Long before Celsius conquered the world, France measured temperature in Réaumur — the scale of René Antoine Ferchault de Réaumur, who in 1730 divided water's freeze-to-boil range into 80 degrees instead of 100. The conversion could not be simpler: multiply Celsius by 4/5. Type 20 into the Celsius (°C) box and the Réaumur (°Ré) readout shows 16; a simmering 80°C candy syrup reads 64°Ré. The scale lingered in French cheesemaking, winemaking, and cooking well into the 20th century — old French recipes calling for "25 degrees" often mean 25°Ré, about 31°C, not a chilly 25°C oven — so bakers and culinary historians still trip over it. The bonus Fahrenheit (°F) box translates everything into American kitchen terms. Réaumur thermometers used alcohol instead of mercury, and the scale's neat 80-division symmetry appealed to pre-revolutionary French sensibilities. It is obsolete in science, but it flavors any kitchen that still cooks from grandmother's French notebooks.

With the pre-filled defaults — Celsius (°C) 100 — the Celsius to Réaumur returns Réaumur (°Ré) 80.0000, 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.