scales, so converting between them is only ever an addition or a subtraction.

At what temperature are Celsius and Fahrenheit the same?

At −40. Setting C equal to F in the conversion and solving gives −40, which is why −40 °C and −40 °F are the same temperature and the only point where the two scales agree.