Fahrenheit to Celsius converter

Why Fahrenheit's zero point looks arbitrary

Daniel Fahrenheit set 0 on his scale using the coldest temperature a salt and ice mixture could reach in his lab in 1724, not a natural constant like water freezing. That is why 0F is not a round number in Celsius: it lands at about -17.78C.

Celsius, introduced in 1742, is built the opposite way: it starts from water's freezing and boiling points, 0 and 100, which is part of why it became the standard for science and for every country that later adopted the metric system.

The two scales cross at exactly one other point besides -40: nowhere else do they share a number, so every other Fahrenheit reading needs the full formula, not a coincidence, to convert correctly.

Oven temperatures in both scales

Recipe sites often round these to the nearest 5 or 10 degrees Celsius, so do not expect a Celsius oven dial to match the converted figure exactly.

Estimating Celsius without a calculator

Subtract 30 from the Fahrenheit number, then divide by 2. For 100F: 100 minus 30 is 70, halved is 35, close to the exact 37.78C. The shortcut drifts a little more at low temperatures, so use the real formula when the result needs to be exact.

At 50F the shortcut is perfect: 50 minus 30 is 20, halved is 10, matching the exact 10C exactly. At -20F it drifts more: the shortcut gives -25, versus the real -28.89C.

Fahrenheit at home, Celsius everywhere else

Fahrenheit is still the everyday scale for weather, ovens and thermostats in the US. Everywhere else, and in every scientific and medical setting worldwide, Celsius is standard, which is why a Fahrenheit reading from a US source usually needs converting before it means anything to a Celsius-only audience.

Weather apps aimed at a US audience often let you switch the display unit, but the underlying forecast data is usually generated in Celsius or Kelvin first and converted to Fahrenheit for display, not the other way around.