Meter

The meter (American spelling; metre in British/international) is the SI base unit of length. One meter equals 100 centimeters or about 39.37 inches.

The definition has evolved four times:

• 1793: one ten-millionth of the distance from equator to North Pole through Paris. Physically meaningful but hard to remeasure.
• 1889: the distance between two scratches on a platinum-iridium bar stored in Sèvres, France. Stable but only one definitive copy.
• 1960: 1,650,763.73 wavelengths of a specific krypton-86 emission line. Reproducible in any lab with the right equipment.
• 1983 (current): the distance light travels in vacuum in 1/299,792,458 of a second. Defines the meter in terms of time, fixing the speed of light by definition.

The current definition reflects the modern SI’s strategy: define base units in terms of fundamental physical constants rather than artifacts. The same revolution redefined the kilogram in 2019 to be based on Planck’s constant rather than a chunk of metal.

Practical equivalents: 1 inch ≈ 2.54 cm exactly (defined relation since 1959); 1 foot = 0.3048 m; 1 yard = 0.9144 m; 1 mile = 1,609.344 m.

Why fixing the speed of light works: the 1983 redefinition flipped the relationship between the metre and the speed of light. Before 1983, the speed of light was a measured quantity (with uncertainty); after 1983, the speed of light is exactly 299,792,458 m/s by definition, and the metre is whatever distance light covers in 1/299,792,458 second. The trick works because the second has its own atomic-clock definition independent of length — anchoring the metre to time + a fixed constant lets both definitions improve together as atomic-clock precision improves, without re-measuring the speed of light. Modern laser interferometers realise the metre to about one part in 10¹². At that precision, fitting kitchen cabinets and surveying property lines remain the same.

The American “meter” spelling — and the lone surviving international body that uses it: the US adopted “meter” in 1828 from Noah Webster’s dictionary reforms (the same wave that gave Americans “color” and “center”). The rest of the English-speaking world uses “metre”. BIPM publications consistently use “metre”; NIST publications consistently use “meter”; international standards (ISO, IEC) use “metre”. The spellings refer to identical units. Convertitive defaults to the American spelling in interface copy and the international spelling in standards citations to match each authority’s preferred form. Related: kilogram, second. Reference: BIPM SI Brochure — The metre.

Worked example

Convert a 6-foot-2-inch person to metres. Total inches: 6 × 12 + 2 = 74 inches. Multiply by the exact inch-to-metre factor: 74 × 0.0254 = 1.8796 m, conventionally rounded to 1.88 m. Going the other way: a 100-metre running track converts to 100 / 0.3048 = 328.084 feet, or 328 ft 1 in — which is why American athletic facilities built around 100-yard fields (91.44 m) feel noticeably shorter than international 100-metre sprints. For surveying, a property line of 2,500 ft is 2500 × 0.3048 = 762 m; submitting that figure on a metric-system permit with extra precision (762.000 m) overclaims accuracy and should be 762 m ± 0.3 m to reflect the input’s real precision.

When and why it matters

Unit-conversion failures have crashed spacecraft (NASA’s 1999 Mars Climate Orbiter, lost because Lockheed software output pound-seconds while JPL expected newton-seconds — a factor of 4.45 in thrust) and grounded planes (the 1983 Air Canada “Gimli Glider” ran out of fuel mid-flight after a kg/lb mix-up at refuelling). For everyday work, the consistent rule is: store the canonical metre value in any database or document and convert at the display layer for the end user. Mixing imperial and metric within a single dataset accumulates rounding error and creates hand-off bugs at every system boundary. If you absolutely must use feet/inches internally (US construction, real estate), document the precise conversion factor used and lock it (1 ft = 0.3048 m exactly; the “US survey foot”, deprecated since 2023, was a different 0.30480061 m and the difference matters over property-scale distances). Reference: NIST — SI Units.