Degree Celsius
SI derived unit · quantity: temperature
01 · Definition
The degree Celsius equals the kelvin: the same step, a shifted zero. A difference of temperatures comes out as the same number in both scales, while the temperatures themselves differ by exactly 273.15.
The scale was historically set by two fixed points: melting ice and boiling water. The lower one holds firm — pressure shifts it by thousandths of a degree. The upper one does not hold at all: water boils at a hundred degrees only at sea level, and noticeably earlier in the mountains. That is why the modern definition gave up boiling and left the Celsius scale a single anchor: it is defined through the kelvin, and since 2019 the kelvin is defined through the Boltzmann constant.
Heating from twenty to thirty degrees Celsius is the same ten kelvins, and in thermal engineering differences are written in kelvins no matter which scale the temperatures themselves are given in. The distinction shows only where temperature enters a formula on its own: in the ideal gas law, in black-body radiation, in the efficiency of the Carnot cycle. Putting degrees Celsius there is a common mistake that yields an absurd answer, because all these laws count from absolute zero.
02 · Conversion
Enter a temperature and the passport will lay it out across the scales
Temperature is converted not by a factor but by a factor with an offset: every scale has its own zero and its own size of degree. That is why zero never maps to zero in any pair except the kelvin and the Rankine.
The Celsius and Fahrenheit scales cross at exactly one point: minus forty means the same in both. It follows from the Fahrenheit degree being five ninths of the Celsius one while the zeros of the scales are thirty-two divisions apart. The kelvin and the Rankine are arranged differently: they share a zero and differ in the size of the degree, so their ratio is constant and equal to one point eight.
| Scale | Name | Value | Where it is met |
|---|---|---|---|
| K | kelvin, the SI base unit | 273.15 | physics, astronomy, colour temperature |
| °C | degree Celsius | 0 | weather, medicine, engineering |
| °F | degree Fahrenheit | 32 | the USA, the Bahamas, Belize, the Cayman Islands |
| °Ra | degree Rankine | 491.67 | thermal engineering in the USA |
| °Ré | degree Réaumur | 0 | 19th-century Russia and France, cheesemaking |
| °De | degree Delisle | 150 | 18th-century Russia, the scale runs backwards |
| °N | degree Newton | 0 | the first attempt at a scale, 1701 |
| K | kelvin | 273.15 | a reference point for comparison |
The kelvin and Celsius differ only by a shift; Fahrenheit differs in the size of the degree as well. In formulas where temperature enters on its own rather than as a difference, only an absolute scale will do
03 · Orders of magnitude
from a nanokelvin to a stellar coreMelting ice, 0 °C
04 · Measuring instruments
What degrees Celsius are measured with
A column of alcohol or galinstan rises with temperature: the scale is set by two fixed points and the interval is divided evenly. Mercury has been driven out of household instruments by the Minamata Convention, yet it was mercury thermometers that for two centuries defined what counted as a degree.
The resistance of pure platinum rises with temperature predictably and reproducibly, which is why this thermometer serves as the standard from hydrogen to silver. An industrial Pt100 gives exactly one hundred ohms at zero degrees.
A junction of two dissimilar wires produces a voltage of a few millivolts, depending on the difference between the temperatures of the junction and of the instrument. It is cheap, small and endures fifteen hundred degrees, so it sits in furnaces, turbines and ovens.
It reads the temperature from a body’s own infrared radiation without touching it. The reading depends on the emissivity of the surface: shiny metal is read too low, and that is the chief source of error in non-contact thermometry.
05 · Writing rules
A space before the symbol, and never “C°”
°C is a compound symbol: the degree sign sits tight against a capital Latin C, while a non-breaking space goes between the number and the symbol, as with any other unit. The kelvin is written with no degree sign at all — that prefix was taken from it in 1967.
The degree sign is its own character, U+00B0, not a superscript zero and not the letter “o”. Unicode also has a ready-made ligature ℃ (U+2103), but the standards recommend typing the two characters: the ligature travels badly between fonts and breaks text search. Negative values are written with the minus sign U+2212, not with a hyphen.
06 · Neighbouring units
The kelvin is the elder of the family: an SI base unit from which Celsius differs only in where the zero sits. Fahrenheit lives a life of its own in three countries; Rankine is an absolute scale with the Fahrenheit degree.
the same degree, one zero lower
everyday temperature
the USA, the Bahamas, Belize
Of the Simetrium passports the degree Celsius is flanked by kelvin as the base unit of temperature, degree Fahrenheit and degree Rankine — the Anglo-American pair, and also joule, through which temperature is tied to the energy of particle motion.
07 · Historical section
The scale that was turned over after its author died
Anders Celsius put zero at the boiling of water and a hundred at the melting of ice: the colder it was, the greater the number. The logic was meteorological — in Uppsala negative readings then almost never arose.
A year after Celsius died the scale was flipped into the form we know. The authorship is credited both to Linnaeus, who ordered a thermometer for his greenhouse, and to Mårten Strömer, Celsius’s successor at the observatory; there is no firm evidence for either.
Until 1948 the scale was called centigrade, but in French and Spanish that word meant a hundredth of a right angle. The General Conference closed the ambiguity by giving the scale its author’s name.
First the scale was redefined through the triple point of water, then ITS-90 laid out practical fixed points from hydrogen to copper, and in 2019 the kelvin was fixed by the Boltzmann constant. The boiling of water ceased to be a definition and became merely a fact.
The zero of the scale is not the melting point
Once it was redefined through the kelvin, zero degrees Celsius stopped being the melting point of ice by definition — it is now simply 273.15 K. At normal pressure ice melts about a ten-thousandth of a degree lower, and that difference is not a measurement error but a consequence of untying the definition from a substance. The water in a triple-point cell must have the isotopic composition of ocean water: heavy isotopes of hydrogen and oxygen shift the point by a quarter of a millikelvin, and the standard specifies the composition separately. Since 2019 even that is gone: the kelvin is set by a numerical value of the Boltzmann constant, and triple-point cells have moved from the definition into the class of working standards, alongside the freezing points of gallium, indium and silver.
Catalogue · units of measurement
A passport for every quantity
Seven SI base units, twenty-two derived units with names of their own, and the non-SI units without which neither engineering nor everyday life gets by. Each has its own sheet: definition, conversion, instruments, writing rules. In 4 languages.
SI derived units with names of their own
the degree Celsius passport is openSI base units
in terracotta — the kelvin, from which Celsius is obtained by a shiftTemperature scales that have gone out of use
Passport language
4 languages. The symbol °C is international, but the everyday scale people are used to is not: in the translations for the USA and Belize the passport opens with the conversion to Fahrenheit.