How to Convert Celsius to Fahrenheit: Formula, Tables and Oven Settings

To convert Celsius to Fahrenheit, multiply by 9/5 (that is, 1.8) and add 32. So 20 °C is 20 × 1.8 + 32 = 68 °F. To go back, subtract 32 and multiply by 5/9. Both formulas are exact, and this guide shows where they come from, a full conversion table, the oven settings US recipes actually use, and why the popular "double it and add 30" shortcut is fine for weather but wrong by 34 °F at a typical baking temperature.

The formulas

• Celsius to Fahrenheit: °F = °C × 9/5 + 32
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Celsius to kelvin: K = °C + 273.15

Two numbers do the work. The 9/5 exists because the two scales split the same physical range into different numbers of steps: between the freezing and boiling points of water there are 100 Celsius degrees and 180 Fahrenheit degrees, and 180/100 reduces to 9/5. The 32 is an offset: water freezes at 0 °C but at 32 °F, so after scaling you shift the zero.

The order matters. Going to Fahrenheit you multiply first, then add. Going back you subtract first, then multiply. Mixing them up is the most common mistake: (20 + 32) × 1.8 gives 93.6, not 68.

A handy symmetric version uses the point where both scales agree, −40: add 40, multiply by 1.8 (or by 5/9 going the other way), then subtract 40. For 180 °C that is (180 + 40) × 1.8 − 40 = 356 °F, the same answer as the standard formula.

Converting a change in temperature

A temperature difference converts without the 32. A rise of 10 °C is a rise of 18 °F, because only the size of the degree differs. If a forecast says tomorrow will be 5 °C warmer, that is 9 °F warmer, not 41 °F. The same rule makes the kelvin easy: one kelvin is exactly the same size as one Celsius degree, so a 10 K change is a 10 °C change.

Worked examples

1. A European oven recipe

A recipe says to bake at 180 °C and your oven only shows Fahrenheit:

1. 180 × 9/5 = 324
2. 324 + 32 = 356 °F
3. US ovens and recipes work in 25 °F steps, so set the dial to 350 °F, the conventional equivalent. The 6 °F difference is a rounding convention, and recipes are written with it in mind.

2. A US weather forecast

The forecast says 72 °F. In Celsius:

1. 72 − 32 = 40
2. 40 × 5/9 = 22.2 °C, a mild day.

3. A thermometer reading

A digital thermometer shows 38 °C and you want the Fahrenheit figure:

1. 38 × 1.8 = 68.4
2. 68.4 + 32 = 100.4 °F

38 °C (100.4 °F) is the figure the NHS uses: a high temperature in adults is "usually considered to be 38C or above". The NHS also notes you can have a high temperature below that reading if you feel hot, cold or shivery, which is why a single conversion is no substitute for medical guidance. The 38 °C threshold is for adults; for babies and children, follow the NHS children's page or your doctor. In the US, the CDC also uses a measured 100.4 °F (38 °C) or higher as a fever.

Celsius to Fahrenheit table

Every value below was computed with the exact formula. Results are exact to one decimal place.

And the reverse, for readers working from Fahrenheit:

Oven temperatures

This is where most people actually need the conversion, and where exact answers can mislead. Recipes do not use exact equivalents: they use round settings in steps of 25 °F, 10 °C or one gas mark. The table pairs each common Celsius setting with its exact value and the setting a US recipe would print. The gas marks follow the usual UK chart, in which each mark is 25 °F hotter than the one before.

Gas mark charts vary slightly: some UK charts give 170 °C rather than 160 °C for gas mark 3.

Two practical notes. First, many European recipes give a separate, lower setting for fan (convection) ovens; check which of the two you are converting. Second, food-safety temperatures are minimum internal temperatures, measured with a food thermometer in the thickest part of the food, not oven settings. The US government's FoodSafety.gov chart lists at least 165 °F (74 °C) for poultry, casseroles and leftovers, 160 °F (71 °C) for ground meats such as beef and pork, and 145 °F (63 °C) plus a 3-minute rest for whole cuts of beef, pork, veal and lamb. Use a food thermometer for these, and use the published pair rather than converting yourself.

The "double it and add 30" shortcut

Doubling the Celsius figure and adding 30 is the classic mental trick. It is exact at 10 °C (both give 50 °F) and drifts by 2 °F for every 10 °C you move away from that point.

For weather between about −10 °C and 30 °C the error stays within 4 °F, which is fine for deciding on a jacket. For an oven it is not: setting 390 °F for a 180 °C cake is more than a full gas mark too hot. For body temperatures the shortcut also misleads, turning 38 °C into 106 °F instead of 100.4 °F.

Where the two scales came from

Anders Celsius proposed his scale in 1742 with the numbers the other way round: 0 for boiling water and 100 for freezing. It was inverted within a few years, and in 1948 the unit was officially renamed from "centigrade" to "degree Celsius". Today the degree Celsius is defined through the kelvin, the SI base unit of temperature: a Celsius temperature is the kelvin temperature minus exactly 273.15.

Fahrenheit is now used as the main scale mainly in the United States and its territories, plus Palau, the Federated States of Micronesia, the Marshall Islands and the Cayman Islands. A few countries, including the Bahamas and Belize, use it alongside Celsius. The United Kingdom, which uses imperial units for some things, uses Celsius for weather and most official purposes; the US system is called US customary, not imperial. For a fuller picture of who uses which units, see our guide to metric vs imperial around the world.

Using the SamemaS temperature converter

The temperature converter has four fields: Celsius, Fahrenheit, kelvin and Rankine (the absolute scale that uses Fahrenheit-sized degrees). Type into any one and the other three update, using the exact formulas above and showing up to ten significant digits. The calculation runs in your browser. It gives exact values, so for an oven you still round to the nearest recipe setting yourself. For volume and weight in recipes, the cooking measurement converter and our guide to US cups to grams cover the rest.

The short version

• °F = °C × 1.8 + 32. °C = (°F − 32) ÷ 1.8. Multiply before adding; subtract before dividing.
• A change of 1 °C is a change of 1.8 °F, with no 32.
• −40 is the same on both scales; 0 °C is 32 °F and 100 °C is 212 °F.
• For ovens, convert exactly and then use the recipe step: 180 °C → 350 °F, 200 °C → 400 °F.
• "Double and add 30" is fine for weather, wrong for ovens and thermometers.
• For fever, go by the official guidance, which the NHS puts at 38 °C (100.4 °F) or above in adults.

Frequently asked questions

Sources

1. Fahrenheit — Wikipedia
2. Celsius — Wikipedia
3. High temperature (fever) in adults — NHS
4. Human body temperature has decreased in United States, study finds — Stanford Medicine
5. Safe Minimum Internal Temperatures chart — FoodSafety.gov
6. Gas mark — Wikipedia