The Quick Answer
To convert Fahrenheit to Celsius, subtract 32 and multiply by 5/9.
Formula: °C = (°F − 32) × 5/9
Example: 72°F → (72 − 32) × 5/9 = 40 × 0.556 = 22.2°C
To convert Celsius to Fahrenheit, multiply by 9/5 and add 32.
Formula: °F = (°C × 9/5) + 32
Example: 25°C → (25 × 1.8) + 32 = 45 + 32 = 77°F
Quick Reference Table
| Fahrenheit | Celsius | What it feels like |
|---|---|---|
| -40°F | -40°C | Extreme cold (scales meet) |
| 0°F | -18°C | Very cold winter day |
| 32°F | 0°C | Water freezes |
| 50°F | 10°C | Cool, light jacket weather |
| 68°F | 20°C | Room temperature |
| 72°F | 22°C | Comfortable indoor temp |
| 77°F | 25°C | Warm day |
| 86°F | 30°C | Hot summer day |
| 98.6°F | 37°C | Normal body temperature |
| 100°F | 37.8°C | Slight fever |
| 104°F | 40°C | High fever |
| 212°F | 100°C | Water boils |
| 350°F | 177°C | Moderate oven |
| 400°F | 204°C | Hot oven |
| 450°F | 232°C | Very hot oven |
Step-by-Step Conversion Examples
Example 1: Convert 98.6°F to Celsius
This is normal human body temperature.
- Start with 98.6°F
- Subtract 32: 98.6 − 32 = 66.6
- Multiply by 5/9: 66.6 × 0.556 = 37°C
Example 2: Convert 350°F to Celsius
A common oven temperature for baking.
- Start with 350°F
- Subtract 32: 350 − 32 = 318
- Multiply by 5/9: 318 × 0.556 = 177°C
Example 3: Convert 25°C to Fahrenheit
A pleasant warm day.
- Start with 25°C
- Multiply by 9/5: 25 × 1.8 = 45
- Add 32: 45 + 32 = 77°F
Example 4: Convert -10°C to Fahrenheit
A cold winter morning.
- Start with -10°C
- Multiply by 9/5: -10 × 1.8 = -18
- Add 32: -18 + 32 = 14°F
Mental Math Shortcuts
Exact conversion requires the full formula, but these shortcuts give quick estimates:
Fahrenheit to Celsius (Approximate)
Subtract 30, then divide by 2.
- 80°F → (80 − 30) ÷ 2 = 25°C (actual: 26.7°C)
- 100°F → (100 − 30) ÷ 2 = 35°C (actual: 37.8°C)
This method is within 2-3 degrees for everyday temperatures.
Celsius to Fahrenheit (Approximate)
Double it and add 30.
- 20°C → (20 × 2) + 30 = 70°F (actual: 68°F)
- 30°C → (30 × 2) + 30 = 90°F (actual: 86°F)
The -40° Rule
At exactly -40°, Fahrenheit and Celsius are equal. This is the only point where the two scales intersect.
Why Do We Have Two Temperature Scales?
Fahrenheit (1724)
Daniel Fahrenheit, a German physicist, created the Fahrenheit scale. He set:
- 0°F as the temperature of a brine solution (the coldest he could reliably produce)
- 96°F as human body temperature (later refined to 98.6°F)
- 32°F as the freezing point of water
- 212°F as the boiling point of water
Fahrenheit is used primarily in the United States, the Bahamas, and a few other countries.
Celsius (1742)
Anders Celsius, a Swedish astronomer, created the Celsius scale based on water:
- 0°C is the freezing point of water
- 100°C is the boiling point of water (at standard atmospheric pressure)
Celsius (sometimes called Centigrade) is used worldwide for science and in most countries for everyday purposes.
Which is Better?
Neither is objectively "better." Celsius makes more intuitive sense for scientific applications since it's based on water's physical properties. Fahrenheit arguably provides more precision for weather without decimals (the 0-100°F range covers most habitable temperatures nicely).
Common Conversion Questions
What is 100°F in Celsius?
100°F = 37.8°C
This is just above normal body temperature, indicating a slight fever if measured in a person.
What is freezing point in both scales?
Water freezes at 32°F or 0°C at standard atmospheric pressure.
What temperature is a fever?
A fever is typically defined as a body temperature above:
- 100.4°F (38°C) in medical contexts
- 99.5°F (37.5°C) is often considered a low-grade fever
What is room temperature?
Room temperature is generally considered:
- 68-72°F (20-22°C) for indoor comfort
- 77°F (25°C) in scientific contexts
What is 0°F in Celsius?
0°F = -17.8°C
This is a very cold temperature, typical of a harsh winter day.
When are Fahrenheit and Celsius the same?
At exactly -40°, both scales read the same value. This is the only temperature where °F = °C.
Oven Temperature Conversions
Cooking often requires temperature conversions. Here are common oven settings:
| Description | Fahrenheit | Celsius | Gas Mark |
|---|---|---|---|
| Very slow | 250°F | 120°C | ½ |
| Slow | 300°F | 150°C | 2 |
| Moderate | 350°F | 175°C | 4 |
| Moderately hot | 375°F | 190°C | 5 |
| Hot | 400°F | 200°C | 6 |
| Very hot | 450°F | 230°C | 8 |
| Extremely hot | 500°F | 260°C | 10 |
Weather Temperature Context
Understanding temperature ranges helps you know what to expect:
| Range (°F) | Range (°C) | Conditions |
|---|---|---|
| Below 0°F | Below -18°C | Extreme cold, frostbite risk |
| 0-32°F | -18 to 0°C | Freezing, ice and snow |
| 32-50°F | 0-10°C | Cold, winter jacket needed |
| 50-68°F | 10-20°C | Cool, light layers |
| 68-77°F | 20-25°C | Comfortable, ideal |
| 77-86°F | 25-30°C | Warm, summer weather |
| 86-95°F | 30-35°C | Hot, stay hydrated |
| Above 95°F | Above 35°C | Very hot, heat advisory |
The Mathematical Relationship
The formulas exist because the two scales have different zero points and different degree sizes.
- The freezing point offset: 32°F = 0°C
- The degree ratio: 180 Fahrenheit degrees = 100 Celsius degrees, so 9°F = 5°C
This gives us the conversion factor 5/9 (or its inverse, 9/5 = 1.8).
Fahrenheit to Celsius: °C = (°F − 32) × 5/9
Celsius to Fahrenheit: °F = (°C × 9/5) + 32
Summary
- F to C: Subtract 32, multiply by 5/9
- C to F: Multiply by 9/5, add 32
- Quick estimate F to C: Subtract 30, divide by 2
- Quick estimate C to F: Double it, add 30
- Equal point: -40° is the same in both scales
- Water freezes: 32°F = 0°C
- Water boils: 212°F = 100°C
- Body temp: 98.6°F = 37°C
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