An anonymous reader writes: As the capabilities of science and engineering expand, they rely more on the precision of measurements. It's vitally important, then, to make sure the standard units underpinning those measurements don't change. This is a problem for the kilogram. For years, it has remained the only SI unit based on a physical object — a small cylinder of platinum and iridium. Scientists have been arguing about how to replace it for decades, but now it looks like their efforts are finally reaching fruition. They finally have all the data they need to define the kilogram with mathematical constants, which solves the problem of the variability of physical objects. "One method, pioneered by an international team known as the Avogadro Project, involves counting the atoms in two silicon-28 spheres that each weigh the same as the reference kilogram. This allows them to calculate a value for Avogadro's constant, which the researchers convert into a value for Planck's constant. Another method uses a device called a watt balance to produce a value for Planck's constant by weighing a test mass calibrated according to the reference kilogram against an eletromagnetic force." Further research has narrowed down the value of Planck's constant, and experimental data from standards bodies is finally matching up. "If they are proved right, in 2018, Le Grand K will join the meter as a museum piece."