SI Units – Length

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 ∆ν_Cs.

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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FAQ: 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:

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FAQ: 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. 

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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.
• 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).
• 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.
• Using a Metric Ruler (WeldNotes, Video).
• Using a Micrometer (University of Toronto).
• Using the Vernier Calipers & Micrometer Screw Gauge (University of Cape Town, Department of Physics).
• Scale of Things Chart (U.S. Department of Energy).
• Examine Cell Size and Scale using an interactive graphic (University of Utah).
• Practice measuring length with centimeters in the Squares and Rectangles activity (PBS).
• 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.
• 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.
• 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!
• How It’s Measured: Calibrating Tape Measures (NIST, Video) – Inside a 60 m long underground tunnel, measurement scientists check the accuracy of tape measures.
• 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.
• Going the Distance on National Tape Measure Day (NIST) – Learn about the significance of tape measures, celebrated on National Tape Measure Day, July 14^th.
• 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.
• 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.
• 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.
• Top 10 Tips for Teaching the Metric System (NIST).

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Credit: J. Wang and B. Hayes/NIST ©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. 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

• SI Units
• Fundamental Physical Constants
• Time – second (s)
• Amount of substance – mole (mole)
• Electric current – ampere (A)
• Temperature – kelvin (K)
• Luminous intensity – candela (cd)
• Mass – kilogram (kg)

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