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Physical Measurement Laboratory

Office of Weights and Measures

SI Units - Mass

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SI Units kilogram banner
Credit: NIST

Resources for Students and Teachers | League of SI Superheroes - Monsieur Kilogram | What is the difference between the terms "mass" and "weight"?

kg - Kilogram - Mass - 2018

The kilogram (kg) is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 ×10−34 when expressed in the unit J s, which is equal to kg m2 s−1, where the meter and the second are defined in terms of c and ∆νCs.

U.S. National Prototype Kilogram
Credit: ©Robert Rathe

The primary standard of mass for this country is United States Prototype Kilogram 20, which is a platinum-iridium cylinder kept at NIST. The kilogram, originally defined as the mass of one cubic decimeter of water at the temperature of maximum density, was known as the Kilogram of the Archives. It was replaced after the International Metric Convention in 1875 by the International Prototype Kilogram which became the unit of mass without reference to the mass of a cubic decimeter of water or to the Kilogram of the Archives. Each country that subscribed to the International Metric Convention was assigned one or more copies of the international standards; these are known as National Prototype Meters and Kilograms. Learn more about the history and current definition of the kilogram.

Among the SI base units, the kilogram (kg) is the only one whose name and symbol, for historical reasons, include a prefix. "Kilo" the SI prefix for 1000 or 103. Names and symbols for decimal multiples and submultiples of the unit of mass are formed by attaching prefix names to the unit name "gram," and prefix symbols to the unit symbol "g." Learn more about this historical quirk.

Units of Mass
10 milligrams (mg)=1 centigrams (cg)
10 centigrams=1 decigrams (dg) = 100 milligrams
10 decigrams=1 gram (g)
10 decigrams=1000 milligrams
10 grams=1 dekagrams (dag)
10 dekagrams=1 hectogram (hg)
10 dekagrams=100 grams
10 hectograms=1 kilogram (kg)
10 hectograms=1000 grams
1000 kilograms=1 megagram (Mg) or 1 metric ton (t)

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Resources for Students and Teachers

Teachers: Top 10 Tips for Teaching the Metric System. (NIST)

 

Blue background. Monsieur Kilogram character. Hearts. Text: I like you a metric ton. To and From lines at bottom.
Credit: ©2020 U.S. Secretary of Commerce. All Rights Reserved.

  • Not sure how to tell that scientific minded person in your life how you feel? Maybe our Superhero Valentine's Day cards will do the trick.
  • Download these fun coloring pages for Monsieur Kilogram and our SI Superheroes.
  • The Metric Meal (option 2) focuses exclusively on length measurements and uses a multidisciplinary approach to build measurement skills.
  • Balancing Act. (PhET, Simulation)- This interactive simulation lets students explore balance, torque, and rotational equilibrium by placing objects on a plank, predicting its motion, and solving puzzles using proportional reasoning.

 

 

Professor Kilogram
Credit: ©2020 U.S. Secretary of Commerce. All Rights Reserved.

 

  • League of SI Superheroes - Monsieur Kilogram
    • This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units.
  • SI Units Card Deck (SP 1297) - this activity offers a fun way to enhance understanding of the International System of Units. (NIST)
  • Metric Estimation Game (SP 1336) - this fun, hands-on activity helps participants become familiar with SI measurements by practicing estimation skills. During this activity, participants will become familiar with using the metric system (International System of Units, SI) measurements on common, everyday items.
  • Metric Kitchen - Cook and Bake with a Kitchen Scale with Grams. (NIST)- For young chefs, these activities offer an opportunity to refine fine motor skills and coordination, practice selecting appropriate cooking tools, opening containers, mixing, pouring without spilling, counting, weighing, as well as reading and interpreting a digital scale readout or oven thermometer.

 

Animation shows the path of a laser moving between large and small spheres and reads: HDYMI? Strength of Gravity
Credit: B. Hayes/NIST

  • Metric Estimation Game (SP 1336) - this fun, hands-on activity helps participants become familiar with SI measurements by practicing estimation skills. During this activity, participants will become familiar with using the metric system (International System of Units, SI) measurements on common, everyday items.
  • SI Base Units Relationship Poster (SP 1247) - 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. (NIST)
  • How Do You Measure the Strength of Gravity? (NIST HDYMI Series)- Gravity is one of the fundamental forces of the universe. It pulls us toward the center of the Earth and keeps our feet on the ground. Understanding the force of gravity makes it possible to predict the orbits of planets and moons in our solar system, enabling us to aim spacecraft precisely enough to land on them.
  • U.S. Coin Specifications. (U.S. Mint)- How much does a dime weigh? What are pennies made of? Find out in the table below, which gives specifications for U.S. Mint legal tender coins presently in production for annual sets.
  • What is the Planck Constant. (NIST)-  The Planck constant is a fundamental number in quantum mechanics that describes how energy is measured in very small amounts called quanta. Learn more about its role in modern science and technology.
  • Everyday Estimation - Mass. (NIST)- This section shows examples of common objects and their masses, helping you understand the weight of everyday items from a small car to a cotton ball.
  • The Name "Kilogram": a Historical Quirk. (BIPM)
  • (Former) International Prototype Kilogram. (BIPM)
Lego animated
Credit: NIST

  • Metric Estimation Game (SP 1336) - this fun, hands-on activity helps participants become familiar with SI measurements by practicing estimation skills. During this activity, participants will become familiar with using the metric system (International System of Units, SI) measurements on common, everyday items.
  • Pirates of the Caribbean (Metric Edition). (NIST)- In 1794, a French scientist’s capture by pirates may have delayed U.S. adoption of the metric system by nearly a century. Learn more about this surprising twist of history
  • The NIST Do-It-Yourself Kibble Balance. (NIST)- The NIST "D.I.Y." Kibble Balance is a classroom science project that teaches the principles behind the effort to redefine the world's basic unit of mass, using LEGO® bricks and some basic electronics.
  • Mass Calibration Brochure. (NIST)- This brochure explains when and how to recalibrate weights following the new kilogram definition, what to expect from NIST calibrations, and answers common questions about uncertainty, stability, and traceability.
  • Elemental Composition of the Human Body. (Visual Capitalist)- The human body is a miraculous, well-oiled, and exceptionally complex machine. Learn more to know what elements makeup the human body.
  • Kibble Balance Principles. (NIST)- Learn how the Kibble balance uses electricity and magnetism to precisely measure mass, replacing the physical kilogram standard by linking it to fundamental constants like the Planck constant.
  • Kilogram: Introduction. (NIST)- For over 100 years, the kilogram was defined by a small metal cylinder kept in a vault in France. Learn more about how it was recently redefined using a constant of nature for greater accuracy.

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Pirates Plunder the Kilogram- NISTory
Pirates Plunder the Kilogram- NISTory
Operating Principles of the NIST-4 Kibble Balance
Operating Principles of the NIST-4 Kibble Balance
How to Build Your NIST D.I.Y. Watt Balance
How to Build Your NIST D.I.Y. Watt Balance

 

NIST in 90: Measuring Planck’s Constant
NIST in 90: Measuring Planck’s Constant

 

FAQ 

What is the difference between the terms "mass" and "weight"?

The mass of a body is a measure of its inertial property or how much matter it contains. The weight of a body is a measure of the force exerted on it by gravity or the force needed to support it. Gravity on earth gives a body a downward acceleration of about 9.8 m/s2. In common parlance, weight is often used as a synonym for mass in weights and measures. For instance, the verb “to weigh” means “to determine the mass of” or “to have a mass of.” The incorrect use of weight in place of mass should be phased out, and the term mass used when mass is meant. The SI unit of mass is the kilogram (kg). In science and technology, the weight of a body in a particular reference frame is defined as the force that gives the body an acceleration equal to the local acceleration of free fall in that reference frame. Thus, the SI unit of the quantity weight defined in this way (force) is the newton (N).

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League of SI Superheroes - Monsieur Kilogram

This comic book-style video animation series has been developed to help middle school students learn about the 7 SI base measurement units. With his immensely strong arms, Monsieur Kilogram is the master of mass. The kilogram is a cylinder of special metal about 39 millimeters wide by 39 millimeters tall that serves as the world's mass standard.

Navigate to more SI base unit information

SI_Superhero_Cards_Monsieur Kilogram_Front
Credit: J. Wang and B. Hayes/NIST ©2020 U.S. Secretary of Commerce. All Rights Reserved.

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EXPLORE THE METRIC PROGRAM

Becoming Familiar with SI | Everyday Estimation | U.S. Metrication | U.S. Metrication FAQs | Prefixes | Metric Kitchen | Metric Estimation Game | SI Education and Training | SI Publications | Understanding Metric | Writing with Metric Units | National Metric Week | NEST-R (STEM Registry) | NIST Education Resources

Metrology, Metric and Weights and measures

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Created March 30, 2011, Updated July 22, 2025
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