The Rankine scale is Fahrenheit's absolute-temperature twin: same degree size as Fahrenheit, but with zero parked at absolute zero instead of at a salt-ice mixture. That makes it the scale of American thermodynamics textbooks, where engineers want absolute temperatures without abandoning familiar degree sizes. Converting to Celsius takes two moves — subtract 491.67 (the Rankine reading of water's freezing point) and multiply by 5/9 — and this converter does both while also reporting Fahrenheit. Enter the schema's freezing example, 491.67, in the Rankine (°R) box and the Celsius (°C) readout lands on exactly 0, with Fahrenheit (°F) at 32. Type 671.67, the boiling point, and you get exactly 100°C and 212°F. Absolute zero itself is 0 °R, which converts to −273.15°C. You will meet Rankine in heat-engine cycles, combustion analysis, and cryogenics papers from US labs; everywhere else, Kelvin does the same job with Celsius-sized degrees. Memorize the two anchor subtractions — 491.67 for the Celsius path, 459.67 for the Fahrenheit path — and the Rankine scale loses every bit of its intimidation.

With the pre-filled defaults — Rankine (°R) 491.67 — the Rankine to Celsius returns Celsius (°C) 0.0000, Fahrenheit (°F) 32.0000. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.