How to Convert Between Celsius, Fahrenheit, and Kelvin
Temperature is one of the few measurements where the three major scales — Celsius, Fahrenheit, and Kelvin — are all in common use today. I find myself reaching for a temperature converter more than almost any other unit tool: whether I'm following a recipe from a US cookbook (Fahrenheit), checking a science textbook (Kelvin), or reading a weather forecast almost anywhere in the world (Celsius). I built this converter so the math is done instantly, without having to remember which formula to use.
The Conversion Formulas
Unlike most unit conversions — where you just multiply by a fixed factor — temperature conversions involve both multiplication and addition because the zero points of the scales are different. Here are the exact 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 + 459.67) × 5/9
Notice that Kelvin uses no degree symbol — it is an absolute scale with no negative values, starting at absolute zero. Celsius and Fahrenheit both allow negative temperatures, which is why you must add or subtract an offset when converting.
Frequently Asked Questions
What is absolute zero?
Absolute zero is the lowest theoretically possible temperature — the point at which all molecular motion stops. It is defined as 0 Kelvin, which equals −273.15 °C or −459.67 °F. Scientists use the Kelvin scale in physics and chemistry precisely because it starts at this natural zero point, making equations simpler to work with.
Why does the United States still use Fahrenheit?
The Fahrenheit scale was the dominant standard in the English-speaking world for most of the 18th and 19th centuries. When the metric system became globally adopted, the US did not mandate the switch for everyday use. As a result, Fahrenheit remains common in weather reporting and cooking in the United States, while the rest of the world uses Celsius.
What are some useful reference temperatures to memorise?
A few anchor points make mental conversions much easier. Water freezes at 0 °C / 32 °F and boils at 100 °C / 212 °F. A comfortable room temperature is roughly 20 °C / 68 °F, and a healthy human body temperature is around 37 °C / 98.6 °F.