Formula
°C = (°F − 32) × 5/9°F = °C × 9/5 + 32K = °C + 273.15°R = °F + 459.67 How it works
Celsius and Fahrenheit differ in both their zero point and the size of a degree. Water freezes at 0 °C (32 °F) and boils at 100 °C (212 °F) at sea level, so 100 Celsius degrees span the same range as 180 Fahrenheit degrees — a ratio of 5 to 9.
Kelvin is the SI unit of temperature. It uses Celsius-sized steps but starts at absolute zero, −273.15 °C, so it never goes negative; scientists write “300 K”, not “300 degrees Kelvin”. Rankine does the same for Fahrenheit and still appears in some US engineering work.
Handy reference points: −40 is the same in °C and °F; room temperature is about 20–22 °C (68–72 °F); normal body temperature is about 37 °C (98.6 °F); a hot oven at 180 °C is 356 °F.
Frequently asked questions
Is there a quick mental shortcut for °C to °F?
Double the Celsius figure and add 30. It is within a couple of degrees for everyday weather (20 °C → 70 °F; the exact answer is 68 °F).
At what temperature are Celsius and Fahrenheit equal?
At −40°. Solve x = x × 9/5 + 32 to see why.