Rømer Temperature Converter

Ole Rømer created his scale in 1701, fixing zero at the temperature of a mixture of ice and salt and choosing 60 for the boiling of water — the round 60 coming from the sexagesimal tradition. On that scale, the freezing of water falls at 7.5, an awkward value that helps explain why it did not survive.

Enter the value and the page converts to Celsius, Fahrenheit and Kelvin at once. The arithmetic for Celsius is subtracting 7.5 and multiplying by 40 over 21 — the fraction comes from the 100 Celsius degrees between freezing and boiling corresponding to 52.5 Rømer degrees, and 100 divided by 52.5 is exactly 40 over 21.

The scale's historical importance is indirect: Daniel Fahrenheit visited Rømer in 1708, adopted the principle and refined it. He multiplied the degrees by four for finer divisions, adjusted the reference points, and arrived at the scale bearing his name. That is why Fahrenheit's two fundamental numbers have seemingly arbitrary origins — they come from an adjustment on top of Rømer's scale.