Celsius (temperature) - History, Temperatures and intervals, The melting and boiling points of water, The special Unicode °C character

A temperature scale that takes the triple point of water to be 0·01°C, which corresponds roughly to taking the freezing point of water as 0°C and the boiling point as 100°C; named after Anders Celsius; a change in temperature of one degree Celsius is equal to a change in temperature of one Kelvin; a temperature in degrees Celsius is still often called by the old name degrees Centigrade.

Celsius temperature conversion formulas

To find	From	Formula
Fahrenheit	Celsius	°F = (°C × 1.8) + 32
Celsius	Fahrenheit	°C = (°F − 32) ÷ 1.8
kelvin	Celsius	K = °C + 273.15
Celsius	kelvin	°C = K − 273.15
For temperature intervals rather than specific temperatures, 1 °C = 1 kelvin and 1 °C = 1.8 °F Comparisons among various temperature scales Conversion calculator for units of temperature

Celsius is, or relates to, the Celsius temperature scale.

Until 1954, 0 °C on the Celsius scale was defined as the melting point of ice and 100 °C was the boiling point of water under a pressure of one standard atmosphere. Today, the unit “degree Celsius” and the Celsius scale are, by international agreement, defined by two points: absolute zero, and the triple point of specially prepared (VSMOW) water. This definition does three things: 1) it fixes the magnitude of the degree Celsius as being precisely 1 part in 273.16 parts the difference between absolute zero and the triple point of water; and 3) it establishes the difference between the two scales’ null points as being precisely 273.15 degrees Celsius (−273.15 °C = 0 K and 0.01 °C = 273.16 K).

	Kelvin	Celsius	Fahrenheit
Absolute zero (precisely, by definition)	0 K	−273.15 °C	−459.67 °F
Melting point of ice	273.15 K	0 °C	32 °F
Water’s triple point (precisely, by definition)	273.16 K	0.01 °C	32.018 °F
Water’s boiling point A	373.1339 K	99.9839 °C	211.9710 °F

A For Vienna Standard Mean Ocean Water at one standard atmosphere (101.325 kPa) when calibrated solely per the two-point definition of thermodynamic temperature. For more about the actual boiling point of water, see The melting and boiling points of water below, as well as VSMOW water in temperature measurement.

History

In 1742, Anders Celsius (1701 – 1744) created a “backwards” version of the modern Celsius temperature scale whereby zero represented the boiling point of water and 100 represented the melting point of ice.

In Celsius’s scale upon receipt of his first thermometer featuring a scale where zero represented the melting point of ice and 100 represented water’s boiling point. Accordingly, the following are permissible ways to express degree Celsius: singular / (plural)

degree Celsius / (degrees Celsius) deg Celsius / (same) degree C / (degrees C) deg C / (same) °C / (same)

As with most other unit symbols and all the temperature symbols, a space is placed between the numeric value and the °C symbol;

Temperatures and intervals

The degree Celsius is a special name for the kelvin for use in expressing Celsius temperatures. 

In science (especially) and in engineering, the Celsius and Kelvin scales are often used simultaneously in the same article (e.g. “…its measured value was 0.01023 °C with an uncertainty of 70 µK…”) Notwithstanding the official endorsements of Resolution 3 of the 13th CGPM (1967/68)] and Resolution 7 of the 9th CGPM (1948), the practice of simultaneously using both “°C” and “K” remains widespread throughout the technical world as the use of SI prefixed forms such as “µ°C” or “millidegrees Celsius” to express a temperature interval has not been well-adopted.

The melting and boiling points of water

The effect of defining the Celsius scale at the triple point of VSMOW water (273.16 kelvins and 0.01 °C), and at absolute zero (zero kelvins and −273.15 °C), is that both the melting and boiling points of water under one standard atmosphere (1013.25 mbar) are no longer the defining points for the Celsius scale. In 1948 when the 9th General Conference on Weights and Measures (CGPM) in Resolution 3 first considered using the triple point of water as a defining point, the triple point was so close to being 0.01 °C greater than water’s known melting point, it was simply defined as precisely 0.01 °C. When calibrated to ITS-90 (a calibration standard comprising many definition points and commonly used for high-precision instrumentation), the boiling point of VSMOW water is slightly less, about 99.974 °C.

This boiling–point difference of 16.1 millikelvins (thousandths of a degree Celsius) between the Celsius scale’s original definition and the current one (based on absolute zero and the triple point) has little practical meaning in real life because water’s boiling point is extremely sensitive to variations in barometric pressure.

The special Unicode °C character

Unicode, which is an industry standard designed to allow text and symbols from all of the writing systems of the world to be consistently represented and manipulated by computers, includes a special “°C” character at U+2103. To better see the difference between the two, below in brown text is the degree Celsius character followed immediately by the two-component version:

℃°C

When viewed on computers that properly support and map Unicode, the above line may be similar to the line below (size may vary):

Depending on the operating system, Web browser, and the default font, the “C” in the Unicode character may be narrower and slightly taller than a plain uppercase C;