Article — Fahrenheit to Celsius Converter
Fahrenheit to Celsius: Formula, History, and Everyday Use
To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9. The formula is °C = (°F − 32) × 5/9. Water freezes at 32 °F (0 °C) and boils at 212 °F (100 °C) at one atmosphere. The two scales meet at one point: −40 °F = −40 °C.
Fahrenheit and Celsius are two names for the same physical quantity, temperature, but they count it differently. Anders Celsius (Sweden, 1742) divided the gap between freezing and boiling water into 100 equal steps. Daniel Gabriel Fahrenheit (1724, born in Gdansk) divided the same gap into 180 steps and set zero at the temperature of a saturated salt-ice brine. The math to translate between them is short, but the order matters and the offsets trip people up.
The exact formula
To go from Fahrenheit to Celsius, the formula is °C = (°F − 32) × 5/9. To go the other way, °F = °C × 9/5 + 32. Both versions are exact, not rounded approximations. The fraction 5/9 = 0.5555… recurring, and 9/5 = 1.8 exactly.
The 32 in the formula is the offset, the gap between the zeros of the two scales. Water freezes at 0 °C and at 32 °F, so any Fahrenheit reading is 32 degrees too high before you start scaling. The 5/9 is the slope: a 9 °F change equals a 5 °C change. Equivalently, 1 °C = 1.8 °F.
Order of operations matters in the Fahrenheit conversion. Multiplying by 5/9 before subtracting 32 gives the wrong answer. For 72 °F: the correct path is (72 − 32) × 5/9 = 22.2 °C. The wrong path is 72 × 5/9 − 32 = 8.0 °C, off by 14 degrees.
Why the world has two temperature scales
Two systems exist because of timing and geopolitics. Fahrenheit published his scale in 1724, with three anchor points: zero at salt-ice brine (the coldest he could reliably reach), 32 at the freezing point of water, and 96 at human body temperature. The 96 was later corrected to 98.6.
Celsius proposed his scale in 1742. His original version was inverted: 0 was boiling, 100 was freezing. Carl Linnaeus flipped it around 1745. The name "centigrade" stuck until 1948, when the General Conference on Weights and Measures renamed it "Celsius" to avoid clashing with the centigrade as a unit of angle.
By the late 1800s, Fahrenheit was entrenched in Britain and its colonies. Britain dropped it during the 1960s and 1970s. The United States never finished its switch — the Metric Conversion Act of 1975 made metric voluntary. About 320 million Americans, plus Liberia, the Bahamas, Belize, the Cayman Islands, Palau, and a few Micronesian states, still use Fahrenheit for daily temperature.
Body temperature and the 98.6 myth
Normal body temperature is widely taught as 98.6 °F or 37.0 °C. That number comes from Carl Wunderlich, a German physician who measured 25,000 patients between 1851 and 1868 and averaged the readings. For more than 150 years it was treated as a constant.
A 2020 Stanford Medicine study reanalyzed 677,423 temperature measurements from the US going back to 1862. Average body temperature has fallen by about 0.05 °F per decade. The modern American average is 97.9 °F (36.6 °C), and the normal range runs from 97.3 to 98.2 °F (36.3 to 36.8 °C). The leading explanation is that humans now carry less background inflammation thanks to vaccines, antibiotics, better sanitation, and lower rates of untreated chronic infection.
The clinical fever threshold has not moved. The World Health Organization defines fever as a measured body temperature at or above 38.0 °C (100.4 °F). Anything between 37.5 and 38.0 °C (99.5 and 100.4 °F) is called a low-grade temperature, common with minor viral infections.
Thermometers in US pharmacies usually show both scales. If you only see Fahrenheit and want Celsius for an international travel form, the easy ones to remember are 98.6 °F = 37.0 °C, 100 °F = 37.8 °C, and 100.4 °F = 38.0 °C (the fever line).
Oven temperatures and baking
American recipes give oven temperatures in Fahrenheit. European recipes use Celsius. Confusing them is more than a minor inconvenience. The most common American baking temperature, 350 °F, equals 176.67 °C — Europeans round it to 175 or 180 °C. The conversion does not scale linearly the way you might guess, because of the +32 offset.
For roasting and high-heat cooking, the key milestones are 400 °F = 204 °C (poultry, roasted vegetables), 425 °F = 218 °C (puff pastry), and 450 °F = 232 °C (pizza, broiling). British recipes sometimes use gas marks instead of degrees: gas mark 4 = 350 °F = 177 °C, gas mark 7 = 425 °F = 218 °C.
A small but real risk: setting a Celsius-capable oven to 350 when the recipe meant 350 °F. That puts the oven at 662 °F (350 °C), hot enough to scorch food and trigger smoke alarms within minutes. Modern ovens often have a unit-toggle button buried in the menu. Verify before preheating.
Weather conversion shortcut
The exact formula is fast on a calculator and slow in your head. Two mental shortcuts handle the weather range (around −20 to 110 °F) with usable accuracy.
The "minus thirty, divide by two" trick: take the Fahrenheit reading, subtract 30, divide by 2. The result is close to the Celsius value. 72 °F → (72 − 30)/2 = 21 °C. The real answer is 22.2 °C, so it is off by about 1 degree. For weather, that is fine.
The reverse, "double and add 30": take the Celsius reading, double it, add 30. 25 °C → 25 × 2 + 30 = 80 °F. The real answer is 77 °F. Again, off by a few degrees, but enough to decide on jacket or no jacket.
°F → °C (°F − 30) ÷ 2°C → °F (°C × 2) + 30°F exact (°F − 32) × 5/9°C exact (°C × 9/5) + 32°C → K °C + 273.15Scientific use and Kelvin
Celsius is the metric scale for everyday use, but the true SI base unit for temperature is the kelvin. Kelvin uses the same step size as Celsius but starts at absolute zero, the lowest physically possible temperature. 0 K = −273.15 °C = −459.67 °F. To convert: K = °C + 273.15.
The kelvin was redefined in 2019 in terms of the Boltzmann constant, k = 1.380 649 × 10⁻²³ J/K, giving it an exact value independent of any physical material. Celsius is now defined through kelvin: t/°C = T/K − 273.15. Scientific papers across all fields use kelvin or Celsius. Fahrenheit shows up only in legacy US engineering documents.
Minus forty: where the scales cross
The two scales meet at exactly one temperature: −40 °F = −40 °C. To see why, set them equal: T = 9/5 × T + 32. Solve for T and you get T = −40. There is only one solution because both formulas are linear with different slopes.
This is not a designed coincidence. Neither Fahrenheit nor Celsius set out to make the scales touch. It falls out of the algebra. The temperature is real and reachable. Northern Manitoba, parts of Siberia, and the Yukon hit −40 most winters. Fairbanks, Alaska records it almost every January. International Falls, Minnesota, the "icebox of the nation", gets close.
Light bulbs, paint, and many adhesives are rated to operate down to −40, the lowest temperature where the labeling is the same in both unit systems. Manufacturers picked −40 specifically so a single number can serve both US and metric markets without ambiguity. A "rated to −40" label means the same thing in Fairbanks as in Yakutsk.
Common conversion mistakes
Order of operations is the big one. The formula has a subtraction before the multiplication. Multiplying first gives an answer that is off by 17.78 — the offset is 32 in Fahrenheit, which scales to 32 × 5/9 = 17.78 in Celsius.
Mixing absolute and relative temperatures is the next trap. A 10 °F change is not a 10 °C change; it is a 5.56 °C change (10 × 5/9). If a forecast says "temperatures will drop by 20 °F overnight", that is an 11.1 °C drop. The 32 offset cancels for differences, but the 5/9 slope does not.
The third mistake is assuming the freezing point of water is the same in both scales. It is not. Water freezes at 32 °F, and the Fahrenheit zero is a brine point with no everyday meaning today.
- −40° the only point where °F and °C agree
- 0 °F = −17.78 °C, a saturated salt-ice brine
- 32 °F = 0 °C, water freezes
- 98.6 °F = 37.0 °C, classic normal body temperature
- 100.4 °F = 38.0 °C, WHO fever threshold
- 212 °F = 100 °C, water boils at 1 atm
- 350 °F = 176.67 °C, default US oven for baking
- −273.15 °C = 0 K = absolute zero
- 9/5 = 1.8, the slope between °F and °C
- 1 °C = 1.8 °F per unit of change