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SI Units: Length

The definition of the meter (m), which is the international unit of length, was once defined by a physical artifact - two marks inscribes on a bar of platinum-iridium. Today, the meter (m) is defined in terms of constant of nature: the length of the path traveled by the light in vacuum during a time interval of 1/299, 792, 458 of a second. The Length - Evolution from Measurement Standard to a Fundamental Constant explains the evolution of the definition of the meter. Follow these changes over time in the NIST Length Timeline.

Historical Standard Platinum Iridium Meter Bar (NIST)
Historical Standard Platinum Iridium Meter Bar (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 to measure fluid volume.

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:


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

FAQ: How do I get a metric ruler?

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.

Educational Resources

meterman_front_webLeague 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.