USMA logo

Metric system temperature
(kelvin and degree Celsius)

metric thermometer

Three temperature scales are in common use in science and industry. Two of those scales are SI metric:

The degree Celsius (°C) scale was devised by dividing the range of temperature between the freezing and boiling temperatures of pure water at standard atmospheric conditions (sea level pressure) into 100 equal parts. Temperatures on this scale were at one time known as degrees centigrade, however it is no longer correct to use that terminology. [The official name was changed from "centigrade degree" to "Celsius degree" by the 9th General Conference on Weights and Measures (CGPM) in 1948.]

The kelvin (K) temperature scale is an extension of the degree Celsius scale down to absolute zero, a hypothetical temperature characterized by a complete absence of heat energy. Temperatures on this scale are called kelvins, NOT degrees kelvin, kelvin is not capitalized, and the symbol (capital K) stands alone with no degree symbol. [The official name was changed to "kelvin" and symbol "K" by the 13th General Conference on Weights and Measures (CGPM) in 1967.]

The degree Fahrenheit (°F) non-metric temperature scale was devised and evolved over time so that the freezing and boiling temperatures of water are whole numbers, but not round numbers as in the Celsius temperature scale.

Some baseline temperatures in the three temperature scales:

temperaturekelvindegree Celsiusdegree Fahrenheit
boiling point of water373.15100.212.
melting point of ice273.150.32.
absolute zero0.-273.15-459.67

Common temperature comparisons:

temperaturedegree Celsiusdegree Fahrenheit
boiling point of water100.212.
average human body temperature37.98.6
average room temperature20. to 25.68. to 77.
melting point of ice0.32.

Temperature conversions between the three temperature scales:

kelvin / degree Celsius conversions (exact):

degree Fahrenheit / degree Celsius conversions (exact):

A degree Celsius memory device:

There are several memory aids that can be used to help the novice understand the degree Celsius temperature scale. One such mnemonic is:

Or, another one to remember:

Quotes about Fahrenheit:

The Fahrenheit temperature scale is "antiquated, clumsy, and still in use in only one industrialized nation in the world today--the United States."
Walter A. Lyons, PhD, The Handy Weather Answer Book, page 41.

The Fahrenheit temperature scale is "plainly silly and its cultural value [is] slim."
Neville Holmes, "The Numerical Dysfunction", Mathematical Intelligencer, 22(1), 2000-winter, page 8.


The U.S. is the only nation that continues to use Fahrenheit temperatures for shelter-level (surface) weather observations. However, as of July 1996 all surface temperature observations in National Weather Service METAR/TAF reports are now transmitted in degrees Celsius.

All upper-air (non-surface) temperatures have always been measured and reported in degrees Celsius by all countries.

Wind chill temperatures in metric units:

Temperature (degree Celsius)
10 °C5 °C0 °C-5 °C-10 °C-15 °C-20 °C-25 °C-30 °C-35 °C-40 °C-45 °C-50 °C
Wind speed
(kilometers per hour)
10 km/h8.62.7-3.3-9.3-15.3-21.1-27.2-33.2-39.2-45.1-51.1-57.1-63.0
15 km/h7.91.7-4.4-10.6-16.7-22.9-29.1-35.2-41.4-47.6-53.7-59.9-66.1
20 km/h7.41.1-5.2-11.6-17.9-24.2-30.5-36.8-43.1-49.4-55.7-62.0-68.3
25 km/h6.90.5-5.9-12.3-18.8-25.2-31.6-38.0-44.5-50.9-57.3-63.7-70.2
30 km/h6.60.1-6.5-13.0-19.5-26.0-32.6-39.1-45.6-52.1-58.7-65.2-71.7
35 km/h6.3-0.4-7.0-13.6-20.2-26.8-33.4-40.0-46.6-53.2-59.8-66.4-73.1
40 km/h6.0-0.7-7.4-14.1-20.8-27.4-34.1-40.8-47.5-54.2-60.9-67.6-74.2
45 km/h5.7-1.0-7.8-14.5-21.3-28.0-34.8-41.5-48.3-55.1-61.8-68.6-75.3
50 km/h5.5-1.3-8.1-15.0-21.8-28.6-35.4-42.2-49.0-55.8-62.7-69.5-76.3
55 km/h5.3-1.6-8.5-15.3-22.2-29.1-36.0-42.8-49.7-56.6-63.4-70.3-77.2
60 km/h5.1-1.8-8.8-15.7-22.6-29.5-36.5-43.4-50.3-57.2-64.2-71.1-78.0

For another report on the new wind chill temperatures see: (See in particular the chart and formula in chapter 3, page 3-8 (page 42 of the entire PDF).

Back to USMA home.

Copyright © 1998-2014, U.S. Metric Association (USMA), Inc. All rights reserved.
Web hosting courtesy of Colorado State University.
Website maintained by USMA Webmaster
Updated: 2014-11-11