Celsius to Fahrenheit: Formula, Shortcuts and a Chart
Temperature scales have different zero points, so conversion needs an offset as well as a factor. Here is the formula and the shortcut.
26 January 2026 · 6 min read · Temperature
Every other everyday conversion is a multiplication. Temperature is not, because Celsius and Fahrenheit disagree about where zero is.
The formulas
°F = (°C × 9/5) + 32 °C = (°F − 32) × 5/9
The 9/5 handles the different degree size; the 32 handles the different zero point.
The mental shortcut
Double it and add 30. For 20 °C: 40 + 30 = 70 °F (true: 68). The error grows at extremes but stays usable across normal weather.
A more accurate variant: double, subtract 10%, add 32. For 20 °C: 40 − 4 + 32 = 68 °F, exactly right.
The anchor points worth memorising
| Celsius | Fahrenheit | Why it matters |
|---|---|---|
| −40 °C | −40 °F | The scales cross here |
| 0 °C | 32 °F | Water freezes |
| 10 °C | 50 °F | Jacket weather |
| 20 °C | 68 °F | Room temperature |
| 37 °C | 98.6 °F | Body temperature |
| 100 °C | 212 °F | Water boils at sea level |
| 180 °C | 356 °F | A moderate oven |
Kelvin and absolute zero
Scientific work uses kelvin, which has the same degree size as Celsius but starts at absolute zero: K = °C + 273.15. Note that there is no degree symbol — it is "300 kelvin", not "300 °K". The Celsius to Kelvin converter does the arithmetic.
Rankine does the same trick for Fahrenheit: °R = °F + 459.67. It still appears in US thermodynamics texts.
A difference is not a temperature
This trips up engineers. A temperature difference of 10 °C equals a difference of 18 °F — you apply the 9/5 factor but not the +32 offset. Convert absolute temperatures with the full formula; convert differences with the ratio only.
Related conversions
This article is for general information. Please verify figures against the relevant standard before using them in critical work. About accuracy