How to Convert Celsius to Fahrenheit (and Back)
Learn the temperature conversion formulas, memory tricks, and common reference points for Celsius and Fahrenheit.
Temperature conversion between Celsius and Fahrenheit is one of the most common conversions needed in daily life. Whether you're traveling, cooking, or checking the weather, understanding these formulas will serve you well.
The Conversion Formulas
Celsius to Fahrenheit:
°F = (°C × 9/5) + 32
Or equivalently: °F = (°C × 1.8) + 32
Fahrenheit to Celsius:
°C = (°F − 32) × 5/9
Or equivalently: °C = (°F − 32) ÷ 1.8
Why These Numbers?
The formulas look arbitrary, but they come from how the scales were defined:
- Celsius: 0° = water freezes, 100° = water boils (at sea level)
- Fahrenheit: 32° = water freezes, 212° = water boils
The 180° span in Fahrenheit (212 - 32) corresponds to 100° in Celsius, giving us the 9:5 ratio (180:100 simplified).
Memory Tricks
Here are some ways to remember without calculating:
- −40 is −40: This is where both scales meet (−40°C = −40°F)
- 16°C = 61°F: Just reverse the digits!
- 28°C = 82°F: Another digit reversal
- Double and add 30: Quick rough estimate (20°C → 40 + 30 = ~70°F)
Common Reference Points
Memorize these touchstones:
- 0°C = 32°F: Water freezes, ice forms
- 10°C = 50°F: Cool weather, light jacket
- 20°C = 68°F: Room temperature, comfortable
- 25°C = 77°F: Warm, pleasant day
- 30°C = 86°F: Hot summer day
- 37°C = 98.6°F: Normal body temperature
- 40°C = 104°F: Heat wave, fever
- 100°C = 212°F: Water boils
Weather Translation
For travelers converting weather forecasts:
- Below 0°C (32°F): Freezing, winter coat, risk of ice
- 0-10°C (32-50°F): Cold, need layers
- 10-15°C (50-59°F): Cool, jacket weather
- 15-20°C (59-68°F): Mild, sweater or light jacket
- 20-25°C (68-77°F): Comfortable, t-shirt weather
- 25-30°C (77-86°F): Warm to hot
- Above 30°C (86°F): Hot, seek shade and hydration
Kelvin: The Scientific Scale
Scientists use Kelvin because it starts at absolute zero — the coldest possible temperature where all molecular motion stops.
- 0 K = −273.15°C = −459.67°F (absolute zero)
- 273.15 K = 0°C = 32°F (water freezes)
- 373.15 K = 100°C = 212°F (water boils)
Conversion: K = °C + 273.15
Note: Kelvin uses no degree symbol — it's just "K" not "°K".
Practical Examples
Example 1: Convert 25°C to Fahrenheit
°F = (25 × 1.8) + 32 = 45 + 32 = 77°F ✓
Example 2: Convert 98.6°F to Celsius
°C = (98.6 − 32) ÷ 1.8 = 66.6 ÷ 1.8 = 37°C ✓
Example 3: Oven at 180°C in Fahrenheit?
°F = (180 × 1.8) + 32 = 324 + 32 = 356°F (usually rounded to 350°F)