How to Convert Temperature — Celsius, Fahrenheit & Kelvin

What are the temperature scales?

Three temperature scales are in common use today, each developed independently and calibrated differently:

Celsius (°C) is the most widely used scale globally. It was defined so that 0°C is the freezing point of water and 100°C is its boiling point (at standard atmospheric pressure). Devised by Swedish astronomer Anders Celsius in 1742, it is the official scale in nearly every country in the world and is used universally in science alongside Kelvin.

Fahrenheit (°F) was developed by German physicist Daniel Gabriel Fahrenheit in 1724. In this scale, water freezes at 32°F and boils at 212°F. It remains in everyday use primarily in the United States. Body temperature is 98.6°F, and comfortable room temperature is around 72°F.

Kelvin (K) is the International System of Units (SI) base unit for thermodynamic temperature. It uses the same degree size as Celsius but starts at absolute zero — the theoretical minimum temperature at which all molecular motion ceases. 0 K = −273.15°C. There is no such thing as a negative Kelvin value. Kelvin is used in physics, chemistry, astrophysics, and engineering, where calculations require a ratio scale with a true zero point.

Temperature conversion formulas

These are the exact, standard formulas for every conversion direction:

• Celsius to Fahrenheit: °F = (°C × 9/5) + 32
• Fahrenheit to Celsius: °C = (°F − 32) × 5/9
• Celsius to Kelvin: K = °C + 273.15
• Kelvin to Celsius: °C = K − 273.15
• Fahrenheit to Kelvin: K = (°F − 32) × 5/9 + 273.15
• Kelvin to Fahrenheit: °F = (K − 273.15) × 9/5 + 32

How to convert temperature — step by step

Follow these steps for accurate temperature conversion:

1. Step 1: Identify the source and target scales. Know which scale your value is in (Celsius, Fahrenheit, or Kelvin) and which scale you need. For example, a US recipe says 350°F — you need to convert to Celsius for your European oven.
2. Step 2: Select the correct formula. For Fahrenheit to Celsius: °C = (°F − 32) × 5/9. Substituting: (350 − 32) × 5/9 = 318 × 0.5556 = 176.7°C. Round to 175°C for practical use.
3. Step 3: Apply the formula or use the converter. For mental math, use the shortcuts below. For precision, enter the value into our Temperature Converter — it handles all six conversion directions simultaneously.
4. Step 4: Check your answer against known reference points. Water freezes at 0°C/32°F, normal body temperature is 37°C/98.6°F, and a comfortable room is about 20°C/68°F. If your converted value seems far off from these landmarks, double-check the formula direction.
5. Step 5: For Kelvin conversions. Add or subtract 273.15 from the Celsius value. Absolute zero is −273.15°C = 0 K. Room temperature (~20°C) = 293.15 K. The surface of the Sun (~5,500°C) = 5,773 K.

Tips and best practices

• Use the "double minus 10% plus 32" shortcut for Celsius to Fahrenheit. Double the Celsius value, subtract 10% of that doubled value, then add 32. Example: 25°C → 50 − 5 + 32 = 77°F (exact answer: 77°F). This gives a close approximation fast.
• Remember key reference points. Memorize a handful of landmarks — 0°C = 32°F, 20°C = 68°F, 37°C = 98.6°F, 100°C = 212°F — and use them to sanity-check conversions and interpolate roughly for values in between.
• Be careful with negative Celsius values. When converting negative Celsius to Fahrenheit, the formula still works but be careful with the signs. −40°C is a special case: it equals −40°F exactly — the only temperature where the two scales coincide.
• Never report Kelvin with a degree symbol. Kelvin values are written as "273.15 K" not "273.15 °K." The degree symbol is reserved for Celsius and Fahrenheit. This is a common notation error in informal writing.

Frequently asked questions

How do I convert Celsius to Fahrenheit?

Use the formula: °F = (°C × 9/5) + 32. Multiply the Celsius value by 9, divide by 5, then add 32. Example: 30°C = (30 × 9/5) + 32 = 54 + 32 = 86°F. For a quick mental estimate, double the Celsius value, subtract 10%, and add 32 — this gets you within 1–2 degrees for most everyday temperatures.

How do I convert Fahrenheit to Celsius?

Use the formula: °C = (°F − 32) × 5/9. Subtract 32 from the Fahrenheit value first, then multiply by 5 and divide by 9. Example: 72°F = (72 − 32) × 5/9 = 40 × 0.5556 = 22.2°C. Always subtract 32 before multiplying — reversing the order gives a completely wrong answer.

What is Kelvin and when is it used?

Kelvin is the SI base unit of thermodynamic temperature, starting at absolute zero (0 K = −273.15°C), the theoretical minimum temperature in the universe. Unlike Celsius and Fahrenheit, it has no negative values. Converting Celsius to Kelvin is simple: K = °C + 273.15. Kelvin is used in all scientific and engineering disciplines where proportional temperature calculations are needed — physics, chemistry, astrophysics, and thermodynamics. Room temperature is approximately 293 K; the Sun's surface is about 5,778 K.

Which countries use Fahrenheit vs. Celsius?

The United States, its territories, and a small number of other jurisdictions use Fahrenheit for everyday temperature reporting — weather forecasts, ovens, and body temperature. The vast majority of the world — including all of Europe, Asia, Africa, South America, and Australia — uses Celsius. In scientific contexts worldwide, both Celsius and Kelvin are standard, while Fahrenheit is almost never used in research or engineering outside the US.

Conclusion

Temperature conversion is a fundamental skill for anyone who cooks international recipes, travels, follows science news, or works across cultural or disciplinary boundaries. The formulas are straightforward once you memorize them, and our free Temperature Converter makes it instant for any value. Bookmark the tool for whenever you encounter an unfamiliar temperature scale, and use the reference points and shortcuts above for quick mental estimates.

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