Fundamentals
The History of the Metric System and Why It's Used Worldwide
· Updated · 4 min read
In short
The metric system was created in 1790s France to replace thousands of inconsistent local measures with one decimal system derived from nature. Its use of powers of ten made conversion trivial, which is the main reason nearly every country adopted it — and why 1 g = 1,000 mg everywhere.
On this page
The reason 1 gram equals exactly 1,000 milligrams — and the reason that is true in every country without exception — is a deliberate design decision taken in France in the 1790s. It replaced a situation almost impossible to imagine today.
The problem it was built to solve
Pre-revolutionary France is estimated to have had something on the order of a quarter of a million distinct units of measure in active use. Units varied by town, by trade, and sometimes by the commodity being weighed. A *livre* in one province was not a *livre* in the next.
This was not merely inconvenient. It made trade across regions difficult, made contracts hard to enforce, and gave whoever controlled the local standard a quiet advantage over everyone using it. Reform of measurement was, in its time, a genuinely political demand.
Two design principles
The Academy of Sciences settled on two ideas that between them define the system still in use.
Derive the units from nature, not from authority
The metre was defined as one ten-millionth of the distance from the equator to the North Pole along the meridian through Paris. The intent was that no country could claim the standard as its own — it belonged, in the phrase of the time, *à tous les temps, à tous les peuples*: for all time, for all people.
Make every relationship a power of ten
This is the decision that matters most in daily use. Sub-units and multiples would be related by factors of ten, and named with consistent prefixes. Converting between them would therefore require no arithmetic beyond moving a decimal point.
Key takeaway
The choice of powers of ten is why milligrams convert to grams by shifting a decimal point three places, and why no conversion factor needs memorising beyond the prefixes themselves.
Where the gram came from
Mass was tied to length through water. The gram was originally defined as the mass of one cubic centimetre of water at the temperature of melting ice — a definition that required no reference object at all, only a procedure anyone could reproduce.
In practice a physical standard was still needed for accurate comparison, and the artefact produced in 1799 was a platinum cylinder of a kilogram — a thousand grams — because a gram-sized reference would have been too small to use precisely. That is the origin of the system's oddest feature: the SI base unit of mass carries a prefix.
| Year | Event |
|---|---|
| 1790 | The French National Assembly commissions a new system of measures |
| 1795 | The metric system is formally defined; the prefixes milli-, centi- and kilo- are introduced |
| 1799 | Platinum standards for the metre and kilogram are deposited in Paris |
| 1875 | The Metre Convention is signed by 17 nations, creating an international framework |
| 1889 | The International Prototype Kilogram becomes the world standard |
| 1960 | The modern SI system is formalised, adding micro- and other prefixes |
| 2019 | The kilogram is redefined in terms of the Planck constant, retiring the physical artefact |
Why the world adopted it
Adoption was neither immediate nor smooth — France itself briefly abandoned the system under Napoleon before reinstating it. What eventually carried it was a combination of practical advantages that compounded over time.
- Conversion is trivial. No factor of 16, 14 or 12 to remember; every step is a power of ten.
- One system covers everything. The same prefixes work for mass, length, volume and more.
- It is nobody's national system. Adopting it did not mean adopting a rival country's units.
- International trade rewards it. Once major trading partners standardised, the cost of not doing so grew.
- Science requires it. Scientific publication has been effectively metric-only for well over a century.
Today the metric system is the official system of measurement in nearly every country. The most-cited exceptions — the United States, Liberia and Myanmar — are more nuanced than usually described: US science, medicine and pharmaceuticals are entirely metric, which is exactly why medicine bottles in American pharmacies are labelled in milligrams.
The 2019 redefinition
For 130 years the kilogram was defined by a single platinum-iridium cylinder held near Paris. Comparisons with its official copies showed drift of tens of micrograms over a century — a tiny amount, but troubling in principle: if the artefact changed, the definition changed with it.
In May 2019 the kilogram was redefined in terms of the Planck constant, whose value was fixed by definition. Mass is now realised through an experimental apparatus rather than a physical object, returning to the original ambition of a standard derived from nature rather than from an artefact.
Did this change any conversions?
No. The redefinition was designed so that the new kilogram matched the old one as closely as measurement allowed. 1 g remains exactly 1,000 mg, and every figure on this site is unaffected.
Grams → Milligrams
1,000mg
1 g = 1,000 mg
Both sides weigh exactly the same — only the unit changes.
A relationship fixed by definition in 1795, and unchanged by every redefinition since.
Quick facts
- The metric system was defined in 1795 in France.
- milli- comes from Latin mille (thousand); micro- from Greek mikrós (small).
- The kilogram is the only SI base unit with a prefix, for historical reasons.
- The Metre Convention of 1875 made the system international.
- Since 2019 the kilogram has been defined by the Planck constant, not an object.
Why any of this matters to a conversion
Because it explains why milligram-to-gram conversion needs no qualification. There is no American gram and no European gram, no gram for medicine and another for jewellery. The relationship was fixed by definition more than two centuries ago and has been protected by international agreement ever since.
The mechanics of the prefix system are covered in the metric prefixes explained, and the comparison with imperial units in grams, milligrams and ounces.
About this article
Written and maintained by the Milligrams to Grams editorial team. Every conversion figure in this article is generated with the same exact-decimal engine that powers the site's calculator, and is verified by an automated test suite rather than typed by hand. Read more about our methodology and accuracy standards.