Celsius to Fahrenheit: Formula, Quick Chart and Mental Math
Convert Celsius to Fahrenheit and back with the exact formula, a quick chart for weather, oven and body temperatures, and a shortcut you can do in your head.
You find a recipe that says “bake at 180 °C”, but your oven dial only shows Fahrenheit. Or a friend overseas says it will be 30 degrees tomorrow, and you can’t tell if that means a sweater or shorts. Celsius and Fahrenheit both measure temperature, but they use different numbers for the same heat, so converting between them comes up more often than you might expect.
This guide gives you the two exact formulas, worked examples, a quick chart with every number calculated to one decimal place, and a mental-math shortcut, along with how far off that shortcut can be. If you’d rather skip the arithmetic, the Deftivo unit converter does it right in your browser.
Where each scale is used
Celsius (°C) sets water’s freezing point at 0 and its boiling point at 100, at normal sea-level pressure. Most of the world uses it for weather forecasts, cooking and medicine, and it is the standard in science.
Fahrenheit (°F) puts freezing at 32 and boiling at 212. It is still the everyday scale in the United States and a few other places, so it turns up in American recipes, weather reports, thermostats and oven dials.
You’ll most often need to convert in three situations:
- Weather: reading a forecast for a country that uses the other scale.
- Oven temperatures: following a recipe written for a different market.
- Body temperature: reading a thermometer or medical advice that uses the scale you aren’t used to.
The exact formulas
Celsius to Fahrenheit:
°F = °C × 9/5 + 32
Fahrenheit to Celsius:
°C = (°F − 32) × 5/9
Since 9/5 is the same as 1.8, you can also write the first formula as °F = °C × 1.8 + 32. The order matters. Going to Fahrenheit, you multiply first and add 32 afterward. Going to Celsius, you subtract 32 first and multiply afterward.
Worked example: 25 °C to Fahrenheit
- Multiply by 9/5: 25 × 9 = 225, and 225 ÷ 5 = 45.
- Add 32: 45 + 32 = 77.
So 25 °C is 77 °F, a warm, pleasant day.
Worked example: 350 °F to Celsius
- Subtract 32: 350 − 32 = 318.
- Multiply by 5/9: 318 × 5 = 1,590, and 1,590 ÷ 9 = 176.67, about 176.7.
So 350 °F is about 176.7 °C. Recipes usually round that to 175 or 180 °C, and either is fine for most baking.
Worked example: 98.6 °F to Celsius
- Subtract 32: 98.6 − 32 = 66.6.
- Multiply by 5/9: 66.6 × 5 ÷ 9 = 37.
So 98.6 °F is exactly 37 °C, the figure traditionally quoted as normal body temperature. Normal temperature varies a little from person to person and over the day, so treat it as a reference point rather than a strict rule.
The “double it and add 30” shortcut
For a quick estimate from Celsius to Fahrenheit, double the number and add 30. It works because doubling is close to multiplying by 1.8, and 30 is close to 32.
The catch is that the error grows as the temperature moves away from 10 °C. The shortcut’s error works out to 0.2 × °C − 2 degrees Fahrenheit. It is exact at 10 °C, a little low below that, and increasingly high above it.
| °C | Exact °F | Shortcut (×2 + 30) | Error |
|---|---|---|---|
| −10 | 14.0 | 10 | −4.0 |
| 0 | 32.0 | 30 | −2.0 |
| 10 | 50.0 | 50 | 0.0 |
| 20 | 68.0 | 70 | +2.0 |
| 30 | 86.0 | 90 | +4.0 |
| 37 | 98.6 | 104 | +5.4 |
| 100 | 212.0 | 230 | +18.0 |
The shortcut works well for weather, where a couple of degrees rarely matters. It is a poor fit for body temperature and oven settings: being off by 5.4 °F at body temperature could make a normal reading look like a fever.
A sharper mental trick: double the Celsius number, subtract one tenth of the result, then add 32. For 25 °C that’s 50, minus 5 is 45, plus 32 is 77. Because doubling and then removing a tenth is the same as multiplying by 1.8, this version is exact.
Going the other way, the rough version is “subtract 30 and halve.” It’s fine for weather but drifts badly at oven temperatures: 350 °F comes out as 160, while the true answer is 176.7.
Quick conversion chart
Every value below is calculated exactly and rounded to one decimal place.
Everyday and body temperatures
| °C | °F | What it feels like |
|---|---|---|
| −10 | 14.0 | Bitterly cold winter day |
| 0 | 32.0 | Water freezes |
| 10 | 50.0 | Cool, jacket weather |
| 20 | 68.0 | Mild room temperature |
| 25 | 77.0 | Warm and pleasant |
| 30 | 86.0 | Hot summer day |
| 37 | 98.6 | Typical body temperature |
| 38 | 100.4 | Often used as a fever threshold |
| 100 | 212.0 | Water boils (at sea level) |
Oven temperatures
| °C | °F |
|---|---|
| 150 | 302.0 |
| 160 | 320.0 |
| 170 | 338.0 |
| 180 | 356.0 |
| 190 | 374.0 |
| 200 | 392.0 |
| 210 | 410.0 |
| 220 | 428.0 |
| 230 | 446.0 |
Common Fahrenheit values in Celsius
| °F | °C |
|---|---|
| 0 | −17.8 |
| 32 | 0.0 |
| 70 | 21.1 |
| 98.6 | 37.0 |
| 100 | 37.8 |
| 325 | 162.8 |
| 350 | 176.7 |
| 375 | 190.6 |
| 400 | 204.4 |
| 425 | 218.3 |
| 450 | 232.2 |
A quick word on Kelvin
Kelvin (K) is the scientific scale. It starts at absolute zero, the coldest temperature possible, and its steps are the same size as Celsius degrees. To convert, add 273.15 to the Celsius value: 0 °C is 273.15 K and 37 °C is 310.15 K. Kelvin is written without a degree sign, and you won’t see it on weather apps or ovens, but it appears in science classes and on some lighting and camera specs.
How to convert with Deftivo
The unit converter runs entirely in your browser, and it shows Celsius, Fahrenheit and Kelvin together.
- Open the unit converter and tap Temperature in the row of categories at the top.
- In the unit menu next to the number box, choose the scale you’re starting from: Celsius (°C), Fahrenheit (°F) or Kelvin (K).
- Type your number into the box. Negative numbers and decimals both work.
- Read the results in the table below. All three scales update instantly as you type, and the row for your starting unit is highlighted.
No button press is needed. To convert another temperature, just change the number or pick a different starting unit.
Tips and pitfalls
- Watch the order of operations. The most common mistake is adding 32 before multiplying, or multiplying before subtracting 32. Both give wrong answers.
- Don’t use the shortcut for fevers or baking. The rough version drifts by several degrees at body temperature and by much more at oven temperatures.
- Check your oven type. Recipes are often written for conventional ovens. Fan (convection) ovens usually need a lower setting, so follow the recipe’s own guidance on that.
- Round sensibly. Oven dials usually move in steps of 5 or 10 degrees, so 176.7 °C becomes 175 or 180. For body temperature, keep the decimal.
- Remember the crossover point. At −40, Celsius and Fahrenheit give the same number, which is a handy way to check your formula.
- Say which scale you mean. “It’s 30 out” means very different weather in each scale, so write the °C or °F.
FAQ
What is the easiest way to convert Celsius to Fahrenheit in my head?
Double the Celsius number and add 30 for a quick estimate. For an exact answer, double it, subtract a tenth of that result, then add 32. The second method takes a few seconds longer but works for any temperature.
Is 38 °C a fever?
38 °C equals 100.4 °F, and that figure is commonly used as the point where a temperature counts as a fever. Normal temperature varies between people and with how it’s measured, so if you’re concerned about someone’s health, follow advice from a medical professional.
What is 180 °C in Fahrenheit for baking?
180 °C is exactly 356 °F. Most American recipes use 350 °F for the same job, which is about 176.7 °C, and the small difference rarely matters for everyday baking.
Why do Celsius and Fahrenheit meet at −40?
The two scales grow at different rates, 9 Fahrenheit degrees for every 5 Celsius degrees, so they have to cross at some point. Solving °C × 9/5 + 32 = °C gives −40, which means −40 °C and −40 °F are the same temperature.