Convert Between Celsius, Fahrenheit, Kelvin and More
India reports the weather in Celsius, but many digital thermometers, older medical charts, American recipes and oven dials use Fahrenheit. Science textbooks add Kelvin to the mix. This temperature converter handles five scales — Celsius, Fahrenheit, Kelvin, Rankine and Réaumur — and shows the exact calculation it used.
How to Use the Temperature Converter
- Enter a temperature in the From box. Negative values are allowed.
- Choose the scale you are converting from and the one you need.
- The answer appears instantly with the formula. Use Copy Result to paste it elsewhere.
Unlike other units, temperature conversion is not a simple multiplication — the scales start at different zero points — which is why a converter is especially handy here.
Temperature Conversion Formulas
| Conversion | Formula |
|---|---|
| Celsius → Fahrenheit | °F = °C × 9/5 + 32 |
| Fahrenheit → Celsius | °C = (°F − 32) × 5/9 |
| Celsius → Kelvin | K = °C + 273.15 |
| Kelvin → Celsius | °C = K − 273.15 |
| Celsius → Rankine | °Ra = (°C + 273.15) × 9/5 |
| Celsius → Réaumur | °Re = °C × 4/5 |
Worked example
Your thermometer reads 101 °F. In Celsius: (101 − 32) × 5/9 = 69 × 0.5556 = 38.3 °C — a mild fever.
Useful Reference Temperatures
| Situation | °C | °F | K |
|---|---|---|---|
| Absolute zero | −273.15 | −459.67 | 0 |
| Celsius equals Fahrenheit | −40 | −40 | 233.15 |
| Water freezes | 0 | 32 | 273.15 |
| Comfortable room | 24 | 75.2 | 297.15 |
| Normal body temperature | 37 | 98.6 | 310.15 |
| Fever threshold | 38 | 100.4 | 311.15 |
| Hot Indian summer day | 45 | 113 | 318.15 |
| Water boils (sea level) | 100 | 212 | 373.15 |
| Baking a cake | 180 | 356 | 453.15 |
Quick Mental Approximation
To estimate Celsius to Fahrenheit without a calculator, double the Celsius value and add 30. For 25 °C that gives 80 °F (the exact answer is 77 °F). It is close enough for weather, but use the exact formula for cooking, medicine or science.
Going the other way, subtract 30 and halve: 90 °F ≈ 30 °C (exact: 32.2 °C).
A Little History
- Fahrenheit (1724) was proposed by Daniel Gabriel Fahrenheit and is still used in the United States.
- Celsius (1742), by Anders Celsius, sets water’s freezing and boiling points 100 degrees apart. It is used almost everywhere else, including India.
- Kelvin (1848), named after Lord Kelvin, begins at absolute zero and is the scientific standard.
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