The next time you buy groceries or fill your tank, you’re relying on the metric system — a global standard made possible by the Treaty of the Metre, signed 150 years ago.
On May 20, 1875, representatives from 17 nations gathered in Paris to sign the Metre Convention, aiming to unify the world’s disjointed measurement systems. At the time, inconsistent units across cities, states, and countries complicated trade and science.
The treaty led to the creation of the International Bureau of Weights and Measures, which first defined the metre and kilogram. Over time, other units were added to establish the International System of Units (SI). Australia joined the treaty in 1947.
The Origins of the Metre
Although the treaty was signed in 1875, the metre’s origin goes back to the French Revolution in the late 18th century. Revolutionaries, seeking to replace systems tied to royalty and religion, wanted a measurement based on nature — not the whims of monarchs.
So, they defined one metre as one ten-millionth of the distance from the North Pole to the equator, measured along the Paris meridian. After seven years of astronomical work, scientists presented their best estimate in 1799, and a platinum bar — the “Metre of the Archives” — was created to serve as the official standard. Later, more stable platinum-iridium metre bars were distributed globally.
Light Becomes the New Ruler
Scientific progress led to further refinements. By the 20th century, researchers discovered that light, with its consistent wave patterns, could serve as an extremely precise ruler. In 1960, the metre was redefined in terms of the wavelength of light emitted by krypton gas.
But even that wasn’t precise enough for modern technology, where tiny components — like those in smartphones — are measured in billionths of a metre. The need for more accurate measurements led scientists to atomic clocks, which use laser-induced atomic oscillations to measure time at incredibly fine scales.
In 1983, the metre was redefined as the distance light travels in a vacuum in 1/299,792,458 of a second — linking it permanently to the speed of light and time measurement.
This high precision allows scientists to do things like measure the distance to the Moon. Thanks to mirrors left on the lunar surface by Apollo astronauts, lasers can be bounced off them to calculate the Moon’s position with remarkable accuracy — even detecting that it’s drifting away from Earth at 3.8 cm per year.
Metric System Adoption: A Mixed Timeline
Despite the metre’s global origins, adoption of the metric system varied widely. Australia signed the treaty in 1947 but didn’t begin its official metric conversion until the 1970 Metric Conversion Act. An advertising campaign helped Australians transition over the following years.
Other countries have been slower. The U.S., a founding signatory of the treaty, still uses imperial units in daily life — though its legal measurement standards are based on the metric system.
In Australia, metric units dominate, but imperial units still appear — like in men’s clothing or screen sizes. Even common cooking measures vary: an Australian tablespoon equals four teaspoons, while in most countries it’s three.
Dr Bruce Warrington, chief metrologist at Australia’s National Measurement Institute, notes these inconsistencies with a touch of humor: “As a professional measurement nerd, I find it slightly frustrating.”
