Celsius to Fahrenheit:
the formula, a chart and a mental shortcut

Celsius to Fahrenheit formula

To convert a temperature from Celsius to Fahrenheit, multiply by 9/5 (which is 1.8) and then add 32. The order matters: the multiplication comes first.

The factor 1.8 comes from the two scales' reference points. Between freezing and boiling water there are 100 Celsius degrees and 180 Fahrenheit degrees, so each Celsius degree is 1.8 Fahrenheit degrees. The 32 is the offset, because Fahrenheit's zero sits below the freezing point of water.

1. 1Write down the temperature in °C, including any minus sign.
2. 2Multiply it by 1.8. If you prefer fractions, multiply by 9 and divide by 5.
3. 3Add 32 to the result.
4. 4Label the answer °F and round to the precision you need.

Most of the world uses Celsius for weather, cooking and medicine, while Fahrenheit remains the everyday scale in the United States. You will meet both when you follow a foreign recipe, read a product manual or travel.

Example: converting 37°C

A second case, with a negative value: −10°C × 1.8 = −18, and −18 + 32 = 14, so −10°C is 14°F. Keep the sign through the multiplication and only then add 32.

One more with a decimal: 21.5°C × 1.8 = 38.7, and 38.7 + 32 = 70.7°F. Decimals do not change the method. Multiply, add, then round at the end to one decimal place for weather and room temperature.

Celsius to Fahrenheit chart (−10°C to 100°C)

Every 10°C step adds exactly 18°F. That is a useful pattern to remember: 20°C is 68°F, 30°C is 86°F, and 40°C is 104°F.

To use the chart for a value that is not listed, find the nearest rows either side and add 1.8°F for each extra degree Celsius. For 27°C, start from 25°C = 77°F and add 2 × 1.8 = 3.6, which gives 80.6°F.

A mental shortcut that stays close

You can estimate Fahrenheit without a calculator: double the Celsius value, subtract 10% of that doubled figure, then add 32. This is the same as multiplying by 1.8.

For 25°C: double to 50, subtract 10% (5) to get 45, add 32 to get 77°F. That is exact. A cruder shortcut of "double and add 30" gives 80°F, which is 3°F too high, and the error grows the hotter it gets.

At 40°C the same shortcut gives 110°F against an exact 104°F, which is 6°F out. The gap widens because the shortcut treats 1.8 as 2 and 32 as 30, and both approximations pull in the same direction once the temperature is above about 10°C.

Body temperature and thermometer readings

Thermometers sold in India usually show Celsius, while foreign medical notes and some imported devices use Fahrenheit. A reading of 38°C is 38 × 1.8 + 32 = 100.4°F, and 39°C is 102.2°F.

• 36.5°C is 97.7°F.
• 37.5°C is 99.5°F.
• 38°C is 100.4°F.
• 39°C is 102.2°F.
• 40°C is 104°F.

This is general information, not medical advice. What counts as a fever depends on age, the way the temperature was taken and the guidance of your doctor, so speak to a qualified professional about any concerning reading.

A Fahrenheit degree is smaller than a Celsius degree: 1°F is about 0.56°C. That is why whole-degree Fahrenheit readings look finer, and why a converted value often needs a decimal place in Celsius to carry the same information.

Oven temperatures and weather

Recipes from different countries use different scales. An oven set to 180°C is 356°F, and 200°C is 392°F. Recipes often round these to 350°F and 400°F, so a small difference between the two is normal.

In the opposite direction, a recipe that says 350°F is (350 − 32) × 5/9 = 176.7°C, usually set as 175°C or 180°C. The Fahrenheit to Celsius formula is °C = (°F − 32) × 5/9.

• 32°F is 0°C.
• 50°F is 10°C.
• 68°F is 20°C.
• 86°F is 30°C.
• 104°F is 40°C.

A day at 24°C is 75.2°F, and one at 45°C is 113°F. For comparisons, a 10°C rise is an 18°F rise, so the temperature difference does not convert with the +32 step.

Fridge and freezer settings are a common reason to convert. A fridge at 4°C is 39.2°F, and a freezer at −18°C is −0.4°F, which is why imported freezers often show a setting of about 0°F.

Mistakes in temperature conversion

• Adding 32 before multiplying. For 25°C this gives (25 + 32) × 1.8 = 102.6°F instead of 77°F.
• Using 5/9 in place of 9/5. That formula goes the other way, from Fahrenheit to Celsius.
• Dropping the minus sign. −10°C is 14°F, not 50°F.
• Adding 32 to a temperature difference. A rise of 10°C is a rise of 18°F, and no 32 is added.
• Rounding too early. 36.7°C is 98.06°F, and rounding to 98°F hides a difference that a thermometer can show.

Where does the −40 come from? It is the one temperature where the two scales agree: −40°C = −40°F, because −40 × 1.8 + 32 = −40. It is a handy anchor: a temperature of −30°C is 10 degrees above −40 on the Celsius scale and 18 above it on the Fahrenheit scale, which gives −22°F.

For length conversions that come up alongside weather and travel, the foot-to-metre converter on this page handles feet and metres.