How to Convert Celsius to Fahrenheit in Your Head

If you're traveling abroad, reading an international recipe, or talking to someone across the globe, you will inevitably run into the great temperature divide: Celsius vs. Fahrenheit.

While the United States primarily uses Fahrenheit, almost the entire rest of the world uses Celsius. Getting caught between the two can be incredibly confusing. Is 30°C hot or cold? (Hint: It's very hot!).

While you can always use a tool, it helps to know a few mental math tricks to approximate the conversion on the fly.

The Official Formula

First, let's look at the actual, mathematically perfect formulas:

Celsius to Fahrenheit: (°C × 1.8) + 32 = °F
Fahrenheit to Celsius: (°F - 32) / 1.8 = °C

Multiplying and dividing by 1.8 in your head isn't easy for most people. Thankfully, there are shortcuts.

The "Times 2, Add 30" Trick (Celsius to Fahrenheit)

If someone tells you it is 20°C outside, you can get a very close Fahrenheit estimate using this simple trick: Multiply by 2, then add 30.

Take the Celsius temperature: 20°C
Multiply by 2: 20 × 2 = 40
Add 30: 40 + 30 = 70°F

The exact temperature is actually 68°F, so your mental estimate of 70°F is impressively close!

The "Subtract 30, Divide by 2" Trick (Fahrenheit to Celsius)

To go the other way, just reverse the process. If someone says it's 80°F outside: Subtract 30, then divide by 2.

Take the Fahrenheit temperature: 80°F
Subtract 30: 80 - 30 = 50
Divide by 2: 50 / 2 = 25°C

The exact temperature is 26.6°C, making your estimate of 25°C extremely helpful for figuring out what to wear.

Common Benchmarks to Memorize

Sometimes it's easier to just memorize the milestones:

0°C = 32°F (Freezing point of water)
10°C = 50°F (A brisk, cool day)
20°C = 68°F (A pleasant, mild day)
30°C = 86°F (A hot summer day)
100°C = 212°F (Boiling point of water)

Need Exact Numbers?

Mental math is great for casual conversation and figuring out whether to bring a jacket. But if you are baking a cake, conducting a science experiment, or setting a thermostat, you need precision.

For exact conversions, instantly flip between the two scales using our free Celsius to Fahrenheit Converter and Fahrenheit to Celsius Converter tools!