Temperature is the one measurement where everyday conversion needs more than multiplication: 0 °C is 32 °F, not 0 °F. The reason is history. Each scale was built by a different person, from different fixed points.
Fahrenheit’s scale (1724)
Daniel Gabriel Fahrenheit, born in Danzig (now Gdańsk), was an instrument maker working in the Dutch Republic who made the first reliable mercury thermometers. In 1724 he described his scale. Its zero was the coldest temperature he could reliably reproduce in the laboratory: a mixture of ice, water and a salt (ammonium chloride). He set the freezing point of plain water at 32 and the temperature of a healthy human body at about 96.
The scale was later recalibrated so that water boils at exactly 212 °F, 180 degrees above freezing. That pushed normal body temperature to the familiar 98.6 °F. Fahrenheit spread through the British Empire and is still the everyday scale in the United States.
Celsius, upside down (1742)
In 1742 the Swedish astronomer Anders Celsius proposed a scale with 100 degrees between the freezing and boiling points of water, but with 0 for boiling and 100 for freezing. Within a few years of his death in 1744 the scale had been turned the right way up. For two centuries it was called “centigrade” (a hundred steps); in 1948 an international conference renamed it degrees Celsius.
Kelvin finds the bottom (1848)
Both scales put zero at a convenient but arbitrary point, so negative temperatures are common. In 1848 William Thomson, later Lord Kelvin, argued for an absolute scale starting at the coldest possible temperature, where thermal motion is at its minimum. That point, absolute zero, is −273.15 °C or −459.67 °F, and nothing can be colder, which is why the temperature converter refuses values below it.
The kelvin uses the same step size as the Celsius degree, so converting is a simple shift: K = °C + 273.15. Scientists write “300 K”, not “300 degrees Kelvin”. Since 2019 the kelvin has been defined by fixing the Boltzmann constant, linking temperature directly to energy.
Converting between them
- °F = °C × 9/5 + 32: multiply first, then add.
- °C = (°F − 32) × 5/9: subtract first, then multiply.
- Quick estimate: double the Celsius figure and add 30 (20 °C ≈ 70 °F; the exact answer is 68 °F).
- The scales cross at −40: −40 °C is exactly −40 °F.
Is 98.6 °F really normal?
The figure goes back to the German physician Carl Wunderlich, who in 1868 published conclusions drawn from about a million readings and reported an average of 37 °C. Converted exactly, that is 98.6 °F, a precision his data never had. Modern studies tend to find a slightly lower average, and normal temperature varies from person to person and through the day.
Try it
- Temperature converter — °C, °F and K on one thermometer