Converting Temperature: Celsius, Fahrenheit, and Kelvin

Conversions · 5 min read

Temperature is measured on three main scales, and moving between them trips up travelers, students, and anyone following a recipe or weather report from another country. The conversions are simple arithmetic once you know the formulas and a few anchor points.

The three scales

Celsius is used by most of the world for everyday temperatures, with water freezing at 0 and boiling at 100 — a clean, intuitive scale. Fahrenheit is used mainly in the United States, with water freezing at 32 and boiling at 212. Kelvin is the scientific scale, starting at absolute zero (the coldest possible temperature) and using the same degree size as Celsius. Each scale suits its purpose.

Celsius to Fahrenheit and back

To convert Celsius to Fahrenheit, multiply by 9/5 and add 32. So 20 degrees Celsius is 20 × 9/5 + 32 = 68 degrees Fahrenheit. To go the other way, subtract 32 first, then multiply by 5/9. So 68 Fahrenheit is (68 − 32) × 5/9 = 20 Celsius. The order of operations matters: for Fahrenheit to Celsius you subtract before multiplying.

Working with Kelvin

Kelvin is the easiest conversion of all because it shares Celsius’s degree size. To convert Celsius to Kelvin, simply add 273.15. So 20 degrees Celsius is 293.15 Kelvin. To go back, subtract 273.15. Kelvin never uses the word "degrees" and never goes negative, because it starts at absolute zero — the point where molecular motion theoretically stops.

Reference points worth memorizing

A few anchors make mental conversion easier. Body temperature is about 37 Celsius or 98.6 Fahrenheit. A comfortable room is around 20 to 22 Celsius or about 68 to 72 Fahrenheit. Freezing is 0 Celsius or 32 Fahrenheit. And there is a neat coincidence worth remembering: −40 is the same on both the Celsius and Fahrenheit scales, the one point where they meet.

A quick estimation trick

For a rough Celsius-to-Fahrenheit estimate without exact math, double the Celsius value and add 30. For 20 Celsius, that gives about 70 Fahrenheit, close to the true 68. It is not precise, but it is good enough to know whether to pack a jacket when reading a foreign weather forecast.

The temperature converter on this site handles all three scales instantly, so you never have to remember which formula runs in which direction.