Meter(m)
The meter (m) is the SI base unit of length. It is defined using the fixed value of the speed of light in vacuum.
Reviewed
- Symbol
- m
- Quantity
- Length
- Measurement system
- SI
- SI classification
- SI base unit
- Reference unit
- SI base unit for length
- Exact relationship
- c = 299,792,458 m/s exactly
Visualizing meter
How the meter is defined
The speed of light in vacuum, c, has the exact value 299,792,458 meters per second. Because the second is independently defined using the caesium-133 atom, this fixes the length of one meter without relying on a physical measuring bar.
Meter relationships
Metric prefixes create larger and smaller units from the meter:
- 1 meter = 100 centimeters
- 1 meter = 1,000 millimeters
- 1 kilometer = 1,000 meters
The international inch is defined as exactly 0.0254 meter, which provides an exact relationship between the SI and inch-based systems.
Where meters are used
Meters are used worldwide for everyday, technical, and scientific measurements. Common examples include room dimensions, building heights, short travel distances, running events, elevation, and engineering specifications.
Meter or metre?
The international SI spelling is metre. Meter is the spelling used by NIST and in American English. The unit symbol is always m, with no period or plural form.
A short history
The meter originated in the French metric system in the late eighteenth century. It was represented by physical standards and later by a wavelength of light before the 1983 definition linked it to the speed of light. The revised SI reformulated the wording in 2019 while preserving the same meter.