The formula
How to convert between the scales
Celsius, Fahrenheit and Kelvin all measure the same thing with different zero points and step sizes. Celsius sets 0° at the freezing point of water; Fahrenheit sets 32° there instead; Kelvin starts at absolute zero, the coldest temperature physically possible, and uses the same step size as Celsius.
The conversion method
Converting Celsius to Fahrenheit is a straight linear formula — multiply by nine-fifths and add 32 — because the two scales share a fixed offset and a fixed ratio between degrees. Kelvin is simpler still: it is Celsius shifted by exactly 273.15, with no multiplication involved, since a change of one degree is the same size in both.
Worked example
At the default of 20 °C: Fahrenheit = 20 × 9⁄5 + 32 = 68 °F, and Kelvin = 20 + 273.15 = 293.15 K.
Reading the result
A useful anchor: 0 °C is 32 °F is 273.15 K (freezing), 37 °C is roughly 98.6 °F is 310.15 K (body temperature), and 100 °C is 212 °F is 373.15 K (boiling, at sea level). Everything else sits on a straight line between and beyond those points. Fahrenheit and Celsius cross at −40° — the one point where both scales read the same number. It looks like a coincidence until you check the algebra: it is the fixed point of the linear conversion formula, and true for every possible pair of linked scales with the same structure.
Fahrenheit predates Celsius by about two decades and was already standard in English-speaking science and commerce when Celsius was proposed in 1742. Most of the English-speaking world switched to metric and Celsius during the 20th century; the US did not complete the switch, so Fahrenheit stayed in everyday use there.
Physics formulas involving temperature — gas laws, thermodynamics, radiation — work directly in absolute terms, where zero means no thermal motion at all. Celsius has no such zero (0 °C is just water freezing, an arbitrary reference), so using it in those formulas would require adding 273.15 by hand every time. Kelvin removes that step.