Rankine and kelvin are the two absolute temperature scales — both start at absolute zero — but they count upward in different-sized steps: Rankine degrees match Fahrenheit, kelvin match Celsius. Since a Fahrenheit degree is 5/9 the size of a Celsius degree, converting is one multiplication: kelvin equals Rankine times 5/9. Enter 491.67 in the Rankine (°R) box and the Kelvin (K) readout shows exactly 273.15, with the Celsius (°C) box at 0. Type 0 and you get 0 K — the two zeros coincide, as they must, since both scales agree on absolute zero. This conversion is the handshake between American engineering papers (Rankine) and international science (kelvin): a combustion study quoting 2,000 °R becomes about 1,111.11 K, a number a European colleague can plug straight into an equation. The companion Celsius box subtracts 273.15 from the kelvin figure, so all three absolute-friendly scales update together as you type.

With the pre-filled defaults — Rankine (°R) 491.67 — the Rankine to Kelvin returns Kelvin (K) 273.1500, Celsius (°C) 0.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.