The United States is one of only a handful of countries still using Fahrenheit as a primary temperature scale. The rest of the world uses Celsius. If you travel, cook with international recipes, or follow science news, you need both — and the formulas to move between them.
The Exact Formulas
- Fahrenheit to Celsius: C = (F − 32) × 5/9
- Celsius to Fahrenheit: F = (C × 9/5) + 32
- Celsius to Kelvin: K = C + 273.15
- Fahrenheit to Kelvin: K = (F − 32) × 5/9 + 273.15
Example: 98.6°F (normal body temperature) = (98.6 − 32) × 5/9 = 66.6 × 0.556 = 37°C. Example: 100°C (boiling water) = (100 × 9/5) + 32 = 180 + 32 = 212°F.
Mental Math Shortcuts
The exact formula is awkward in your head. These approximations work for everyday use:
- Quick F→C: Subtract 30, then divide by 2. (86°F: 86−30=56, 56÷2=28°C. Exact is 30°C — close enough for weather.)
- Quick C→F: Double it, then add 30. (25°C: 25×2=50, 50+30=80°F. Exact is 77°F.)
💡 The one temperature that's the same in both scales: −40°. At −40°F = −40°C, the two scales intersect. A useful anchor point for the conversion formula.
Key Reference Temperatures
- 0°C / 32°F — Water freezes
- 20°C / 68°F — Comfortable room temperature
- 37°C / 98.6°F — Normal human body temperature
- 100°C / 212°F — Water boils (at sea level)
- 180°C / 356°F — Typical baking temperature
Why Does the US Still Use Fahrenheit?
The US attempted to metricate (including Celsius adoption) in the 1970s after Congress passed the Metric Conversion Act. The effort was largely voluntary and never gained traction. Science, medicine, and most US industries already use Celsius or Kelvin. Only everyday weather, cooking, and body temperature discussions remain Fahrenheit-dominant in the US — and even that is slowly changing.
What About Kelvin?
Kelvin is the SI base unit of temperature used in physics and chemistry. It starts at absolute zero (−273.15°C) — the point at which all molecular motion theoretically stops. Unlike Fahrenheit and Celsius, Kelvin has no negative values. It's used in thermodynamics, astronomy, and any calculation involving the laws of physics, where negative temperatures would break equations.