How much does a kilogram weigh? More than it used to, but unfortunately that doesn't mean we weigh any less.
A cylindrical hunk of platinum-iridium alloy stored in a vault in France, whose weight is defined as the world standard kilogram, is getting heavier.
Experts from the University of Newcastle say contaminants have built up on the surface of the 'International Prototype Kilogram', meaning it could be as many as tens of micrograms heavier than it was when it was first made, back in 1875.
There are 40 replicas of the matchbox-sized weight around the world, and each is likely to have gained different amounts of weight. Each country has its own methods and timetables for keeping their weights clean, adding to the growing discrepancy.
"It doesn't really matter what it weighs as long as we are all working to the same exact standard, the problem is there are slight differences," says Prof Peter Cumpson.
"Around the world, the IPK and its 40 replicas are all growing at different rates, diverging from the original.
"We're only talking about a very small change – less than 100 micrograms – so, unfortunately, we can't all take a couple of kilograms off our weight and pretend the Christmas overindulgence never happened."
A major problem with the varying gains in weight is that different countries would now have slightly different definitions of the kilogram.
"Mass is such a fundamental unit that even this very small change is significant and the impact of a slight variation on a global scale is absolutely huge," says Prof Cumpson.
"There are cases of international trade in high-value materials – or waste – where every last microgram must be accounted for."
The problem isn't restricted to the kilogram – if it changes, so must other units of measurement which are defined by their relation to the kilogram, such as the joule, a measurement of energy.
In the future, it's expected the kilogram will be redefined as a calculation based on a fundamental law of nature, reports Fox News.
"I think the definition will be changed in the next five to 10 years," says metrologist Richard Davis, a consultant for the International Bureau of Weights and Measures.