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Unit Conversions
Temperature Converter — Fahrenheit to Celsius, Kelvin & Back
The everyday math behind weather, ovens, fevers and science class
In the United States, temperature lives in two worlds at once. The thermostat, the weather app, your oven dial and your kid's fever all read in Fahrenheit (°F) — but recipes from abroad, science homework, lab work and almost every other country use Celsius (°C), and physics class throws in Kelvin (K) on top of that. Knowing how to switch between them quickly saves you from a ruined cake, a misread forecast, or a wrong answer on a test.
The two formulas you actually need are short. To go from Fahrenheit to Celsius, subtract 32 and multiply by 5/9: °C = (°F − 32) × 5/9. To go the other way, multiply by 9/5 and add 32: °F = (°C × 9/5) + 32. Kelvin is just Celsius shifted by the absolute-zero offset: K = °C + 273.15.
Here's a worked example. Normal body temperature is 98.6°F. Subtract 32 to get 66.6, multiply by 5/9, and you get 37°C — exactly the number you'll see on a European thermometer. Add 273.15 and that's 310.15 K. Going the other way, boiling water at 100°C becomes 100 × 9/5 + 32 = 212°F, and freezing at 0°C becomes 32°F.
The single most common mistake is multiplying before subtracting (or adding before multiplying). The order matters. For °F → °C you must subtract 32 first, then scale by 5/9. Do it backwards and a mild 70°F afternoon turns into nonsense. The other classic slip is forgetting that the two scales cross at −40°: −40°F equals exactly −40°C, the one point where both readings match.
A few reference points worth memorizing: water freezes at 32°F / 0°C and boils at 212°F / 100°C; a comfortable room is around 68–72°F (20–22°C); a fever starts near 100.4°F (38°C); and a hot summer day at 95°F is 35°C. This converter handles all three scales at once — type any value in any unit and instantly see it in Fahrenheit, Celsius and Kelvin together, with the exact arithmetic shown so you can check the work.
📖 See also: Ovulation and Your Fertile Window, Explained
Calculator
Fill in the fields and click "Calculate" for instant results.
📰 Formula
• °F → °C: C = (F − 32) × 5 / 9 • °C → °F: F = (C × 9 / 5) + 32 • °C → K: K = C + 273.15 • K → °C: C = K − 273.15 • Note: −40°F = −40°C (the scales meet there)
📰 Formula
• °F → °C: C = (F − 32) × 5 / 9 • °C → °F: F = (C × 9 / 5) + 32 • °C → K: K = C + 273.15 • K → °C: C = K − 273.15 • Note: −40°F = −40°C (the scales meet there)
🧪 Worked examples
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Example 2
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Example 3
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Example 4
⚠️ Common mistakes
- Multiplying by 5/9 before subtracting 32 (the order of operations matters).
- Adding 32 before multiplying when converting °C to °F.
- Confusing the 5/9 and 9/5 factors — use 5/9 going to Celsius, 9/5 going to Fahrenheit.
- Using 273 instead of 273.15 for the Kelvin offset (small but real).
💡 Tips
- Quick mental estimate: double the °C and add 30 to get a rough °F (20°C ≈ 70°F).
- Remember the anchors — 0°C/32°F freezing and 100°C/212°F boiling — to sanity-check any answer.
- Kelvin never goes negative; the lowest possible value is 0 K (absolute zero, −273.15°C).
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❓ Frequently asked questions
How do I convert Fahrenheit to Celsius?
Subtract 32, then multiply by 5/9. Example: (68 − 32) × 5/9 = 36 × 5/9 = 20°C.
How do I convert Celsius to Fahrenheit?
Multiply by 9/5, then add 32. Example: 25 × 9/5 + 32 = 45 + 32 = 77°F.
What is normal body temperature in Celsius?
98.6°F equals 37°C. A reading of 100.4°F (38°C) or higher is generally considered a fever.
What is 0 degrees Celsius in Fahrenheit?
0°C is 32°F — the freezing point of water. And 100°C is 212°F, the boiling point at sea level.
How do I convert Celsius to Kelvin?
Add 273.15. So 25°C = 298.15 K, and 0°C = 273.15 K. To go back, subtract 273.15.
Why is −40°F the same as −40°C?
It's the one point where the two formulas give the same number. The Fahrenheit and Celsius lines cross exactly at −40°.
What is room temperature in Celsius and Fahrenheit?
Comfortable room temperature is about 68–72°F, which is roughly 20–22°C.
What is a quick way to estimate Celsius to Fahrenheit in my head?
Double the Celsius value and add 30. 20°C → 40 + 30 ≈ 70°F (the exact answer is 68°F), close enough for weather.
What is absolute zero?
Absolute zero is 0 K, the coldest temperature possible. It equals −273.15°C, or about −459.67°F. Kelvin readings never go below it.