Celsius, Fahrenheit & Kelvin Explained
Temperature is one of the few measurements where you cannot just multiply by a factor to convert. Celsius, Fahrenheit, and Kelvin each start counting from a different zero and, in two of the three cases, step in differently sized degrees. Once you understand where each scale begins and how big its degree is, the conversion formulas make complete sense.
The Celsius scale
Celsius is the everyday scale for most of the world. It is anchored to water at standard atmospheric pressure: 0 degrees Celsius is the freezing point and 100 degrees Celsius is the boiling point. That clean 0-to-100 span makes Celsius intuitive for weather, cooking, and science alike. A comfortable room sits around 20 to 22 degrees Celsius, and normal human body temperature is about 37 degrees Celsius.
The Fahrenheit scale
Fahrenheit is the standard scale for daily use in the United States. On this scale water freezes at 32 degrees and boils at 212 degrees, so the same span of water covers 180 degrees rather than 100. That means a Fahrenheit degree is smaller than a Celsius degree: each Celsius degree equals 1.8 Fahrenheit degrees. Body temperature is about 98.6 degrees Fahrenheit, and a warm summer day might read 86 degrees Fahrenheit.
The Kelvin scale
Kelvin is the scale of physics and chemistry. Its degrees are exactly the same size as Celsius degrees, but it starts at absolute zero, the coldest temperature physically possible, where molecular motion is at its minimum. Absolute zero is 0 Kelvin, which equals minus 273.15 degrees Celsius. Because Kelvin never goes negative, it is ideal for equations where temperature must be a positive quantity. Note that Kelvin values are written without the word degree: you say 300 Kelvin, not 300 degrees Kelvin.
The reference points side by side
Lining up the same physical events across all three scales makes the relationships obvious:
- Absolute zero: -273.15 C = -459.67 F = 0 K
- Water freezes: 0 C = 32 F = 273.15 K
- Room temperature: about 21 C = about 70 F = about 294 K
- Body temperature: 37 C = 98.6 F = 310.15 K
- Water boils: 100 C = 212 F = 373.15 K
The conversion formulas
Here are the exact formulas. Notice that converting between Celsius and Fahrenheit needs both a multiplication (because the degrees differ in size) and an addition (because the zeros differ), while Celsius and Kelvin only need an addition.
- Celsius to Fahrenheit: F = (C x 9/5) + 32
- Fahrenheit to Celsius: C = (F - 32) x 5/9
- Celsius to Kelvin: K = C + 273.15
- Kelvin to Celsius: C = K - 273.15
- Fahrenheit to Kelvin: K = (F - 32) x 5/9 + 273.15
Worked examples
Let us convert a normal body temperature of 37 degrees Celsius into the other two scales step by step.
- To Fahrenheit: multiply 37 by 9/5 to get 66.6, then add 32 to get 98.6 degrees Fahrenheit.
- To Kelvin: add 273.15 to 37 to get 310.15 Kelvin.
Now the other direction. Suppose a US weather report says 68 degrees Fahrenheit and you want Celsius. Subtract 32 to get 36, then multiply by 5/9 to get 20 degrees Celsius, a pleasant room temperature.
A handy mental shortcut
For a quick Fahrenheit-to-Celsius estimate without a calculator, subtract 30 and halve the result. A reading of 70 degrees Fahrenheit becomes roughly (70 - 30) / 2 = 20 degrees Celsius, which is very close to the exact answer. To go from Celsius to Fahrenheit, double the Celsius value and add 30. These shortcuts are accurate enough for everyday weather but should not be used when precision matters.
When precision matters
Cooking, medicine, and lab work all depend on getting temperature exactly right, and the mental shortcuts can be off by a few degrees. When the number counts, use the exact formulas or let a converter handle them for you. Our Temperature converter applies these formulas precisely and switches between Celsius, Fahrenheit, and Kelvin instantly, so you can focus on the result rather than the arithmetic.