History of the Meter: How the Metric Base Unit Was Defined
The meter began as a fraction of the Earth’s meridian and is now defined by the speed of light. Learn the key milestones from 1793 to 1983.
The Meter Started With the Earth
The meter was introduced in France in the 1790s as a unit of length based on the size of the Earth. It was originally defined as one ten-millionth of the distance from the equator to the North Pole along the meridian through Paris. A platinum bar was cut to that length and kept as the standard.
Our inches to cm converter still applies the modern result of that effort: 1 inch = 2.54 cm.
Key Milestones in the Meter’s Definition
From a bar to a constant
1799: The first platinum "metre des Archives" bar is deposited in Paris.
1889: A platinum-iridium International Prototype Metre bar becomes the standard, with copies sent to other countries.
1960: The meter is redefined using the wavelength of light from krypton-86, removing the dependence on a physical bar.
1983: The meter is redefined by the speed of light: the distance light travels in 1/299,792,458 of a second.
Why the Meter Matters Today
Because the meter is now tied to the unchanging speed of light, it can be realized anywhere with the right laser and clock, with no reference object to drift or damage. It is the base unit of length in the International System of Units (SI), and every other SI length unit is built from it.
For the mass counterpart, see our what is the kilogram guide, which follows the same path from a physical artifact to a fixed constant.
Frequently Asked Questions
When was the meter invented?
In the 1790s in France, as part of the new metric system.
How is the meter defined now?
Since 1983, as the distance light travels in vacuum in 1/299,792,458 of a second.
Was the meter once a physical bar?
Yes. A platinum bar (1799) and later a platinum-iridium International Prototype Metre (1889) served as the standard.
Why redefine the meter using light?
A constant-based definition is stable, exact, and reproducible anywhere without a physical artifact.
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Quick Summary
The meter was first defined in 1790s France as one ten-millionth of the Earth’s meridian quadrant, kept as a platinum bar from 1799 and an international prototype from 1889, redefined by krypton-86 light in 1960, and since 1983 by the speed of light (1/299,792,458 second).
Related Guide
What Is the Kilogram? Definition, History, and the SI System
The kilogram is the SI base unit of mass. Learn how it was once a physical artifact, how it is now defined by Planck's constant, and how it links to pounds.
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Disclaimer
Unit conversions on this site use internationally recognized, fixed conversion factors (for example, 1 inch = 2.54 cm, 1 pound = 0.45359237 kg, 1 °F = (°C × 9/5) + 32). They are provided for general, informational, and educational use. For applications where accuracy is critical — such as medical dosing, engineering, scientific research, trade, or legal compliance — always verify results against authoritative sources such as the U.S. National Institute of Standards and Technology (NIST) and the official International System of Units (SI) brochure, and follow any applicable local standards.
Exact Constants & Authoritative Sources
Every conversion on this page uses the fixed, internationally agreed constants below — drawn from NIST and the SI Brochure. No values are estimated or rounded as if exact. See the full list on our Sources & Standards page.
- EXACT1 inch (in) = 25.4 millimetres (exact)
- EXACT1 foot (ft) = 0.3048 metres (exact, 12 in)
- EXACT1 yard (yd) = 0.9144 metres (exact, 3 ft)
- EXACT1 mile (mi) = 1609.344 metres (exact, 5280 ft)
- EXACT1 centimetre (cm) = 0.01 metre (definition)
Common Pitfalls & How to Avoid Them
US survey foot is deprecated
The old US survey foot (0.304800609601 m) was retired in 2023 in favor of the international foot (0.3048 m). Modern conversions use the international foot; mixing them drifts over long distances.
Inches vs. centimetres for screens
Display and wheel sizes are often rounded to the nearest convenient number; a "27-inch" monitor is nominal, not a precise 68.58 cm panel.
A unit-conversion reference site has no manufacturer-specific or device calibration parameters, so "layered tuning" does not apply here. The only real error-handling is input validation: the converter rejects non-numeric, empty, or out-of-range entries and shows a clear message, then falls back to exact constants above. No estimated or rounded figures are ever presented as exact.
Authoritative sources for this page
- NIST (U.S. National Institute of Standards and Technology) — NIST SP 330 — The International System of Units (SI), 2019 edition
- BIPM (International Bureau of Weights and Measures) — SI Brochure — The International System of Units (9th edition)
Last Updated 2026 | This article was reviewed for accuracy