Convert between Celsius, Fahrenheit and Kelvin.
°F = °C × 9/5 + 32 | °C = (°F − 32) × 5/9 | K = °C + 273.15
Celsius and Fahrenheit are both anchored to arbitrary reference points — Celsius to water's freezing and boiling points at standard pressure (0°C and 100°C), Fahrenheit to a mix of historical reference mixtures — which is exactly why both scales extend into negative numbers: there's nothing stopping a temperature from going below an arbitrarily chosen zero point. Kelvin is different by design. In 1848, Lord Kelvin (William Thomson) proposed an absolute scale anchored not to a substance's behavior at one pressure, but to the theoretical point where molecular motion itself stops entirely — absolute zero, calculated at -273.15°C by extrapolating how a gas's volume shrinks as it cools. Nothing can be colder than absolute zero, so 0 K is a genuine physical floor, not an arbitrary marker, which is why Kelvin temperatures are never negative.
The practical relationship is simple once you know it: Kelvin and Celsius move in identical-sized steps (a 1-degree change is the same magnitude in both), just offset by exactly 273.15 — so 0°C (water's freezing point) equals 273.15 K, and 100°C equals 373.15 K. Fahrenheit, by contrast, uses smaller degree increments than either (180 Fahrenheit degrees span the same range as 100 Celsius degrees between freezing and boiling), which is why Fahrenheit-to-Celsius conversion needs both a multiplication and an addition/subtraction, while Celsius-to-Kelvin needs only addition. Scientists and engineers work almost exclusively in Kelvin for exactly this reason: gas laws, thermodynamics and most physics equations are written in terms of absolute temperature, and using Celsius or Fahrenheit in those formulas would require constantly re-deriving the offset.
Because 0 Kelvin is defined as absolute zero — the theoretical point where all molecular motion stops — which is a genuine physical floor, not an arbitrary reference point like Celsius's 0° (water's freezing point) or Fahrenheit's 0°. Nothing can physically be colder than absolute zero, so there's no valid negative Kelvin temperature.
In 1848, Lord Kelvin extrapolated how an ideal gas's volume shrinks as it cools at constant pressure. Following that relationship to its logical endpoint — the temperature at which the gas's volume would theoretically reach zero — gives -273.15°C, remarkably close to the modern precisely measured value.
Most physics and thermodynamics formulas (gas laws, blackbody radiation, and others) are written in terms of absolute temperature from true zero. Using Celsius or Fahrenheit in those equations would require constantly adding or subtracting an offset; Kelvin lets the math work directly without that extra step, since it starts from the real physical zero point.
Because Fahrenheit and Celsius differ in both their zero point and their degree size — 180 Fahrenheit degrees span the same temperature range as 100 Celsius degrees between water's freezing and boiling points. Converting between them needs a scaling factor (5/9 or 9/5) for the different degree sizes, plus an offset (32) for the different zero points. Celsius to Kelvin only needs the offset, since their degree sizes are identical.