Home SCIENCE The American Kilogram – The Metric System

The American Kilogram – The Metric System

The US signed the metre convention and bases all customary units on SI standards. As an aside, the Utah constitution from 1895 required the metric system to be taught in schools. This requirement was repealed in 1987.

image credit: Veritasium 

Back in 1875 The US signed the Metre Convention, which basically committed the country to use the metric system. In return, French scientists sent two platinum-iridium cylinders that weigh 1kg to the US in 1889 (known by their designations K4 and K20 from a set of 40 identical objects that were produced and sent around the world).

image credit: Veritasium 

So even though everything you see and buy in the US is usually reported in pounds, all weights are traceable back to the K20 kilogram (by applying a conversion factor to get to pounds).


Anyone who wants a definitive answer to the question of how much a kilogram weighs has to visit NIST and order a stainless steel cylinder calibrated against the precise weight of K20 or K4.

“Virtually every mass that has been accurately measured in the US over the past 130 years can trace its measurement back to this one kilogram hunk of 90 percent platinum and 10 percent iridium,” explains host Derek Muller.

As Derek explains, the kilogram is the last metric unit still defined by a physical artefact rather than a fundamental constant – that is, a fixed property found in nature – and that’s causing a problem for scientists.

Every few decades, K4, K20, and all the other kilograms made in the 1800s regularly take a trip back to Paris so their weights can be compared against the original International Prototype of the Kilogram.