Tool

Celsius to Fahrenheit Converter

68^°F

Temperature

From

How do you convert Celsius to Fahrenheit?

Multiply the Celsius value by 9/5 and add 32. To go the other way, subtract 32 from the Fahrenheit value and multiply by 5/9. For a quick mental estimate, double the Celsius number and add 30 — that lands within a couple of degrees across ordinary outdoor temperatures.

The formulas

Celsius to Fahrenheit: °F = (°C × 9/5) + 32

Fahrenheit to Celsius: °C = (°F − 32) × 5/9

Two things are happening in each. The 9/5 accounts for the degrees being different sizes — a Fahrenheit degree is smaller, so there are 180 of them between freezing and boiling where Celsius has 100. The 32 accounts for the scales starting in different places: water freezes at 0 °C and 32 °F.

Get the order wrong and you get a plausible-looking wrong answer, which is the usual failure. Multiply first, then add.

The reference table

The values people actually look up, in one place.

Common conversions across the range of ordinary outdoor weather.
Celsius	Fahrenheit	What it is
−40 °C	−40 °F	The two scales meet
−20 °C	−4 °F	Bitter
−10 °C	14 °F	Hard frost
−6 °C	21 °F	Pipes start to be at risk
0 °C	32 °F	Water freezes
10 °C	50 °F	Minimum for most exterior paint
13 °C	55 °F	Minimum for an empty house in winter
20 °C	68 °F	Common indoor target
25 °C	77 °F	Warm summer day
30 °C	86 °F	Hot
40 °C	104 °F	Heat wave

Doing it in your head

Three methods, in ascending order of accuracy.

Double and add 30. The one worth memorising. 20 °C → 70 (true 68). 25 °C → 80 (true 77). 5 °C → 40 (true 41). Off by up to three degrees in the middle of the range, more at the extremes, and entirely good enough for deciding on a coat.

Double, subtract a tenth, add 32. Closer, because subtracting a tenth of the doubled value turns ×2 into ×1.8, which is 9/5 exactly. 20 °C → 40 − 4 + 32 = 68, correct.

Anchor points. Remember 0 °C = 32 °F, 10 °C = 50 °F, 20 °C = 68 °F, 30 °C = 86 °F, and interpolate. Every 5 °C step is 9 °F, so the intermediate values come out quickly.

Going the other way, halve the Fahrenheit figure and subtract 15: 70 °F → 20 (true 21). Same trade-off, same usefulness.

Where the two scales came from

Fahrenheit is the older of the two, from 1724. Daniel Fahrenheit set zero at the coldest temperature he could reliably reproduce — a brine mixture of ice, water and salt — and fixed the upper part of his scale against the warmth of a human being. The freezing point of water landed at 32 as a consequence, not as a design decision.

Celsius arrived in 1742, and Anders Celsius originally had it upside down: 0 for boiling water and 100 for freezing. It was inverted after his death into the form everybody now uses. It is sometimes still called centigrade, which describes the hundred divisions between the two fixed points.

Kelvin, from 1848, keeps the Celsius degree size but moves zero to absolute zero — the point where there is no thermal energy left to remove. So kelvin and Celsius differ by a constant 273.15 and nothing else, which is why converting between them is addition rather than multiplication.

The values people search for

There are individual pages for the specific conversions that get looked up most, each with the formula worked through for that number and the range either side of it. Everything from bitter cold to a heat wave is covered, and the panel at the top of this page handles anything they do not.

Common questions

What is the quickest way to convert in my head?

Double the Celsius figure and add 30. At 20 °C that gives 70 against a true 68 °F, and at 0 °C it gives 30 against 32 °F. It drifts a little at the extremes but stays within about three degrees across the whole range of ordinary outdoor weather, which is close enough to decide what to wear.

At what temperature are Celsius and Fahrenheit the same?

At −40. That is the one point where the two scales read the same number, which is why −40 °C and −40 °F need no conversion. It falls out of the arithmetic: the scales have different zero points and different degree sizes, and −40 is where the two differences cancel.

Why does the United States still use Fahrenheit?

Largely inertia. Fahrenheit arrived first in the American colonies and became embedded in weather reporting, cooking and everyday speech before metrication efforts began. The US did formally adopt the metric system as preferred in 1975, but the change was voluntary, and everyday temperature stayed where it was.

Should there be a space before °C?

The international standard writes 20 °C, with a space between the number and the unit and the degree sign bound to the letter. Common American style closes the gap and writes 20°C. Both are widely accepted. What is not correct is 20° C, with the space between the degree sign and the letter.

Does kelvin use a degree sign?

No. Kelvin is written as a bare K — 293 K, never 293 °K. It was redefined in 1967 as an absolute unit rather than a scale of degrees, and the degree sign went with that change. Celsius and Fahrenheit keep theirs because they remain scales measured in degrees.

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