Definition · Plain-language
Temperature scales
Temperature scales are systems for measuring how hot or cold something is; the main ones are Celsius, Fahrenheit and the kelvin, the SI base unit of temperature.
The step most authors miss
Doing CRediT right? Don’t stop at the statement.
A CRediT statement credits you inside one paper. The recognition CRediT was built for happens when those roles are tied to you, persistently. Sign in with your ORCID — free — and claim your CRediT contributions on casrai.org, the home of the standard. They become a verified, portable part of your identity, not a line that disappears into one PDF.
Free: claim your contributions, then export a journal-ready CRediT statement, schema.org structured data, JATS XML, CSV or BibTeX — and preview your public profile. A membership publishes that profile publicly and verifies the journals you serve.
Celsius, Fahrenheit and kelvin
Three scales dominate. The Celsius scale, used in most of the world, sets 0 degrees at the freezing point of water and 100 at its boiling point under standard conditions, dividing the interval into a hundred degrees. The Fahrenheit scale, common mainly in the United States, places these landmarks at 32 and 212 degrees. The kelvin is the scientific scale and the SI base unit of temperature; it shares the Celsius degree size but starts its count from absolute zero, the coldest temperature possible, rather than from the freezing point of water.
Absolute zero and the kelvin
What makes the kelvin special is its zero. Absolute zero, 0 kelvin, is the point at which a system has the minimum possible thermal energy — it cannot be made colder. Because the kelvin scale begins there, it has no negative temperatures, which makes it the natural scale for physics, where many laws depend on absolute temperature. Zero kelvin corresponds to about minus 273.15 degrees Celsius. Since the 2019 redefinition, the kelvin is defined by fixing the value of the Boltzmann constant, tying temperature directly to energy rather than to the properties of water.
Converting between scales
Converting is straightforward once the offsets and step sizes are clear. To go from Celsius to kelvin, add 273.15; to go back, subtract it — the degree size is the same, so only the starting point shifts. Celsius and Fahrenheit differ in both: to convert Celsius to Fahrenheit, multiply by 9/5 and add 32; to reverse it, subtract 32 and multiply by 5/9. The two scales happen to read the same value, minus 40 degrees, at one point. A change of one kelvin and one degree Celsius are identical, which is why scientists often quote temperature differences in either.
Key facts
At a glance
- Definition: systems for measuring how hot or cold something is
- Celsius: water freezes at 0 °C, boils at 100 °C
- Fahrenheit: water freezes at 32 °F, boils at 212 °F
- Kelvin: the SI base unit, starting at absolute zero
- Absolute zero: 0 K ≈ −273.15 °C, the coldest possible
- Step size: 1 K equals 1 °C; Fahrenheit degrees are smaller
Common misconceptions
What people often get wrong
Often heard: The kelvin scale uses degrees, written °K.
Actually: It does not. The unit is simply the kelvin, symbol K, with no degree sign. We say "300 kelvin", not "300 degrees kelvin".
Often heard: A change of one degree Celsius is bigger than a change of one kelvin.
Actually: They are identical. The Celsius degree and the kelvin are the same size; the scales differ only in where their zero sits.
Often heard: You can reach temperatures below absolute zero.
Actually: Absolute zero is the lower limit of thermodynamic temperature — the minimum possible thermal energy. The kelvin scale has no negative values for this reason.
Going deeper
