Every science class eventually hits the moment where Celsius stops being enough: gas laws, thermodynamics, and black-body radiation all demand an absolute temperature scale, one that starts at absolute zero instead of at water's freezing point. That scale is the kelvin, the SI unit of temperature, and converting is the simplest shift in all of thermometry — add 273.15. Type 25 into the Celsius (°C) box and the Kelvin (K) readout shows 298.15; the bonus Fahrenheit (°F) box keeps the familiar 77°F alongside. Water freezes at 273.15 K and boils at 373.15 K, and because the kelvin uses the same degree size as Celsius, a change of 10°C is exactly a change of 10 K — only the starting line moves. Note the style point scientists care about: it is "kelvin," not "degrees Kelvin," and the symbol is a plain K. Whether you are plugging numbers into the ideal gas law or just decoding a weather report from a physics paper, this one addition is the whole conversion.

With the pre-filled defaults — Celsius (°C) 100 — the Celsius to Kelvin returns Kelvin (K) 373.1500, 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.