Fahrenheit to Celsius Converter — Convert Fahrenheit to Celsius

Temperature

98.6°F

Formula

C = (F − 32) × 5/9

1. 1

   Subtract 32 from the Fahrenheit value.

   98.6 − 32 = 66.6
2. 2

   Multiply by 5/9.

   66.6 × 5/9 = 333/9 = 37

98.6°F = 37.0°C — normal human body temperature.

The arithmetic is exact here: 66.6 × 5/9 = 37 with no rounding. This is why 98.6°F became the standard US figure — it is the exact Fahrenheit conversion of the Celsius measurement.

Temperature

0°F

1. 1

   Subtract 32.

   0 − 32 = −32
2. 2

   Multiply by 5/9.

   −32 × 5/9 = −160/9 = −17.7̅ (repeating)
3. 3

   Round to one decimal place.

   ≈ −17.8°C

0°F = −17.8°C. Well below water’s freezing point — dangerous exposure territory.

The repeating decimal arises because 32 is not divisible by 9 (or 1.8). There is no exact terminating decimal; −17.778°C is rounded from −17.7777…°C.

1. [1]BIPM SI Brochure — Thermodynamic temperature and the kelvin · Bureau International des Poids et Mesures · 2019
2. [2]NIST SP 811 — Temperature conversions and ITS-90 · National Institute of Standards and Technology · 2008

Degree Fahrenheit (°F)

The US customary temperature unit. Water freezes at 32°F, boils at 212°F. A Fahrenheit degree is 5/9 the size of a Celsius degree.

Proposed by Daniel Gabriel Fahrenheit in 1724. Still used in daily weather, cooking, and body temperature reporting in the United States. All other major countries use Celsius for everyday temperatures.

Degree Celsius (°C)

The SI-accepted temperature unit. Zero is the freezing point of water; 100 is the boiling point at 1 atm pressure.

Formally defined as T(K) − 273.15, where T(K) is the thermodynamic temperature in kelvins. A change of 1°C is identical to a change of 1 K. The universal language of science and medicine for temperature.

The 32°F Offset

The Fahrenheit reading at 0°C (the freezing point of water). Must be subtracted before scaling.

Fahrenheit placed his zero at a brine-ice mixture, not at water’s freezing point. The resulting 32°F offset is why F-to-C conversions require subtraction before scaling, while the reverse requires scaling before addition.

The 5/9 Scale Factor

The ratio of Celsius degree size to Fahrenheit degree size. Multiply the offset-corrected Fahrenheit value by 5/9 to get Celsius.

Derived from the fact that 180°F = 100°C spans the same physical temperature range (freezing to boiling at 1 atm). So 100/180 = 5/9. One Fahrenheit degree is only 55.6% as large as one Celsius degree.

Kelvin (K)

The SI base unit of temperature. 0 K = absolute zero = −273.15°C = −459.67°F.

Required for thermodynamics, radiation physics, and gas laws. To convert Fahrenheit to Kelvin: K = (F − 32) × 5/9 + 273.15. There is no degree symbol for Kelvin — it is written as “273 K” not “273°K.”

ITS-90

International Temperature Scale of 1990. The global calibration standard linking thermometer readings to fundamental physics.

ITS-90 defines temperature through 17 fixed calibration points (such as the triple point of water at 273.16 K) rather than properties of specific materials. This is why all thermometers worldwide agree with each other to high precision.

Triple Point of Water

0.01°C (273.16 K / 32.018°F). The one pressure and temperature where ice, liquid water, and steam coexist.

Used as a primary fixed point in ITS-90. It differs from the melting point of ice (0.00°C at 1 atm) by exactly 0.01°C because the melting point shifts with pressure. The triple point occurs only at 611.657 Pa.

Frigorific Mixture

Fahrenheit’s 0°F reference: a brine-ice-salt solution that stabilizes at a reproducible cold temperature.

Before ice-point thermometry was standard, Fahrenheit used a mixture of ammonium chloride, ice, and water as a reproducible cold reference. The eutectic of this mixture stabilizes near −17.8°C (0°F) — which is why the Fahrenheit zero has that seemingly arbitrary Celsius value.