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SI Units – Length

SI Units meter banner
Credit: NIST
m - Meter - Length - 2018

The meter (m) is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s−1, where the second is defined in terms of ∆νCs.

The meter was once defined by a physical artifact - two marks inscribed on a platinum-iridium bar, like these from the NIST Museum.

From the meter, several other units of measure are derived such as the:

  • unit of speed is the meter per second (m/s). The speed of light in vacuum is 299 792 458 meters per second.
  • unit of acceleration is the meter per second per second (m/s2).
  • unit of area is the square meter (m2).
  • unit of volume is the cubic meter (m3). The liter (1 cubic decimeter), although not an SI unit, is accepted for use with the SI and is commonly used when measuring fluid volume, but is also used when measuring gases and solids.

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Units of Length
10 millimeters (mm)=1 centimeter (cm)
10 centimeters=1 decimeter (dm)
10 centimeters=100 millimeters
10 decimeters=1 meter (m)
10 decimeters=1000 millimeters
10 meters=1 dekameter (dam)
10 dekameters=1 hectometer (hm)
10 dekameters=100 meters
10 hectometers=1 kilometer (km)
10 hectometers=1000 meters

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Resources for Students and Teachers

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

Meter man character. Hearts. Text: I like you this much. To and From lines.
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 Meter Man and our SI Superheroes.
  • The Metric Meal (option 2) focuses exclusively on length measurements and uses a multidisciplinary approach to build measurement skills.
  • Practice measuring length with centimeters in the Squares and Rectangles activity (PBS). k-2
  • Calculate Focal Length (The Optical Society) – Use this hands-on activity to explore an important concept that’s used in STEAM tools like microscopes, telescopes, and cameras. K-

 

Meter Man
Credit: ©2020 U.S. Secretary of Commerce. All Rights Reserved.

 

  • League of SI Superheroes - Meter Man
    • 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 Ruler (SP 376) (NIST) - an educational tool designed to teach SI) length measurements. 3
  • What is a Nanometer? (IEEE) – In this activity, develop an understanding of how small a nanometer really is. During the lesson, students will measure common classroom objects and convert the results to nanometers. 3
  • Length Column Game (Quintessential Instructional Archive) – Become familiar with equivalent metric length measurements. Draw a line to connect between like measurements. Look carefully, because some items do not have a match! 3
  • The Wright Way: Innovation Through Engineering (NASA) - This educational poster includes metric activities associated with the Wright Brothers' flight experiments and include constructing and testing a sled kite, analyzing the resulting data, and assembling a metric cube. The student worksheet includes the learning objectives and academic standards (Pre-K to 12th grades), the sled kite template, worksheet and metric cube. 3
  • SI Area (NIST) – Explore resources to become familiar with units used to measure area, including the hectare.
  • SI Volume (NIST) – Explore resources to become familiar with units used to measure volume, including the liter.
Animated illustration says "HDYMI? Ocean Depth" with swimming shark, bobbing ship, satellite.
Credit: N. Hanacek/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) (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.
  • Historic Meter Stick (Smithsonian National Museum of American History). This historic meter stick was used to teach length. One side is graduated in centimeters (cm) by alternate markings of black paint. One side is divided into decimeters, with alternate decimeters (dm) in plain wood or black paint.
  • Examine Cell Size and Scale using an interactive graphic (University of Utah).
  • How Do You Ensure That a Tape Measure is Accurate (NIST HDYMI Series) – Learn how a laser interferometer is used to precisely measure distances along a measuring tape.
  • How Do You Measure the Depth of the Ocean (NIST HDYMI Series) – Scientists and researchers can use sonar, radar and satellite methods to measure ocean depth. The average ocean depth is 3.7 km, but the deepest part ever recorded is located in the Mariana Trench, at a depth of around 11 km.
  • Garden Layout (University of California at Berkley, University of Nottingham) – Design, plan, and draw a garden to scale using a metric ruler.
  • SI Area (NIST) – Explore resources to become familiar with units used to measure area, including the hectare.
  • SI Volume (NIST) – Explore resources to become familiar with units used to measure volume, including the liter.
  • Calculate Circumference, Area and Volume (NIST) – Become familiar with methods used to calculate the circumference, area, and volume of common objects. 
Animated illustration shows a tape measure with a laser beam moving along it and says "HDYMI? Tape Measure."
Credit: N. Hanacek/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 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.
  • Metric Ruler (SP 376) (NIST) - an educational tool designed to teach SI) length measurements.
  • Meter (NIST) – Whether it’s the interminable distance to Grandma’s house, a span of cloth, the distance to the track and field race finish line, or the space between the unfathomably small transistors on a computer chip, length is one of the most familiar units of measurement.
  • National Prototype Meter No. 27 (NIST)- An image of the U.S. National Prototype Meter, a platinum-iridium bar created in the late 1800s as one of 31 international standards.
  • Using a Micrometer (University of Toronto)- This resource offers a detailed explanation of how to accurately measure small dimensions using a micrometer.
  • Using the Vernier Calipers & Micrometer Screw Gauge (University of Cape Town, Department of Physics)- This detailed instructional guide explains how to read and use Vernier calipers and micrometer screw gauges for precise length measurements.
  • Scale of Things Chart (U.S. Department of Energy)- This visual chart from the DOE's Office of Basic Energy Sciences illustrates the relative sizes of objects from the macroscopic to the atomic scale.
  • Going the Distance on National Tape Measure Day (NIST) – Learn about the significance of tape measures, celebrated on National Tape Measure Day, July 14th.
  • SI Area (NIST) – Explore resources to become familiar with units used to measure area, including the hectare.
  • SI Volume (NIST) – Explore resources to become familiar with units used to measure volume, including the liter.

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How It’s Measured: Calibrating Tape Measures
How It’s Measured: Calibrating Tape Measures
Inside a 60-meter-long underground tunnel, measurement scientists check the accuracy of tape measures.
How to Read a Metric Ruler
How to Read a Metric Ruler
Bob and Sparky explain how to read a metric ruler.  They look at meters, centimeters and millimeters.  Also features Bob speaking fluent French.

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FAQ

NIST metric ruler measures 3 autumn leaves
Credit: NIST

How do I get a metric ruler?

A metric ruler is a valuable tool for science, technology, engineering, art, and math (STEAM) classrooms. Metric rulers are available from many

 retail vendors, which can be identified by using search terms such as "metric rule," "meter stick," or "metric stick." Printable rulers, such as the centimeter Color-square rules, can be color printed on to overhead transparency sheets to make inexpensive metric rulers. The NIST Metric Ruler (SP 376) is available within the NIST SI Teacher Kit, a curated collection of instructional measurement resources. 

When did the metric redefinition of the inch occur?

In 1958, a conference of English-speaking nations agreed to unify their standards of length and mass, and define them in terms of metric measures. The American yard was shortened and the imperial yard was lengthened as a result. The new conversion factors were announced in 1959 in Federal Register Notice 59-5442 (June 30, 1959), which states the definition of a standard inch: The value for the inch, derived from the value of the Yard effective July 1, 1959, is exactly equivalent to 25.4 mm.

The conversion factor can be determined:

$$\left.\frac{0.9144\; \mathrm{m}}{1\; \mathrm{yd}}\right|\frac{1\; \mathrm{yd}}{36\; \mathrm{in}} \quad = \quad \frac{0.0254\; \mathrm{m}}{\mathrm{in}}$$

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League of SI Superheroes – Meter Man:

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 sharp eyes and stretchy ruler arms, no distance is too big or small for Meter Man to measure. A meter is the distance light travels in a tiny fraction of a second.

Navigate to more SI base unit information

graphic image of SI Superhero, Meter Man
Credit: J. Wang and B. Hayes/NIST ©2020 U.S. Secretary of Commerce. All Rights Reserved.

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Contacts

Created April 12, 2010, Updated July 24, 2025
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