How to Convert Celsius to Fahrenheit (And Vice Versa)

The Temperature Tug-of-War

Unlike length or weight, temperature doesn't start at zero in both systems (except for absolute zero). This makes temperature the most confusing unit to convert. In Celsius, water freezes at 0° and boils at 100°. In Fahrenheit, water freezes at 32° and boils at 212°.

Whether you're checking the weather for a vacation to London or trying to set an oven for a French recipe, here is how to handle the heat.

The Exact Science: The Formula

To get a 100% accurate conversion, you need these two formulas:

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

Fahrenheit to Celsius: (Fahrenheit - 32) × 5/9 = Celsius

The "Quick Hack" for Travelers

If you're in a car in Canada and the sign says 20°C, you probably don't want to do "20 times 1.8 plus 32" in your head. The Hack: Double it and add 30. - 20°C doubled = 40. - 40 + 30 = 70°F (The actual answer is 68°F). This hack is usually within 2-3 degrees—perfect for deciding if you need a jacket.

Key Temperature Benchmarks

Description	Celsius (°C)	Fahrenheit (°F)
Freezing Water	0°	32°
Room Temperature	20°	68°
Human Body	37°	98.6°
Hot Summer Day	30°	86°
Boiling Water	100°	212°

Why Does Fahrenheit Start at 32?

Daniel Gabriel Fahrenheit, the inventor of the mercury thermometer, wanted a scale where 0° was the freezing point of a specific brine solution (ice, water, and ammonium chloride) and 100° was roughly human body temperature. By the time the scale was refined, the freezing point of pure water landed at 32.

Anders Celsius, on the other hand, created a scale based entirely on the properties of water at sea level. Interestingly, his original scale had 0 as boiling and 100 as freezing! It was reversed after his death to the system we use today.

FAQ: Temperature

Is -40 the same in both?

Yes! -40°C is exactly equal to -40°F. This is the only point where the two scales intersect.

What is Kelvin?

Kelvin is the primary unit of temperature in the physical sciences. It starts at "Absolute Zero" (the point where all molecular motion stops). 0 Kelvin is -273.15°C.

Why does the U.S. use Fahrenheit?

Like the rest of the Imperial system, it’s a matter of infrastructure and cultural inertia. Fahrenheit is arguably more precise for weather because a 1-degree change in Fahrenheit is smaller and more "feelable" to a human than a 1-degree change in Celsius.