Celsius vs. Fahrenheit: The Conversion Formula and the History Behind Two Temperature Scales
The formula for converting between Celsius and Fahrenheit, how each scale was defined, and why the United States still measures temperature in Fahrenheit while most of the world uses Celsius.
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Celsius and Fahrenheit are both linear temperature scales, so converting between them is a matter of a fixed formula: multiply a Celsius value by 9/5 and add 32 to get Fahrenheit, or subtract 32 and multiply by 5/9 to go from Fahrenheit back to Celsius. The scales differ because they were defined around different reference points roughly two decades apart, not because either one is more scientifically fundamental than the other.
How each scale was defined
Fahrenheit: built around a lab mixture and body temperature
Daniel Gabriel Fahrenheit, a German physicist, proposed his scale around 1724. He set 0°F near the freezing point of a saturated brine (salt and ice) solution, one of the coldest stable temperatures he could reliably produce in a laboratory at the time. He separately set roughly 96°F near human body temperature. Water's freezing and boiling points fell out of that calibration at 32°F and 212°F — 180 degrees apart, which is why the scale has that somewhat arbitrary-looking spread.
Celsius: built around water
Anders Celsius, a Swedish astronomer, proposed his scale in 1742 using water's freezing and boiling points directly: originally 0° for boiling and 100° for freezing, later reversed to the 0°C freezing / 100°C boiling convention used today. Because it is defined around round numbers for a universally available reference substance, Celsius became the basis for the metric system's temperature unit and, later, the kelvin.
The conversion formula
Both scales are linear, so a single formula converts between them in either direction:
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F − 32) × 5/9
The two scales intersect at −40 degrees — −40°C is exactly −40°F, the only point where both scales agree.
Why the US still uses Fahrenheit
Fahrenheit remains the everyday standard in the United States (and is used alongside Celsius in a few territories) largely because of inertia: by the time the metric system spread globally in the 19th and 20th centuries, Fahrenheit was already deeply embedded in US weather reporting, home thermostats, and cooking. Many Americans also find Fahrenheit's finer granularity convenient for describing everyday weather, since it uses a wider range of whole numbers (roughly 0–100) to describe typical outdoor temperatures than Celsius does.
Frequently asked questions
- What is the formula to convert Celsius to Fahrenheit?
- Multiply the Celsius temperature by 9/5 (1.8) and add 32: °F = (°C × 9/5) + 32. To go the other way, subtract 32 and multiply by 5/9: °C = (°F − 32) × 5/9.
- Why does water freeze at 32°F instead of 0°F?
- Daniel Fahrenheit built his scale around a laboratory brine mixture (0°F) and, separately, human body temperature (originally set near 96°F). Water's freezing and boiling points landed at 32°F and 212°F almost incidentally, rather than being chosen as round numbers the way Celsius chose 0 and 100.
- Is there a temperature where Celsius and Fahrenheit are equal?
- Yes: −40°C and −40°F are the same temperature, which is the point where the two linear scales cross.
- Which countries still use Fahrenheit?
- The United States is the main country that uses Fahrenheit for everyday temperatures, along with a handful of territories such as the Bahamas, Belize, and the Cayman Islands. Nearly every other country uses Celsius.