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SI Units – Temperature

SI Units kelvin banner
Credit: NIST
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




Boiling Point of Water




Body Temperature




Sweltering Day




Hot Day




Room Temperature




Cold Day




Freezing Point of Water




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 - 32) / 1.8 

(°F - 32) / 1.8 + 273.15

Celsius (°C)

(°C * 1.8) + 32


°C + 273.15

Kelvin (K)

(K - 273.15) * 1.8 + 32

K - 273.15


Resources for Students and Teachers

  • SI Base Units Relationship Poster (SP 1247) (NIST) – a colorful poster illustrating the relationships of the International System of Units (SI) derived units with special names and symbols and the seven traditional base units.
  • SI Units Card Deck (SP 1297) (NIST) – this activity offers a fun way to enhance understanding of the International System of Units, including the defining constants, base units, derived units with special names, and prefixes.
  • Kelvin: Introduction (NIST) – Temperature is one of the most important and ubiquitous measurements in human life.
  • How Do You Measure Air Temperature Accurately (NIST HDYMI Series) – 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.
  • Culinary Temperature (NIST Metric Kitchen) – Food temperature is key to a successful recipe. Explore oven temperature, common cooking temperatures, candy making temperatures, and safe minimum internal temperatures for a variety of foods.
  • Absolutely Hot (Jason Learning) –  Analyze a common chemical reaction. Explore heat and energy and learn how they are measured.
  • Kelvin Temperatures and Very Cold Things! (NASA) – To keep track of some of the coldest things in the universe, scientists use the Kelvin temperature scale.
  • Building a Thermometer (GLOBE Program) – Build an instrument to measure water temperature.
  • Thermometer Model (JavaLab) – A thermometer is a device used to measure temperature. Explore the interactive model. Review thermometers types.
  • Grow Snow Crystals (NOAA SciJinks) – Use a web application to explore how snowflake crystals take shape depending on the temperature (degree Celsius).
  • Precipitation Simulator (NOAA SciJinks) – Make it rain and snow using this precipitation simulator by setting the air temperature and dew point (degree Celsius).
  • Top 10 Tips for Teaching the Metric System (NIST).
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:



Created April 12, 2010, Updated January 16, 2024