How to Convert Celsius to Fahrenheit
Celsius and Fahrenheit measure the same heat with different numbers. Converting between them takes one short formula. Here it is, with a worked example, the reverse direction, and a mental shortcut for quick estimates.
The short version: multiply the Celsius reading by 9/5, then add 32. The multiply step rescales the degree size, and the add step lines up the two starting points.
The Celsius to Fahrenheit formula
One line covers it. The 9/5 is the same as 1.8, so you can use whichever is easier.
°F = (°C × 9 / 5) + 32
Why two steps? A Fahrenheit degree is smaller than a Celsius degree, so the 9/5 stretches the scale. And the two scales start counting from different points, so the +32 shifts the result to match. Skip either step and the answer drifts off.
A worked example, step by step
Say it is 25 °C, a warm spring day, and you want the Fahrenheit reading.
- Multiply: 25 × 9 / 5 = 45.
- Add 32: 45 + 32 = 77.
So 25 °C is 77 °F. The same steps work for any reading. A hot 200 °C oven, for instance, is 200 × 9 / 5 = 360, plus 32, which is 392 °F.
Going the other way
To convert Fahrenheit back to Celsius, reverse the steps. Subtract 32 first, then multiply by 5/9.
°C = (°F − 32) × 5 / 9
Say a thermostat reads 50 °F. Subtract: 50 − 32 = 18. Then multiply: 18 × 5 / 9 = 10. So 50 °F is 10 °C. The order flips, since you undo the add before you undo the multiply.
A quick mental shortcut
When you only need a ballpark, double the Celsius figure and add 30. For 25 °C that gives 25 × 2 + 30 = 80, close to the exact 77. The trick runs a little high, but it is fine for deciding what to wear.
The gap grows at the extremes, so use the full formula for cooking, science, or anything precise. For everyday weather, the shortcut gets you within a few degrees in your head.
Why the world uses two scales
Celsius is the everyday scale across most of the world and the standard for weather outside the United States. It anchors 0 at the freezing point of water and 100 at the boiling point, which makes round numbers easy to picture.
Fahrenheit stays the household scale in the United States, with freezing at 32 and boiling at 212. Its smaller degrees give finer steps without decimals, which some people like for weather. Neither scale is more correct. They just count from different points, which is exactly why the conversion needs both a multiply and an add.
Handy reference points
- Water freezes at 0 °C, which is 32 °F.
- A mild 10 °C is 50 °F.
- A warm 25 °C is 77 °F.
- Water boils at 100 °C, which is 212 °F.
Notice the two scales meet at −40, where −40 °C equals −40 °F. It is the one reading that is the same on both, a handy fact to remember.
Converting oven temperatures
Recipes are a frequent reason to convert, since cookbooks use whichever scale their country favors. The same formula handles it. A hot 220 °C oven is 220 × 9 / 5 = 396, plus 32, which is 428 °F. A gentler 160 °C is 160 × 9 / 5 = 288, plus 32, or 320 °F.
Cooks often round to a familiar mark, so 428 °F may appear as 425 °F on a dial. The few degrees rarely change the result, but for baking, where precision counts more, convert exactly and then set the nearest available number.
Common mistakes to avoid
- Forgetting the +32. Multiplying alone gives the wrong starting point.
- Adding 32 before multiplying. Multiply by 9/5 first, then add.
- Reversing the steps incorrectly. To go back, subtract 32 first, then multiply by 5/9.
- Using the rough shortcut for precise work. It runs high, so use the full formula when accuracy matters.
Frequently asked questions
What is the exact formula? Multiply Celsius by 9/5 and add 32. To reverse it, subtract 32 from Fahrenheit and multiply by 5/9.
Is the double-and-add-30 trick accurate? It is a quick estimate, not exact. It tends to read a few degrees high, which is fine for weather but not for cooking or science.
At what temperature do the scales match? At −40. That single reading is −40 in both Celsius and Fahrenheit, since the formulas cross there.
Why do the scales start differently? Celsius sets 0 at the freezing point of water. Fahrenheit puts freezing at 32, so the formula adds 32 to bridge the gap.
Do I need to do this by hand? No. The formula is good to know, but a converter is faster and handles Kelvin too. Use the one below to switch scales in a second.