Convert fahrenheit to celsius

°C = (°F − 32) × 5/9

The subtraction must come first. Dividing 70 by 1.8 and then subtracting 32 gives 6.9 °C — a cold day — instead of the correct 21.1 °C.

• 32 °F = 0 °C — freezing
• 50 °F = 10 °C — cool
• 70 °F = 21 °C — comfortable
• 90 °F = 32 °C — hot
• 100 °F = 38 °C — a heatwave

A reverse shortcut: subtract 30 and halve. 80 °F becomes 25 °C against the true 26.7 °C.

Daniel Gabriel Fahrenheit built his scale in the 1720s around reference points he could reproduce in a laboratory. Zero was the coldest temperature he could reach with a mixture of ice, water and salt. He placed the human body at about 96 degrees. Water happened to freeze at 32 on that scale and, after later recalibration, boil at 212 — exactly 180 degrees apart, which is where the 9/5 ratio in the formula comes from.

American oven settings are round Fahrenheit numbers: 350 °F is 177 °C, 375 °F is 191 °C, 425 °F is 218 °C. Most metric ovens step in 10 °C increments, so set 180, 190 and 220 °C respectively. Fan ovens usually run 20 °C lower than the recipe states.

The scientific scale that starts at absolute zero is kelvin: add 273.15 to any Celsius figure.

From the size of the degrees. NIST Special Publication 811, the US guide to the SI states that a degree Fahrenheit is exactly 5/9 of a kelvin, the same size as 5/9 of a degree Celsius, with 32 °F at the freezing point of water. The Celsius scale itself is tied to the kelvin in the SI Brochure published by the International Bureau of Weights and Measures (BIPM), which in turn has been defined by the Boltzmann constant since 2019.