The three scales
Celsius sets 0 at water's freezing point and 100 at its boiling point. Fahrenheit puts those at 32 and 212, giving 180 degrees across the same span — so a Fahrenheit degree is smaller. Kelvin uses Celsius-sized degrees but starts at absolute zero.
To convert C to F, multiply by 9/5 and add 32. The two scales cross at −40, which is the same temperature in both.
Why the United States kept Fahrenheit
Fahrenheit was the established scale across the British Empire when the US adopted it, and it simply never got displaced. Britain and its other former colonies moved to Celsius during the metrication drives of the 1960s and 70s; the US metrication effort of 1975 was made voluntary and quietly stalled.
Fahrenheit does have one practical merit for weather: its degrees are finer, so ordinary outdoor temperatures span roughly 0 to 100 rather than −18 to 38, which some find more intuitive as a scale of human comfort.
Absolute zero and the Kelvin scale
Kelvin measures from absolute zero — the point at which particles have minimum possible thermal energy — which is why negative Kelvin temperatures are not physically meaningful. It is the SI base unit for temperature and the standard throughout physics and chemistry.
Because Kelvin uses Celsius-sized degrees, temperature differences are numerically identical in both. A 10-degree rise in Celsius is a 10-Kelvin rise, though it is an 18-degree rise in Fahrenheit.
Frequently asked questions
What is a quick mental conversion? +
Double the Celsius figure and add 30. It is a few degrees off but close enough for weather: 20°C gives 70 (actual 68), 30°C gives 90 (actual 86).
At what temperature are Celsius and Fahrenheit equal? +
At −40 degrees. It is the single point where the two scales cross, which makes it a useful check on any conversion formula.
What is a normal body temperature? +
About 37°C or 98.6°F, though healthy individuals vary by roughly half a degree Celsius and body temperature fluctuates through the day.