Fahrenheit to Celsius Calculator

Background.

Fahrenheit readings turn up wherever American sources do: a forecast of 86 °F, a recipe that bakes at 350 °F, a thermometer bought in the US showing 101.5 °F. Turning them into Celsius takes two steps, in a fixed order. First remove the 32-degree head start Fahrenheit has at the freezing point of water, then shrink what is left by 5/9, because a Fahrenheit degree is only five-ninths the size of a Celsius one.

Those steps give a pattern worth remembering: every 18 °F is exactly 10 °C. From 50 °F, which is 10 °C, the familiar marks follow — 68 °F is 20 °C, 86 °F is 30 °C, 104 °F is 40 °C. Below freezing it runs the same way: 0 °F is about −17.78 °C, and at −40 the two scales finally agree.

The result is exact, not an approximation, and the kelvin equivalent is shown alongside for science and engineering work. For a body temperature the conversion is all you get: whether 38.6 °C counts as a fever, and what to do about it, depends on age, symptoms, where the reading was taken and the guidance you are following, none of which a unit conversion can judge.

What is fahrenheit to celsius calculator?

Fahrenheit and Celsius number the same temperatures from different starting points and in different step sizes. Water freezes at 32 °F and 0 °C and boils at 212 °F and 100 °C at standard pressure, so 180 Fahrenheit degrees cover the same interval as 100 Celsius degrees. Both scales are now tied to the kelvin, the SI unit of temperature: a Celsius degree is exactly one kelvin in size with 0 °C at 273.15 K, and absolute zero, 0 K, falls at −273.15 °C and −459.67 °F.

How to use this calculator.

1. 01Type the Fahrenheit reading, including a minus sign for winter temperatures below 0 °F.
2. 02Read the Celsius result; for a thermometer, one decimal place matches what most clinical devices show.
3. 03Use the kelvin figure for gas-law calculations, physics coursework or equipment specified in kelvin.
4. 04For a change in temperature rather than a reading, skip the tool and multiply the difference by 5/9.

The formula.

°C = (°F − 32) × 5/9. The 32 lines up the freezing points; the 5/9 is 100 ÷ 180, the ratio of the two degree sizes. Order matters: subtracting after multiplying would turn 212 °F into about 85.8 °C instead of 100 °C. The kelvin value then adds a pure offset with no scaling, K = °C + 273.15. Both relations are fixed by definition, as set out in NIST Special Publication 811 and the SI Brochure, so the only limit on accuracy is the reading you start with. Inputs below −459.67 °F are rejected because nothing can be colder than absolute zero; the upper limit is 1,500 °F.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology (2008). NIST Special Publication 811 — Guide for the Use of the International System of Units (SI), Appendix B: Conversion Factors.
2. [2]Bureau International des Poids et Mesures (2019). The International System of Units (SI), 9th edition — defines the kelvin via the fixed Boltzmann constant and the exact offset between the Kelvin and Celsius scales.