Surprisingly enough, nowadays, one kilogram weighs less than it did a century ago…about 50 micrograms less. The 118-year-old international prototype for the metric mass, kept tightly under lock outside Paris, is mysteriously losing weight.

"The reference kilo appears to have lost 50 micrograms compared with the average of dozens of copies. The mystery is that they were all made of the same material, and many were made at the same time and kept under the same conditions, and yet the masses among them are slowly drifting apart. We don't really have a good hypothesis for it.", said American physicist Richard Davis of the International Bureau of Weights and Measures in Sevres, southwest of Paris.

The kilogram's shift could affect even countries that did not adopt the metric system, as it is the ultimate weight standard for the U.S. customary system, a kilogram corresponding to 2.2 pounds. But the change also worries researchers, as it impairs measurements like electricity generation. "They depend on a mass measurement and it's inconvenient for them to have a definition of the kilogram which is based on some artifact," said Davis.

But for people who are on a diet, this won't turn their 'calculations' upside down, as 50 micrograms is the mass of, for example, a fingerprint. "For the lay person, it won't mean anything. The kilogram will stay the kilogram, and the weights you have in a weight set will all still be correct.", said Davis.

The Sevres standard kilogram is the only one that counts amongst the world's kilograms, heavily guarded in a triple-locked safe and rarely put out, mainly for comparison with other cylinders from around the world.

"It's not clear whether the original has become lighter, or the national prototypes have become heavier. But by definition, only the original represents exactly a kilogram." said Michael Borys, a senior researcher with Germany's national measures institute in Braunschweig.

The cylinder, an ultimate in technology for 1889, was made of platinum and iridium alloy, having 1.54 inches (4 cm) in diameter and height. The most probable alternative for a 21st-century kilogram is a sphere built of a Silicon-28 isotope crystal, containing a single atom species and a fixed mass.