Celsius and Fahrenheit: Quick Conversions for Cooking, Weather and Fever
Most of the world measures temperature in degrees Celsius, while the United States and a few other places use Fahrenheit. You meet the difference when following a recipe from another country, reading a weather forecast while travelling, or checking a child's temperature with a thermometer set to the "wrong" scale. This guide gives the exact formulas, a quick mental method and tables for the three most common situations.
The exact formulas
°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9
The two scales use different zero points and different degree sizes. Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level, so 100 Celsius degrees span 180 Fahrenheit degrees: one Celsius degree is 1.8 Fahrenheit degrees. The scales meet at −40, where −40 °C equals −40 °F. For instant results, use the Celsius to Fahrenheit or Fahrenheit to Celsius converter.
A quick mental estimate
- Celsius to Fahrenheit: double it and add 30. 20 °C → 70 °F (exact: 68 °F).
- Fahrenheit to Celsius: subtract 30 and halve. 80 °F → 25 °C (exact: 26.7 °C).
The shortcut is close for everyday weather but drifts at high temperatures, so do not use it for ovens or medical readings.
Oven temperatures
| °C | °F (exact) | Often written as | Typical use |
|---|---|---|---|
| 150 | 302 | 300 °F | Slow baking, meringues |
| 160 | 320 | 325 °F | Rich cakes |
| 180 | 356 | 350 °F | Most cakes and biscuits |
| 190 | 374 | 375 °F | Muffins, pastry |
| 200 | 392 | 400 °F | Roasting vegetables |
| 220 | 428 | 425 °F | Bread, crispy roasting |
| 230 | 446 | 450 °F | Pizza in a home oven |
Recipes round to the nearest 25 °F, so 180 °C becomes 350 °F although the exact value is 356 °F. Fan (convection) ovens usually need about 20 °C (around 25 °F) less than conventional ovens; check whether a recipe already assumes a fan oven.
Weather
| °C | °F | How it feels |
|---|---|---|
| −10 | 14 | Very cold, frost |
| 0 | 32 | Freezing point |
| 10 | 50 | Cool, jacket weather |
| 20 | 68 | Mild, room temperature |
| 25 | 77 | Warm |
| 30 | 86 | Hot |
| 35 | 95 | Very hot |
| 40 | 104 | Extreme heat |
A handy rhyme for Celsius users: "30 is hot, 20 is nice, 10 is cool and 0 is ice."
Body temperature and fever
| °C | °F | Usual meaning (adults) |
|---|---|---|
| 36.5 | 97.7 | Normal |
| 37.0 | 98.6 | Classic "normal" average |
| 37.5 | 99.5 | Upper normal / slightly raised |
| 38.0 | 100.4 | Fever threshold commonly used |
| 39.0 | 102.2 | High fever |
| 40.0 | 104.0 | Very high fever, seek medical advice |
Normal body temperature varies between people, through the day and with the measuring site (mouth, ear, armpit or rectum). For babies and young children, follow your doctor's or health service's guidance on when a temperature needs medical attention.
Kelvin for science
Scientists use the kelvin scale, which starts at absolute zero (−273.15 °C). A kelvin step equals a Celsius degree, so K = °C + 273.15: room temperature of 20 °C is 293.15 K. See the Celsius to Kelvin converter.
Common mistakes
- Forgetting the +32 or −32. Multiplying alone gives answers that are far off.
- Converting temperature differences with the offset. A rise of 10 °C is a rise of 18 °F, not 50 °F; for differences, only multiply by 1.8.
- Using the mental shortcut for ovens or fever. Use the exact formula or the converter.