Celsius to Fahrenheit in Your Head (and When Not To)

Fahrenheit and Celsius disagree about two things — where zero sits and how big a degree is. The formula handles both at once, and a two-step shortcut gets you close enough for weather.

ConversionBy Jun 14, 20265 min read
Celsius to Fahrenheit in Your Head (and When Not To) — ListCalc

The exact formula

°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9

The 9/5 (= 1.8) rescales the degree size — a Fahrenheit degree is smaller, so it takes 180 of them to cover the 100°C between freezing and boiling. The +32 shifts the zero point, because water freezes at 32°F. Both scales famously meet at one temperature: −40° is the same in each.

Temperatures worth memorizing

°C°FWhat it is
−40−40The crossover point
032Water freezes
1050Jacket weather
2068Room temperature
2577Warm day
3086Hot day
3798.6Body temperature
40104Heat wave
100212Water boils (at sea level)

The double-and-add-30 shortcut

For weather, skip the 1.8 and use: double it, add 30. 20°C → 40 + 30 = 70°F (exact: 68). 10°C → 50°F (exact: 50 — dead on). The shortcut is exact at 10°C and drifts by 2° for every 10°C away from there: at 30°C it says 90 against a true 86, and at oven temperatures it is useless. Weather: fine. Cooking and anything medical: use the real formula or the Celsius to Fahrenheit converter.

Oven temperatures

Recipes from metric countries give oven settings in Celsius, and the gap to Fahrenheit is big enough to ruin dinner:

°C°FTypical use
160320Slow bake
180356 (set 350)Standard bake — cakes, cookies
200392 (set 400)Roasting vegetables
220428 (set 425)Pizza, crusty bread
250482 (set 475–500)Maximum heat

Home ovens only offer 25°F steps, so the “set” values are the accepted equivalents — 180°C and 350°F is the classic pairing, off by only 6°F.

Liters to gallons: the travel companion

The other conversion travelers hit at the pump. A US gallon is 3.78541 liters:

US gallons = liters ÷ 3.78541
liters = US gallons × 3.78541
LitersUS gallons
10.264
102.64
40 (small tank)10.57
50 (typical tank)13.21
60 (large tank)15.85

Watch for the imperial gallon: the UK gallon is 4.54609 liters, about 20% bigger than the US one, which is one reason UK “miles per gallon” figures always look better than American ones for the same car. The liters to gallons converter handles both, and the general temperature converter adds Kelvin when you need it.

Run your own numbers

Celsius to Fahrenheit Liters to gallons

FAQ

What is the formula to convert Celsius to Fahrenheit?
Multiply by 9/5 (that is, by 1.8) and add 32. So 25°C is 25 × 1.8 + 32 = 77°F. To go the other way, subtract 32 first and then multiply by 5/9.
What is 180°C in Fahrenheit for baking?
356°F exactly — in practice you set the oven to 350°F, the nearest standard mark. The 6-degree difference is well within a home oven's normal temperature swing.
How accurate is the double-and-add-30 trick?
It is exact at 10°C (50°F) and drifts about 2°F for every 10°C away from that point. At 30°C it gives 90°F against a true 86°F — fine for weather talk, not for cooking or medicine.
At what temperature are Celsius and Fahrenheit equal?
At −40 degrees. Setting C = F in the formula gives F = 1.8F + 32, which solves to F = −40.
How many liters are in a US gallon?
3.78541 liters. The imperial (UK) gallon is larger at 4.54609 liters — about 20% more — so always check which gallon a figure uses.

Sources

Primary references used for the figures and rules on this page.

  1. Temperature scales and conversions — NIST
  2. Units of volume: gallon definitions — NIST Handbook 44