Celsius scale, previously known as the centigrade scale, is
a temperature scale used by the
International System of Units
International System of Units (SI). As
an SI derived unit, it is used by all countries in the world, except
the United States, Myanmar, and Liberia. It is named after the Swedish
Anders Celsius (1701–1744), who developed a similar
temperature scale. The degree
Celsius (symbol: °C) can refer to a
specific temperature on the
Celsius scale as well as a unit to
indicate a temperature interval, a difference between two temperatures
or an uncertainty. Before being renamed to honor
Anders Celsius in
1948, the unit was called centigrade, from the Latin centum, which
means 100, and gradus, which means steps.
Before 1954, the
Celsius scale was based on 0 °C for the
freezing point of water and 100 °C for the boiling point of
water at 1 atm pressure following a change introduced in 1743 by
Jean-Pierre Christin to reverse the
Celsius thermometer scale (from
water boiling at 0 degrees and ice melting at 100 degrees). This scale
is widely taught in schools today.
By international agreement, since 1954 the unit "degree Celsius" and
Celsius scale are defined by absolute zero and the triple point of
Vienna Standard Mean Ocean Water
Vienna Standard Mean Ocean Water (VSMOW), a specially purified water.
This definition also precisely relates the
Celsius scale to the Kelvin
scale, which defines the
SI base unit
SI base unit of thermodynamic temperature
with symbol K. Absolute zero, the lowest temperature possible, is
defined as being exactly 0 K and −273.15 °C. The
temperature of the triple point of water is defined as exactly
273.16 K (0.01 °C; 32.02 °F). Thus, a temperature
difference of one degree
Celsius and that of one kelvin is exactly the
same, with the null point of the
Kelvin scale (0 K) at exactly
−273.15 °C, and the null point of the
(0 °C) at exactly 273.15 K.
1.1 Centigrade, hectograde and Celsius
1.2 Common temperatures
2 Name and symbol typesetting
3 Temperatures and intervals
4 Coexistence of
5 Melting and boiling points of water
6 See also
9 External links
An illustration of Anders Celsius's original thermometer. Note the
reversed scale, where 100 is the freezing point of water and 0 is its
In 1742, Swedish astronomer
Anders Celsius (1701–1744) created a
temperature scale which was the reverse of the scale now known by the
name "Celsius": 0 represented the boiling point of water, while 100
represented the freezing point of water. In his paper Observations
of two persistent degrees on a thermometer, he recounted his
experiments showing that the melting point of ice is essentially
unaffected by pressure. He also determined with remarkable precision
how the boiling point of water varied as a function of atmospheric
pressure. He proposed that the zero point of his temperature scale,
being the boiling point, would be calibrated at the mean barometric
pressure at mean sea level. This pressure is known as one standard
atmosphere. The BIPM's 10th General Conference on Weights and Measures
(CGPM) later defined one standard atmosphere to equal precisely
1013250dynes per square centimetre (101.325 kPa).
In 1743, the Lyonnais physicist Jean-Pierre Christin, permanent
secretary of the Académie des sciences, belles-lettres et arts de
LyonFR, working independently of Celsius, developed a scale where zero
represented the freezing point of water and 100 represented the
boiling point of water. On 19 May 1743 he published the design
of a mercury thermometer, the "
Thermometer of Lyon" built by the
craftsman Pierre Casati that used this scale.
In 1744, coincident with the death of Anders Celsius, the Swedish
Carl Linnaeus (1707–1778) reversed Celsius's scale. His
custom-made "linnaeus-thermometer", for use in his greenhouses, was
made by Daniel Ekström, Sweden's leading maker of scientific
instruments at the time and whose workshop was located in the basement
of the Stockholm observatory. As often happened in this age before
modern communications, numerous physicists, scientists, and instrument
makers are credited with having independently developed this same
scale; among them were Pehr Elvius, the secretary of the Royal
Swedish Academy of Sciences (which had an instrument workshop) and
with whom Linnaeus had been corresponding; Daniel Ekström[SV], the
instrument maker; and Mårten Strömer (1707–1770) who had studied
astronomy under Anders Celsius.
The first known Swedish document reporting temperatures in this
Celsius scale is the paper Hortus Upsaliensis dated
16 December 1745 that Linnaeus wrote to a student of his, Samuel
Nauclér. In it, Linnaeus recounted the temperatures inside the
orangery at the University of Uppsala Botanical Garden:
...since the caldarium (the hot part of the greenhouse) by the angle
of the windows, merely from the rays of the sun, obtains such heat
that the thermometer often reaches 30 degrees, although the keen
gardener usually takes care not to let it rise to more than 20 to 25
degrees, and in winter not under 15 degrees...
Centigrade, hectograde and Celsius
Since the 19th century, the scientific and thermometry communities
worldwide referred to this scale as the centigrade scale. Temperatures
on the centigrade scale were often reported simply as degrees or, when
greater specificity was desired, as degrees centigrade (symbol: °C).
Because the term centigrade was also the Spanish and French language
name for a unit of angular measurement (1/10000 of a right angle) and
had a similar connotation in other languages, the term centesimal
degree (known as the gradian, "grad" or "gon": 1ᵍ = 0.9°, 100ᵍ =
90°) was used when very precise, unambiguous language was required by
international standards bodies such as the BIPM. More properly, what
was defined as "centigrade" then would now be "hectograde".
Furthermore, in the context here, centigrade/hectograde is referring
to the whole 0–100 range, not the given part thereof, hence "20°
centigrade" means "20ᵍ per 100 gradians" (or 20% hectograde), not
its literal description, "0.2 gradians".
(To be descriptively correct, "20° centigrade" should be "20°
hectocentigrade", or just "20 gradians" 20ᵍ .)
To eliminate such confusion, the 9th
CGPM and the CIPM (Comité
international des poids et mesures) formally adopted "degree Celsius"
in 1948,[a] formally keeping the recognized degree symbol, rather
than adopting the gradian/centesimal degree symbol.
For scientific use, "Celsius" is the term usually used, with
"centigrade" otherwise continuing to be in common but decreasing use,
especially in informal contexts in English-speaking countries. It
was not until February 1985 that the forecasts issued by the BBC
switched from "centigrade" to "Celsius".
Some key temperatures relating the
Celsius scale to other temperature
scales are shown in the table below.
Key scale relations
(precisely, by definition)
Boiling point of liquid nitrogen
Sublimation point of dry ice.
Melting point of H2O (purified ice)
Water's triple point
(precisely, by definition)
Normal human body temperature
Normal human body temperature (approximate average)
Water's boiling point at 1 atm (101.325 kPa)
(approximate: see Boiling point)[b]
Name and symbol typesetting
The "degree Celsius" has been the only SI unit whose full unit name
contains an uppercase letter since the
SI base unit
SI base unit for temperature,
the kelvin, became the proper name in 1967 replacing the term degrees
Kelvin. The plural form is degrees Celsius.
The general rule of the International Bureau of Weights and Measures
(BIPM) is that the numerical value always precedes the unit, and a
space is always used to separate the unit from the number, e.g. "30.2
°C" (not "30.2°C" or "30.2° C"). Thus the value of the
quantity is the product of the number and the unit, the space being
regarded as a multiplication sign (just as a space between units
implies multiplication). The only exceptions to this rule are for the
unit symbols for degree, minute, and second for plane angle (°, ′,
and ″, respectively), for which no space is left between the
numerical value and the unit symbol. Other languages, and various
publishing houses, may follow different typographical rules.
Unicode provides the
Celsius symbol at codepoint U+2103 ℃ degree
celsius. However, this is a compatibility character provided for
roundtrip compatibility with legacy encodings. It easily allows
correct rendering for vertically written East Asian scripts, such as
Unicode standard explicitly discourages the use of this
character: "In normal use, it is better to represent degrees Celsius
"°C" with a sequence of U+00B0 ° degree sign + U+0043 C latin
capital letter c, rather than U+2103 ℃ degree celsius. For
searching, treat these two sequences as identical."
Shown below is the degree
Celsius character followed immediately by
the two-component version:
When viewed on computers that properly support Unicode, the above line
may be similar to the image in the line below (enlarged for clarity):
The canonical decomposition is simply an ordinary degree sign and "C",
so some browsers may simply display "°C" in its place due to Unicode
Temperatures and intervals
Celsius is a special name for the kelvin for use in
Celsius temperatures. The degree
Celsius is also
subject to the same rules as the kelvin with regard to the use of its
unit name and symbol. Thus, besides expressing specific temperatures
along its scale (e.g. "
Gallium melts at 29.7646 °C" and "The
temperature outside is 23 degrees Celsius"), the degree
also suitable for expressing temperature intervals: differences
between temperatures or their uncertainties (e.g. "The output of the
heat exchanger is hotter by 40 degrees Celsius", and "Our standard
uncertainty is ±3 °C"). Because of this dual usage, one
must not rely upon the unit name or its symbol to denote that a
quantity is a temperature interval; it must be unambiguous through
context or explicit statement that the quantity is an interval.[c]
This is sometimes solved by using the symbol °C (pronounced "degrees
Celsius") for a temperature, and C° (pronounced "
for a temperature interval, although this usage is non-standard.
What is often confusing about the
Celsius measurement is that it
follows an interval system but not a ratio system; that it follows a
relative scale not an absolute scale. This is put simply by
illustrating that while 10 °C and 20 °C have the same
interval difference as 20 °C and 30 °C, the temperature
20 °C is not twice the air heat energy as 10 °C. As this
example shows, degrees
Celsius is a useful interval measurement but
does not possess the characteristics of ratio measures like weight or
In science and in engineering, the
Celsius scale and the
are often used in combination in close contexts, e.g. "...a measured
value was 0.01023 °C with an uncertainty of 70 µK...".
This practice is permissible because the magnitude of the degree
Celsius is equal to that of the kelvin, but referring to a
"microdegree Celsius" would be awkward.
Notwithstanding the official endorsement provided by decision #3 of
Resolution 3 of the 13th CGPM, which stated "a temperature interval
may also be expressed in degrees Celsius", the practice of
simultaneously using both °C and K remains widespread throughout the
scientific world as the use of SI-prefixed forms of the degree Celsius
(such as "µ°C" or "microdegrees Celsius") to express a temperature
interval has not been well-adopted.
Melting and boiling points of water
One effect of defining the
Celsius scale at the triple point of Vienna
Standard Mean Ocean Water (VSMOW, 273.16 K and 0.01 °C),
and at absolute zero (0 K and −273.15 °C), is that
neither the melting nor boiling point of water under one standard
atmosphere (101.325 kPa) remains a defining point 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. However, current measurements
show that the difference between the triple and melting points of
VSMOW is actually very slightly (<0.001 °C) greater than
0.01 °C. Thus, the actual melting point of ice is very slightly
(less than a thousandth of a degree) below 0 °C. Also, defining
water's triple point at 273.16 K precisely defined the magnitude
of each 1 °C increment in terms of the absolute thermodynamic
temperature scale (referencing absolute zero). Now decoupled from the
actual boiling point of water, the value "100 °C" is hotter than
0 °C – in absolute terms – by a factor of precisely
373.15/273.15 (approximately 36.61% thermodynamically hotter). When
adhering strictly to the two-point definition for calibration, the
boiling point of
VSMOW under one standard atmosphere of pressure is
actually 373.1339 K (99.9839 °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
is slightly less, about 99.974 °C.
This boiling-point difference of 16.1 millikelvin between the Celsius
scale's original definition and the current one (based on absolute
zero and the triple point) has little practical meaning in common
daily applications because water's boiling point is very sensitive to
variations in barometric pressure. For example, an altitude change of
only 28 cm (11 in) causes the boiling point to change by one
Celsius temperature conversion formulae
[°F] = [°C] × 9⁄5 + 32
[°C] = ([°F] − 32) × 5⁄9
[K] = [°C] + 273.15
[°C] = [K] − 273.15
[°R] = ([°C] + 273.15) × 9⁄5
[°C] = ([°R] − 491.67) × 5⁄9
For temperature intervals rather than specific temperatures,
1 °C = 1 K = 9⁄5 °F = 9⁄5 °R
Comparisons among various temperature scales
Comparison of temperature scales
Degrees of frost
^ According to The Oxford English Dictionary (OED), the term "Celsius'
thermometer" had been used at least as early as 1797. Further, the
Celsius or Centigrade thermometer" was again used in
reference to a particular type of thermometer at least as early as
1850. The OED also cites this 1928 reporting of a temperature: "My
altitude was about 5,800 metres, the temperature was 28° Celsius."
However, dictionaries seek to find the earliest use of a word or term
and are not a useful resource as regards to the terminology used
throughout the history of science. According to several writings of
Dr. Terry Quinn CBE FRS, Director of the
BIPM (1988–2004), including
Temperature Scales from the early days of thermometry to the 21st
century" (PDF). (146 KiB) as well as
Edition/1990/Academic Press/0125696817), the term
connection with the centigrade scale was not used whatsoever by the
scientific or thermometry communities until after the CIPM and CGPM
adopted the term in 1948. The
BIPM was not even aware that "degree
Celsius" was in sporadic, non-scientific use before that time. It is
also noteworthy that the twelve-volume, 1933 edition of OED didn't
even have a listing for the word
Celsius (but did have listings for
both centigrade and centesimal in the context of temperature
measurement). The 1948 adoption of
Celsius accomplished three
1. All common temperature scales would have their
units named after someone closely associated with them; namely,
Kelvin, Celsius, Fahrenheit, Réaumur and Rankine.
2. Notwithstanding the important contribution of
Linnaeus who gave the
Celsius scale its modern form, Celsius' name was
the obvious choice because it began with the letter C. Thus, the
symbol °C that for centuries had been used in association with the
name centigrade could continue to be used and would simultaneously
inherit an intuitive association with the new name.
3. The new name eliminated the ambiguity of the term
"centigrade", freeing it to refer exclusively to the French-language
name for the unit of angular measurement.
Vienna Standard Mean Ocean Water
Vienna Standard Mean Ocean Water at one standard atmosphere
(101.325 kPa) when calibrated solely per the two-point definition of
thermodynamic temperature. Older definitions of the
Celsius scale once
defined the boiling point of water under one standard atmosphere as
being precisely 100 °C. However, the current definition results
in a boiling point that is actually 16.1 mK less. For more about
the actual boiling point of water, see
VSMOW in temperature
measurement. There is a different approximation using
approximates the temperature to 99.974 °C
^ In 1948, Resolution 7 of the 9th
CGPM stated, "To indicate a
temperature interval or difference, rather than a temperature, the
word 'degree' in full, or the abbreviation 'deg' must be used." This
resolution was abrogated in 1967/1968 by Resolution 3 of the 13th CGPM
which stated that ["The names "degree Kelvin" and "degree", the
symbols "°K" and "deg" and the rules for their use given in
Resolution 7 of the 9th
CGPM (1948),] ...and the designation of the
unit to express an interval or a difference of temperatures are
abrogated, but the usages which derive from these decisions remain
permissible for the time being." Consequently, there is now wide
freedom in usage regarding how to indicate a temperature interval. The
most important thing is that one's intention must be clear and the
basic rule of the SI must be followed; namely that the unit name or
its symbol must not be relied upon to indicate the nature of the
quantity. Thus, if a temperature interval is, say, 10 K or
10 °C (which may be written 10 kelvin or 10 degrees Celsius), it
must be unambiguous through obvious context or explicit statement that
the quantity is an interval. Rules governing the expressing of
temperatures and intervals are covered in the BIPM's "SI Brochure, 8th
edition" (PDF). (1.39 MiB).
Celsius temperature scale". Encyclopædia Britannica. Retrieved 19
Celsius temperature scale, also called centigrade
temperature scale, scale based on 0° for the freezing point of water
and 100° for the boiling point of water at 1 atm pressure.
^ Helmenstine, Anne Marie (December 15, 2014). "What Is the Difference
Celsius and Centigrade?". Chemistry.about.com. About.com.
Retrieved March 16, 2015.
^ "SI brochure, section 22.214.171.124". International Bureau of Weights and
Measures. Archived from the original on 26 September 2007. Retrieved 9
^ "Essentials of the SI: Base & derived units". Retrieved 9 May
^ Celsius, Anders (1742) "Observationer om twänne beständiga grader
på en thermometer" (Observations about two stable degrees on a
thermometer), Kungliga Svenska Vetenskapsakademiens Handlingar
(Proceedings of the Royal Swedish Academy of Sciences), 3 :
171–180 and Fig. 1.
^ "Resolution 4 of the 10th meeting of the
^ Don Rittner; Ronald A. Bailey (2005): Encyclopedia of Chemistry.
Facts On File, Manhattan, New York City. pp. 43.
^ Smith, Jacqueline (2009). "Appendix I: Chronology". The Facts on
File Dictionary of Weather and Climate. Infobase Publishing.
p. 246. ISBN 978-1-4381-0951-0. 1743 Jean-Pierre Christin
inverts the fixed points on Celsius' scale, to produce the scale used
Mercure de France
Mercure de France (1743): MEMOIRE sur la dilatation du Mercure dans
le Thermométre. Chaubert; Jean de Nully, Pissot, Duchesne, Paris. pp.
^ Journal helvétique (1743): LION. Imprimerie des Journalistes,
Neuchâtel. pp. 308–310.
^ Memoires pour L'Histoire des Sciences et des Beaux Arts (1743): DE
LYON. Chaubert, París. pp. 2125–2128.
^ Citation: Uppsala University (Sweden), Linnaeus' thermometer
^ Citation for Daniel Ekström, Mårten Strömer, Christin of Lyons:
The Physics Hypertextbook, Temperature; citation for Christin of
Lyons: Le Moyne College, Glossary, (
Celsius scale); citation for
Linnaeus' connection with Pehr Elvius and Daniel Ekström: Uppsala
University (Sweden), Linnaeus' thermometer; general citation: The
Uppsala Astronomical Observatory, History of the
^ Citations: University of Wisconsin–Madison, Linnæus & his
Garden and; Uppsala University, Linnaeus' thermometer
^ "CIPM, 1948 and 9th CGPM, 1948". International Bureau of Weights and
Measures. Retrieved 9 May 2008.
^ "centigrade, adj. and n". Oxford English Dictionary. Oxford
University Press. Retrieved 20 November 2011.
^ 1985 BBC Special: A Change In The Weather on YouTube
^ Lide, D.R., ed. (1990–1991). Handbook of Chemistry and Physics.
71st ed. CRC Press. p. 4–22.
^ The ice point of purified water has been measured to be
Celsius – see Magnum,
B.W. (June 1995). "Reproducibility of the
Temperature of the Ice Point
in Routine Measurements" (PDF). Nist Technical Note. 1411. Archived
from the original (PDF) on July 14, 2007. Retrieved 11 February
^ Elert, Glenn (2005). "
Temperature of a Healthy Human (Body
Temperature)". The Physics Factbook. Retrieved 22 August 2007.
^ "Unit of thermodynamic temperature (kelvin)". The
NIST Reference on
Constants, Units, and Uncertainty: Historical context of the SI.
National Institute of Standards and Technology
National Institute of Standards and Technology (NIST). 2000. Retrieved
16 November 2011.
^ BIPM, SI Brochure, Section 5.3.3.
^ For more information on conventions used in technical writing, see
the informative SI Unit rules and style conventions by the
well as the BIPM's SI brochure: Subsection 5.3.3, Formatting the value
of a quantity.
^ "22.2". The
Unicode Standard, Version 9.0 (PDF). Mountain View, CA,
Unicode Consortium. July 2016. ISBN 978-1-936213-13-9.
Retrieved 20 April 2017.
^ Note (e) of SI Brochure, Section, 2.2.2, Table 3
^ Decision #3 of Resolution 3 of the 13th CGPM
^ H.D. Young, R.A. Freedman (2008). University Physics with Modern
Physics (12th ed.). Addison Wesley. p. 573
^ This fact is demonstrated in the book Biostatistics: A Guide to
Design, Analysis, and Discovery By Ronald N. Forthofer, Eun Sul Lee
and Mike Hernandez
^ "Resolution 3 of the 9th
CGPM (1948)". International Bureau of
Weights and Measures. Retrieved 9 May 2008.
^ Citation: London South Bank University, Water Structure and
Behavior, notes c1 and c2
Celsius in Wiktionary, the free dictionary.
NIST, Basic unit definitions: Kelvin
The Uppsala Astronomical Observatory, History of the Celsius
London South Bank University, Water, scientific data
BIPM, SI brochure, section 126.96.36.199, Unit of thermodynamic temperature
TAMPILE, Comparison of temperature scales
C to F converter,
Scales of temperature
Conversion formulas and comparison
International System of Units
International System of Units (BIPM)
with special names
Other accepted units
degree of arc
minute of arc
second of arc
Conversion of units
Systems of measurement