Celsius, Fahrenheit and Kelvin: How the Temperature Scales Work
Most of the world says it is 20 degrees outside, the United States says 68, and a physicist might say 293. All three are describing the same pleasant spring day.
Temperature is unusual among units because its scales don’t just differ in size, they also start from different zero points. This guide explains where each scale comes from, how to convert exactly and in your head, and the one conversion mistake that even careful people make.
Three scales, three starting points
Celsius (°C) sets 0° at the freezing point of water and 100° at its boiling point at sea level. Anders Celsius proposed the scale in 1742. Oddly, his original version ran the other way, with water boiling at 0° and freezing at 100°, and it was reversed soon after his death.
Fahrenheit (°F) was defined by Daniel Gabriel Fahrenheit in the early 1700s. On his scale, water freezes at 32° and boils at 212°, so there are 180 Fahrenheit degrees between the two points instead of 100. Today Fahrenheit is used mainly in the United States and a handful of other countries.
Kelvin (K) is the scientific scale. It starts at absolute zero, the lowest temperature physically possible, where particles have the minimum possible energy. One kelvin is the same size as one degree Celsius, so the scales are simply shifted: 0 K is −273.15 °C. Since 1968 the unit has been written as “kelvin” without “degree”, so it is 293 K, not 293 °K.
| Reference point | Celsius | Fahrenheit | Kelvin |
|---|---|---|---|
| Absolute zero | −273.15 °C | −459.67 °F | 0 K |
| Celsius equals Fahrenheit | −40 °C | −40 °F | 233.15 K |
| Water freezes | 0 °C | 32 °F | 273.15 K |
| Comfortable room | 20 °C | 68 °F | 293.15 K |
| Human body | 37 °C | 98.6 °F | 310.15 K |
| Water boils (sea level) | 100 °C | 212 °F | 373.15 K |
The exact formulas
Because the zero points differ, converting between Celsius and Fahrenheit needs both a multiplication and an addition:
°F = °C × 9/5 + 32
°C = (°F − 32) × 5/9
Kelvin only needs an offset:
K = °C + 273.15
°C = K − 273.15
Example: convert 25 °C to Fahrenheit.
25 × 9/5 + 32 = 45 + 32 = 77 °F
Example: convert 50 °F to Celsius.
(50 − 32) × 5/9 = 18 × 5/9 = 10 °C
The Celsius to Fahrenheit and Fahrenheit to Celsius converters show the result for any value instantly.
A mental shortcut for the weather
For everyday temperatures, double the Celsius value and add 30:
20 °C → 20 × 2 + 30 = 70 °F (exact: 68 °F)
To go the other way, subtract 30 and halve:
80 °F → (80 − 30) ÷ 2 = 25 °C (exact: 26.7 °C)
The shortcut is within a few degrees between about 0 °C and 30 °C, which covers most weather. It gets worse at the extremes, so don’t use it for cooking, medicine or anything technical.
The mistake almost everyone makes: temperature differences
Suppose a weather report says it will be 10 °C warmer tomorrow. How much warmer is that in Fahrenheit?
Plugging 10 into the formula gives 50 °F, but that is wrong. The formula converts a temperature reading, and the “+ 32” exists only because the zero points differ. A difference between two temperatures has no zero point, so only the size of the degree matters:
A change of 10 °C = a change of 10 × 9/5 = 18 °F
The same applies to tolerances, oven calibration and climate data. A global temperature rise of 1.5 °C is a rise of 2.7 °F, not 34.7 °F. Between Celsius and kelvin, a difference is the same number in both: a rise of 1 °C is a rise of 1 K.
Why scientists use kelvin
Many physical laws depend on absolute temperature, measured from absolute zero. In the ideal gas law, for example, doubling the kelvin temperature of a gas at constant volume doubles its pressure. Doubling a Celsius temperature means nothing physically: 20 °C is not “twice as hot” as 10 °C. In kelvin the comparison is 293 K versus 283 K, only about 3.5 % warmer.
Since 2019, the kelvin has been defined by fixing the value of the Boltzmann constant, which links temperature to energy, rather than by the properties of water. For everyday purposes nothing changed: water still freezes at about 0 °C.
Key takeaways
- Celsius and Fahrenheit have different zero points and different degree sizes; kelvin has the same degree size as Celsius but starts at absolute zero.
- °F = °C × 9/5 + 32; K = °C + 273.15.
- For weather, “double and add 30” gets you within a few degrees.
- Convert temperature differences by the degree size only: 1 °C of change = 1.8 °F of change.
- −40 is the one temperature that is the same in Celsius and Fahrenheit.
Convert any temperature with the temperature converter, including kelvin and the less common Rankine and Réaumur scales.