Currently, it is defined by the weight of a platinum-based ingot called "Le Grand K" which is locked away in a safe in Paris.
On Friday, researchers meeting in Versailles voted to get rid of it in favour of defining a kilogram in terms of an electric current.
The decision was made at the General Conference on Weights and Measures.
But some scientists, such as Perdi Williams at the National Physical Laboratory in the UK, have expressed mixed feelings about the change.
"I haven't been on this project for too long but I feel a weird attachment to the kilogram," she said.
"I think it is such an exciting thing and this is a really big moment. So I'm a little bit sad about [the change]. But it is an important step forward and so the new system is going to work a lot better. It is also a really exciting time, and I can't wait for it to happen."
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