Fahrenheit vs. Celsius: How to Convert Temperature

Instantly convert weather forecasts and recipes using our Temperature Converter, or read on to understand the complex formulas behind Celsius and Fahrenheit.

If an American travels to Europe and sees the weather forecast says "30 degrees," they might pack a heavy winter coat, expecting freezing weather. In reality, 30 degrees Celsius is a hot, sweltering summer day (86°F).

The United States is one of the only countries on Earth that still relies on the Fahrenheit scale for everyday temperature measurement. The rest of the world uses Celsius.

The History of the Scales

Why do the two scales exist, and why do they measure heat so differently?

Fahrenheit (1724)

Invented by physicist Daniel Gabriel Fahrenheit. He set 0 degrees as the freezing point of a specific brine solution (water, ice, and salt). He then set the average human body temperature around 96 degrees. (We now know body temperature is closer to 98.6°F). On this scale, pure water freezes at 32°F and boils at 212°F.

Celsius (1742)

Invented by astronomer Anders Celsius. He wanted a scale based entirely on the metric system (Base-10). He logically set the freezing point of pure water at exactly 0°C, and the boiling point of pure water at exactly 100°C.

The Conversion Formulas

Because the two scales have completely different starting points (0 vs 32) and completely different increment sizes (there are 180 degrees between freezing and boiling in Fahrenheit, but only 100 degrees in Celsius), you cannot just add or subtract a flat number to convert between them.

You must use a formula involving fractions.

Converting Celsius to Fahrenheit

Formula: F = (C × 9/5) + 32

Example: Convert 20°C to Fahrenheit.

1. Multiply by 9/5 (or 1.8): 20 × 1.8 = 36
2. Add 32: 36 + 32 = 68 Answer: 68°F

Converting Fahrenheit to Celsius

Formula: C = (F - 32) × 5/9

Example: Convert 100°F to Celsius.

1. Subtract 32: 100 - 32 = 68
2. Multiply by 5/9 (or divide by 1.8): 68 / 1.8 = 37.77 Answer: 37.77°C

Mental Math Shortcuts

Doing complex fractional math in your head is difficult. If you are traveling and just need a rough estimate, use these mental shortcuts:

Quick C to F: Double the Celsius number, then add 30. Example: 20°C. Double it (40) + 30 = 70°F. (Actual is 68°F. Very close!)

Quick F to C: Subtract 30 from the Fahrenheit number, then cut it in half. Example: 80°F. Subtract 30 (50) and cut in half = 25°C. (Actual is 26.6°C).

FAQ

What is the Kelvin scale? Kelvin is a scientific temperature scale used in physics and astronomy. Unlike Fahrenheit and Celsius, Kelvin has no negative numbers. It starts at "Absolute Zero" (0 K), which is the theoretical point where all molecular movement completely stops. It uses the exact same increment size as Celsius, but starts much lower. 0 K = -273.15°C.

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