Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

SI Units – Temperature

K - Kelvin - Temperature - 2018

The kelvin (K) is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 ×10−23 when expressed in the unit J K−1, which is equal to kg m2 s−2 K−1, where the kilogram, meter and second are defined in terms of h, c and ∆νCs. The temperature 0 K is commonly referred to as "absolute zero." On the widely used Celsius temperature scale, water freezes at 0 °C and boils at about 100 °C. One Celsius degree is an interval of 1 K, and zero degrees Celsius is 273.15 K. An interval of one Celsius degree corresponds to an interval of 1.8 Fahrenheit degrees on the Fahrenheit temperature scale.

The standard temperature at the triple point of water is provided by a special cell, an evacuated glass cylinder containing pure water. When the cell is cooled enough so that a mantle of ice forms around the reentrant well, the temperature at the interface of solid, liquid, and vapor is 273.16 K. Thermometers to be calibrated are placed in the reentrant well.

Celsius Temperature Poem

Collage of ice cubes, gold tulips, park benches with fall foliage in background, and colored plastic float in pool
Four Seasons.
Credit: Pixabay

Activity:  Estimate the Celsius temperature in each scene (ice crystals, tulip flowers, park bench, and swimming pool). Recite and reflect on the Celsius Temperature Poem. Use the hints provided by the poem.

  • 30 °C is hot
  • 20 °C is nice
  • 10 °C is cold
  • 0 °C is ice
Common Temperature Reference Points
  Celsius (°C) Kelvin (K) Fahrenheit (°F)
Surface of the Sun

5600

5900

10100

Boiling Point of Water

100

373

212

Body Temperature

37

310.2

98.6

Sweltering Day

40

313

104

Hot Day

30

303

86

Room Temperature

20

293

68

Cold Day

10

283

50

Freezing Point of Water

0

273

32

Oven Temperature Equivalencies
Description °F °C
Cool 200 90
Very Slow 250 120
Slow 300 to 325 150 to 160
Moderately Slow 325 to 350 160 to 180
Moderate 350 to 375 180 to 190
Moderately Hot 375 to 400 190 to 200
Hot 400 to 450 200 to 230
Very Hot 450 to 500 230 to 260
Temperature Conversion (Exact)
From To Fahrenheit To Celsius To Kelvin

Fahrenheit (°F)

°F

(°F - 32) / 1.8 

(°F - 32) / 1.8 + 273.15

Celsius (°C)

(°C * 1.8) + 32

°C

°C + 273.15

Kelvin (K)

(K - 273.15) * 1.8 + 32

K - 273.15

K

    Resources for Students and Teachers

    • Kelvin: Introduction – Temperature is one of the most important and ubiquitous measurements in human life. (NIST)
    • How Do You Measure Air Temperature Accurately – For purposes of safety, accuracy and convenience, air temperature is most often measured using electronic thermometers, which detect the changes in the resistance to the flow of electrical current through a metal.
    • Absolutely Hot –  Analyze a common chemical reaction. Explore heat and energy and learn how they are measured. (Jason Learning)
    • Kelvin Temperatures and Very Cold Things! – To keep track of some of the coldest things in the universe, scientists use the Kelvin temperature scale. (NASA)
    • Building a Thermometer – Build an instrument to measure water temperature. (GLOBE Program)
    • Thermometer Model – A thermometer is a device used to measure temperature. Explore the interactive model. Review thermometers types. (JavaLab)
    Cartoon older gentleman. Bald. White bushy beard and mustache. Wearing long jacket and striped pants. Holding a thermometer in one hand and a ball of fire in the other
    Credit: J. Wang and B. Hayes/NIST

    League of SI Superheroes – Dr. Kelvin

    This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. With the ability to speed up or slow down particles, Dr. Kelvin can measure any temperature. The kelvin temperature scale begins at absolute zero, the coldest possible temperature and the point at which even atoms would stand perfectly still.

    Navigate to more SI base unit information:

    Resources

    Contacts

    Created April 12, 2010, Updated September 2, 2022