The kilogram is changing, for accuracy's sake.
Previously it's been based on the weight of a metal block sitting in a hermetically sealed room in Paris. Now it'll be based on a quantum physics calculation derived from the Planck constant, a fundamental constant of nature and inherently stable.
The old block is believed to be mostly accurate - but it has been vulnerable to damage through environmental factors.
"It's so precious that people don't want to use it, physicist Stephan Schlamminger of the National Institute of Standards and Technology in the United States said in 2016.
Since 1879, the block has only been removed from its case a few times to check that it still weighs the same.
In 1992, it was discovered the block - called Le Grand K - had either shrunk, or its replicas around the world had increased in mass - but as Le Grand K was literally the benchmark, no one could tell for sure which had happened. It was decided in 2011 to link the kilogram to a physical constant so this wouldn't happen again.
The Planck constant is a tiny number that relates to a quantum particle's wave frequency to its energy. It never changes, unlike a slab of metal.
The new measurement, which comes into effect Monday morning, is expected to be particularly helpful for pharmaceutical research.
But don't expect the scales to change when you step on them - the new kilogram is designed to be exactly the same as the old, it just won't have to rely on an old metal block sitting in a room in Paris.
Other measurements changing today include the ampere (electric current) and kelvin (temperature).
New Zealand voted in favour of the switch to quantum physics last year.
"The change to quantum physics will open up a whole new world of innovation opportunities," said Fleur Francois, director of the New Zealand's national metrology institute, the Measurements Standards Laboratory.
"For me, when I think about space exploration - and who knows, we might colonise Mars one day - we're going to need to be able to measure to greater accuracy using standards that are more universal than a single piece of metal in Paris."