How do I convert Celsius to Fahrenheit in my head?

Double the Celsius and add 30. It is approximate but within a degree or two for everyday temperatures.

Is the double-and-add-30 method accurate?

It is a fast estimate, most accurate near 10–20°C; the exact formula is multiply by 9/5, then add 32.

Everyday math

Celsius to Fahrenheit: A Memory Trick That Works

3 min readEveryday

You land in a country where the forecast is in Celsius, or a recipe says "bake at 180," and your Fahrenheit brain freezes. The exact formula — °F = °C × 9/5 + 32 — is precise but slow. For real life you only need a faster friend: double it, then add 30.

The shortcut in action

Taking 20°C: double to 40, add 30, get 70°F. The exact answer is 68 — close enough to dress. Take 37°C (body temp): double to 74, add 30, get 104... but the real answer is 98.6. The trick drifts as numbers climb, so know where it shines and where it wanders.

Where the estimate holds up

It is most faithful in the everyday band of about 10 to 20°C (50–68°F), where the rule lands within a degree or two. That happens to cover most comfortable outdoor weather, so for "do I need a jacket" decisions it is excellent.

"Mental shortcuts trade two degrees of accuracy for ten seconds of freedom. In weather and cooking, that trade is nearly always worth it."

When to reach for the real formula

Use the exact conversion whenever the answer needs precision: an oven dial, a fever check, a laboratory reading. There the converter shows the true number across every scale instantly. Reserve the shortcut for the split-second estimates where a rounding error changes nothing about your coat.

A second anchor to hang the range on

Memorize two fixed points to sanity-check the shortcut: freezing is 0°C = 32°F and a comfortable day is 20°C ≈ 68°F. If your estimate lands far from those anchors, redo the double-add. Two reference pins plus one fast rule is all a traveler's brain needs.