Temperature is one of the few measurements that uses genuinely different scales rather than just different unit sizes. Converting between centimeters and inches is a matter of multiplication. Converting between Celsius and Fahrenheit requires both multiplication and addition because the scales have different zero points and different degree sizes. This makes temperature conversion less intuitive than most other unit conversions, which is why even people who are otherwise comfortable with unit arithmetic tend to reach for a calculator when temperatures come up.

Three scales account for the vast majority of temperature references you will encounter: Celsius, Fahrenheit and Kelvin. Each was developed for different purposes and each is still in active use in specific contexts. Understanding why each exists and what it is good for makes the conversions more meaningful than memorizing formulas alone.

The Celsius scale

The Celsius scale, sometimes still called centigrade, sets 0 degrees at the freezing point of water and 100 degrees at the boiling point of water at standard atmospheric pressure. This alignment with water's phase transitions makes the scale intuitive for everyday purposes because water is the liquid humans interact with most and the one most relevant to weather, cooking and health contexts.

Most countries use Celsius as the standard for everyday temperature reference. Weather forecasts, cooking recipes, medical measurements and scientific publications in most of the world use Celsius. The scale has the practical advantage that common temperature ranges for human experience, roughly minus 10 to 40 degrees, map to a reasonable two-digit number range without requiring negative numbers for most situations outside of cold winters.

One degree Celsius represents the same temperature change as one Kelvin, which makes converting between Celsius and scientific measurements expressed in Kelvin straightforward. The relationship between Celsius and Kelvin is a simple offset of 273.15, with 0 Kelvin corresponding to minus 273.15 Celsius.

The Fahrenheit scale

The Fahrenheit scale was developed by Daniel Gabriel Fahrenheit in the early 18th century. He calibrated 0 degrees to the coldest temperature he could reliably reproduce in his laboratory using a salt and ice mixture, and calibrated 96 degrees to approximately human body temperature. The modern standard adjusts these slightly, placing the freezing point of water at 32 degrees and the boiling point at 212 degrees.

The United States is the most notable country that uses Fahrenheit as the primary everyday temperature scale, along with a handful of other countries. Americans working in science or medicine use Celsius or Kelvin like everyone else, but weather reports, home thermostats, cooking temperatures and casual temperature references in the US use Fahrenheit.

One degree Fahrenheit is a smaller temperature change than one degree Celsius. Specifically, one Celsius degree equals 1.8 Fahrenheit degrees. This means Fahrenheit provides finer gradations for everyday temperature ranges, which some people find useful for distinguishing between, say, a warm day and a hot day. A change from 70 to 72 Fahrenheit is a noticeable but modest change, whereas the equivalent in Celsius is a change from about 21.1 to 22.2 degrees.

The Kelvin scale

Kelvin is the SI unit of temperature and the scale used in scientific contexts. Its zero point is absolute zero, the theoretical temperature at which all thermal motion ceases. There is no temperature below 0 Kelvin, which makes it the only temperature scale that cannot go negative. This property makes Kelvin useful for scientific calculations because it eliminates the need to handle negative temperatures in equations where temperature appears in ratios or products.

Kelvin does not use the degree symbol. You say 300 Kelvin, not 300 degrees Kelvin. The scale was named after Lord Kelvin, the physicist who proposed the concept of absolute zero and argued for a temperature scale based on it. Each unit of Kelvin is the same size as one degree Celsius, so converting between them is simply adding or subtracting 273.15.

Everyday contexts where Kelvin appears include color temperature of light sources, where daylight is approximately 5500 to 6500 Kelvin and warm incandescent light is around 2700 Kelvin. Photographers working with white balance and videographers working with color grading encounter Kelvin as a standard unit for describing the character of light sources.

The conversion formulas

Converting from Celsius to Fahrenheit multiplies the Celsius temperature by 9/5 and adds 32. Converting from Fahrenheit to Celsius subtracts 32 and then multiplies by 5/9. Converting from Celsius to Kelvin adds 273.15. Converting from Kelvin to Celsius subtracts 273.15.

A useful rough approximation for mental math from Celsius to Fahrenheit doubles the Celsius temperature and adds 30. This is not precise but gives a close enough estimate for casual reference. 20 Celsius becomes approximately 70 Fahrenheit by this method, which is close to the actual 68 Fahrenheit. 35 Celsius becomes approximately 100 Fahrenheit, close to the actual 95 Fahrenheit.

A few reference points are worth memorizing for quick orientation: 0 Celsius is 32 Fahrenheit, 100 Celsius is 212 Fahrenheit, 37 Celsius is 98.6 Fahrenheit (normal body temperature), and minus 40 Celsius equals exactly minus 40 Fahrenheit, the one point where the two scales coincide.

Practical contexts where you need temperature conversion

Cooking recipes from different countries use different scales. American recipes use Fahrenheit for oven temperatures. European and most other recipes use Celsius. Baking in particular requires accurate temperature for consistent results, so converting without rounding significantly is worth doing carefully.

Medical temperature references vary by country and context. A fever is described as above 38 Celsius or above 100.4 Fahrenheit. Knowing both reference points helps when reading medical information from sources written for different audiences.

1. Open the Temperature Converter below.
2. Enter the temperature value you want to convert.
3. Select the scale you are converting from.
4. See instant results in all three scales simultaneously.