Celsius to Fahrenheit: Formula & Conversion Guide

The formulas

Celsius to Fahrenheit: F = C x 9/5 + 32. Fahrenheit to Celsius: C = (F - 32) x 5/9. The 9/5 (or 1.8) accounts for Fahrenheit degrees being smaller, and the 32 shifts the zero point, since the scales do not start in the same place.

Example: 20 C is 20 x 1.8 + 32 = 68 F. Going back, 68 F is (68 - 32) x 5/9 = 20 C. The Temperature Converter does both instantly.

A mental shortcut

For a quick estimate, double the Celsius value and add 30: 20 C becomes about 70 F (the exact answer is 68). It is a few degrees off at the extremes but close enough to know whether to grab a jacket. The reverse: subtract 30 and halve.

Reference points worth knowing

• 0 C = 32 F — water freezes
• 37 C = 98.6 F — normal body temperature
• 100 C = 212 F — water boils at sea level
• -40 C = -40 F — the one point where the scales meet

Why they cross at -40

Because the two scales have different sizes and different zero points, there is exactly one temperature where they read the same number: -40. Set C equal to F and solve, and -40 falls out — a neat fact, and a good check that you have the formula right.

Frequently asked questions

Which countries use which? Most of the world uses Celsius; the United States and a few others use Fahrenheit for everyday weather.

Why is body temperature an odd number in Fahrenheit? Because 37 C converts to 98.6 F.

Is there a degree sign for Kelvin? No — Kelvin is written without one, as in 300 K.