The Three Temperature Scales — Celsius, Fahrenheit and Kelvin Explained

Three scales measure temperature, each invented for different purposes. Here's what they are, why they differ, and when you'd actually use each one.

Celsius is the world's default temperature scale for everyday use. Defined by the freezing point of water (0°C) and the boiling point of water (100°C) at standard atmospheric pressure, it was designed to be intuitive for human-scale temperatures. Most of the world uses Celsius for weather, cooking and everyday conversation.

Fahrenheit was developed in 1724 by Daniel Gabriel Fahrenheit. He set 0°F as the lowest temperature he could achieve with a brine solution, and 96°F as human body temperature. The result is a scale that doesn't map to water's phase transitions as neatly, but which has the advantage (for some applications) of giving more decimal precision in the range of typical outdoor temperatures. The United States, its territories and a few other countries still use Fahrenheit for weather and cooking.

Kelvin is the scientific standard, used in physics, chemistry, astronomy and engineering. It starts at absolute zero — the coldest possible temperature, at which all molecular motion stops, equivalent to -273.15°C. Kelvin uses the same scale divisions as Celsius, which makes converting between the two simple: K = °C + 273.15. Scientists prefer Kelvin because it avoids negative numbers in thermodynamic calculations, which simplifies the mathematics considerably.

For practical purposes: if someone tells you the temperature is 100°F, they mean a hot summer day (about 38°C). If they say 100°C, water is boiling. If they say 100K, you are in the realm of liquid nitrogen (-173°C), far beyond anything encountered in daily life.