Temperature Conversion: Celsius, Fahrenheit & Kelvin
Temperature scales cause confusion whenever you read a foreign weather forecast, follow a recipe from another country, or work on a science problem. The three scales in common use — Celsius, Fahrenheit, and Kelvin — measure the same physical quantity but use completely different reference points and step sizes. This guide explains each scale, gives you the exact conversion formulas, and helps you build an intuitive feel for the numbers.
The Three Temperature Scales
- Celsius (°C): the metric scale, used by virtually every country except the United States. Water freezes at 0 °C and boils at 100 °C at sea level. Most weather forecasts, cooking recipes, and medical contexts outside the US use Celsius.
- Fahrenheit (°F): the scale used in the United States (and, historically, a few other countries). Water freezes at 32 °F and boils at 212 °F. The range 32–212 °F equals 0–100 °C.
- Kelvin (K): the SI unit of thermodynamic temperature, used in physics, chemistry, and engineering. It uses the same step size as Celsius but starts at absolute zero (−273.15 °C), the theoretical minimum temperature where molecular motion stops. Kelvin values are never written with a degree symbol.
Conversion Formulas
| From | To | 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 |
| Fahrenheit | Kelvin | K = (°F + 459.67) × 5/9 |
For instant results without memorising formulas, use the dedicated converters: Celsius to Fahrenheit, Fahrenheit to Celsius, Celsius to Kelvin, Kelvin to Celsius, and Fahrenheit to Kelvin.
Worked Examples
Example 1: Body temperature in Fahrenheit to Celsius. Normal body temperature is 98.6 °F. Converting: (98.6 − 32) × 5/9 = 66.6 × 0.5556 = 37.0 °C.
Example 2: Oven temperature in Celsius to Fahrenheit. A recipe says 180 °C. Converting: (180 × 9/5) + 32 = 324 + 32 = 356 °F (most ovens would be set to 350 °F as the nearest standard mark).
Example 3: Absolute zero in Kelvin to Celsius. 0 K − 273.15 = −273.15 °C. This is the coldest temperature physically possible.
Quick Reference: Common Temperature Benchmarks
| Description | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15 °C | −459.67 °F | 0 K |
| Water freezes | 0 °C | 32 °F | 273.15 K |
| Room temperature | ~22 °C | ~72 °F | ~295 K |
| Body temperature | 37 °C | 98.6 °F | 310.15 K |
| Water boils | 100 °C | 212 °F | 373.15 K |
Practical Tips for Each Context
Cooking: Most international recipes use Celsius; US recipes use Fahrenheit. A quick mental rule: double the Celsius value and add 30 to get an approximate Fahrenheit figure (e.g., 200 °C → 2×200+30 = 430 °F; actual is 392 °F, so use a converter for precision).
Weather: A Celsius temperature around 0 °C means freezing conditions; 20 °C is pleasant; 35 °C is very hot. For Fahrenheit, think 32 °F for freezing, 70 °F for comfortable, 95 °F for extreme heat.
Science: Kelvin is required for gas law calculations (e.g., the Ideal Gas Law PV = nRT) because equations break down if temperature can be zero or negative. Always convert to Kelvin before plugging values into thermodynamic formulas.
Common Mistakes
- Forgetting to apply the offset before scaling. The formula °F = °C × 1.8 alone is wrong — you must also add 32.
- Using 273 instead of 273.15. For most practical purposes 273 is fine, but in precise scientific work use 273.15.
- Writing “°K” instead of “K”. The Kelvin unit has no degree symbol.
Frequently Asked Questions
At what temperature are Celsius and Fahrenheit the same?
They are equal at −40°. Both scales read −40 at that point, which you can verify with the formula: (−40 × 9/5) + 32 = −72 + 32 = −40.
Why does the US still use Fahrenheit?
Historical inertia and the enormous cost of changing infrastructure, signage, and public conventions. The US is one of only three countries (with Liberia and Myanmar) that have not officially adopted the metric system for everyday use.
Can I use the Temperature Converter for all three scales at once?
Yes — the Temperature Converter accepts input in any scale and displays the equivalent in all three simultaneously, so you never need to do two separate conversions.