Science textbooks quote temperatures in kelvin — 273.15 K, 373.15 K — numbers that look alien until you learn the one-step secret: subtract 273.15. The kelvin scale starts at absolute zero, the coldest anything can theoretically get, while Celsius starts at water's freezing point. Those two starting points are 273.15 degrees apart, and the degrees themselves are identical in size. So converting is pure addition or subtraction, no multiplication needed: room temperature of 293 K is 20°C, and boiling water at 373.15 K is exactly 100°C.

Type your value in the Kelvin (K) box and the Celsius (°C) readout subtracts 273.15, with a bonus Fahrenheit (°F) column for everyday context. The three worked examples are the anchors worth memorizing: 0 K (absolute zero) is -273.15°C, 273.15 K (freezing) is 0°C, and 373.15 K (boiling) is 100°C. Chemistry students converting gas-law temperatures, weather nerds reading scientific papers, and cooks following sous-vide guides written by food scientists all do this subtraction constantly. One caution: never write "degrees kelvin" — it is just "kelvin," symbol K, no degree sign. And remember that while the offset is 273.15 exactly, quick estimates can use 273; the 0.15 only matters in precise lab work.

With the pre-filled defaults — Kelvin (K) 373.15 — the Kelvin to Celsius returns Celsius (°C) 100.0000, Fahrenheit (°F) 212.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.