The Delisle scale is the contrarian of thermometry: it runs backwards. Invented in 1732 by French astronomer Joseph-Nicolas Delisle for a St. Petersburg observatory, it sets 0°De at the boiling point of water and climbs as things get colder — room temperature sits near 100°De and ice water hits 150°De. The formula is (100 − °C) × 3/2, so every Celsius degree is worth one and a half Delisle degrees, counted downward. Type 20 into the Celsius (°C) box and the Delisle (°De) readout shows 120; boiling water at 100°C reads a satisfying 0°De. Russia used this scale for roughly a century, which is why old Russian scientific texts quote temperatures that look absurd until you convert them. The bonus Fahrenheit (°F) box keeps you oriented in familiar units. Nobody measures weather in Delisle today, but it is a favorite of trivia nights and history-of-science courses — and a vivid reminder that temperature scales are human inventions, not laws of nature.

With the pre-filled defaults — Celsius (°C) 100 — the Celsius to Delisle returns Delisle (°De) 0.0000, 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.