Celsius to Fahrenheit: An Affine Conversion
Temperature conversion is affine, not a simple ratio: multiply Celsius by 1.8 and then add 32. The two scales do not share a zero, so multiplying alone would turn 0 °C into 0 °F.
Open the tool：Celsius to Fahrenheit
Weather, ovens and body temperature
A 30 °C summer day is 86 °F, which is what a friend in the US needs to hear; a -5 °C winter morning is 23 °F, nine degrees below the 32 °F freezing point. Each Celsius degree is 1.8 Fahrenheit degrees, but the scales start 32 degrees apart.
English recipes call for 350 °F, 375 °F or 425 °F, which are about 176.7 °C, 190.6 °C and 218.3 °C. Home ovens usually offer 175, 180, 190 or 200, so pick the nearest; body temperature of 37 °C is 98.6 °F, the familiar normal reading.
- Share the weather with friends using Fahrenheit
- Set an oven from an English recipe
- Convert body or water temperature
The affine formula and its order
The formula is °F = °C × 1.8 + 32. The 1.8 comes from 100 Celsius degrees spanning 180 Fahrenheit degrees, and the 32 from water freezing at 32 °F. Multiply first, then add; reversing the order gives a completely different answer.
The scales coincide only at -40: -40 °C = -40 °F. The tool accepts decimals and negatives and shows the result in the usual decimal form. Temperature differences use a different rule: a 10 °C gap is an 18 °F gap, with no 32 added.
- °F = °C × 1.8 + 32, multiply then add
- -40 °C = -40 °F, the only crossing point
- For a difference, multiply by 1.8 only
Temperature ratios do not work
Saying 20 °C is twice as hot as 10 °C has no physical meaning, because Celsius zero is an arbitrary water-ice point rather than absolute zero. Scientific work uses kelvin, where 0 K = -273.15 °C, and only then do ratios make sense.
The tool converts the number only. It does not account for wind chill, humidity, apparent temperature or the real gradient inside an oven. A 180 °C setting does not mean every spot is 180 °C, so use an oven thermometer for baking.
- Neither scale is absolute, so ratios are meaningless
- Science uses kelvin: 0 K = -273.15 °C
- Apparent and oven temperatures need measurement
FAQ
How do you convert Celsius to Fahrenheit?
°F = °C × 1.8 + 32. The formula follows from the two fixed points: water freezes at 0 °C / 32 °F and boils at 100 °C / 212 °F, which gives both the 1.8 factor and the 32 offset. So 37 °C reads 98.6 °F.
Why can't you just multiply by 1.8?
Because the scales do not share a zero. Celsius puts water's freezing point at 0, while Fahrenheit placed 0 at the temperature of an ice-and-salt mixture, which pushes water's freezing point to 32. Multiplying alone would turn 0 °C into 0 °F.
Do the scales meet at -40?
Yes. -40 °C equals -40 °F, the only point where the two numbers agree. Check it with the formula: -40 × 1.8 + 32 = -72 + 32 = -40. Above or below that point, the numbers diverge again.
Related tool：Celsius to Fahrenheit