Maths📅 24 March 2025⏱ 4 min read
How to Convert Between Celsius, Fahrenheit and Kelvin
Temperature conversion trips people up constantly. Here are the exact formulas, quick mental maths shortcuts, and a reference table for the most common values.
JW
James WhitfieldPersonal Finance & Maths WriterJames has written about personal finance, health metrics, and everyday mathematics for over six years. He holds a BSc in Mathematics from the University of Leeds.
Whether you're baking from an American recipe, reading a scientific paper, or just confused by a weather forecast, temperature conversion is a skill worth having locked in. Here are all three scales and how to move between them.
The Three Scales
Celsius (°C) is the international standard used in science and most of the world. Water freezes at 0°C and boils at 100°C.
Fahrenheit (°F) is used primarily in the United States. Water freezes at 32°F and boils at 212°F.
Kelvin (K) is the scientific absolute temperature scale. 0K is absolute zero — the theoretical temperature at which all molecular motion stops. There are no negative Kelvin values. Water freezes at 273.15K.
The Conversion Formulas
Celsius to Fahrenheit:
°F = (°C × 9/5) + 32
Fahrenheit to Celsius:
°C = (°F − 32) × 5/9
Celsius to Kelvin:
K = °C + 273.15
Kelvin to Celsius:
°C = K − 273.15
Fahrenheit to Kelvin:
K = (°F − 32) × 5/9 + 273.15
Quick Mental Maths Shortcuts
The exact formulas require a calculator. These shortcuts are close enough for everyday use:
- °C to °F (rough): Double the Celsius, then add 30. (Exact answer is multiply by 1.8 and add 32, but doubling and adding 30 is close enough for weather.)
- °F to °C (rough): Subtract 30, then halve it.
- Key anchor points to memorise: -40 is the same in both scales. 16°C ≈ 61°F. 28°C ≈ 82°F.
Essential Reference Table
SituationCelsiusFahrenheit
Absolute zero−273.15°C−459.67°F
Water freezes0°C32°F
Cold winter day−5°C23°F
Cool room18°C64°F
Comfortable room21°C70°F
Warm summer day28°C82°F
Hot day35°C95°F
Body temperature37°C98.6°F
Fever38.5°C101.3°F
Oven: gas mark 4180°C356°F
Oven: gas mark 7220°C428°F
Water boils (sea level)100°C212°F
Cooking Temperature Conversions
American recipes use Fahrenheit oven temperatures. Here are the most common, including fan vs conventional adjustments:
FahrenheitCelsius (Conventional)Celsius (Fan)
300°F150°C130°C
325°F160°C140°C
350°F175°C155°C
375°F190°C170°C
400°F200°C180°C
425°F220°C200°C
450°F230°C210°C
Fan-assisted ovens circulate air and cook more efficiently. The standard adjustment is to reduce the temperature by 20°C (about 25°F) compared to a conventional oven setting.