The kilogram is about to change. Image Courtesy BIPM What does a kilogram weigh? Middle school science classes often teach that the unit is based on the weight of water—specifically a cube of water, a tenth of a meter on each side, at just above freezing. This used to be the case, but it isn’t actually true anymore—since 1875, the kilogram has been defined by one specific platinum cylinder, known affectionately as “Le Grande K” and officially as “the International Prototype Kilogram,” or IPK. It stands stands an inch-and-a-half high and wide and is housed in a vault outside Paris, inside three concentric glass containers to protect it from dust and other weight-altering debris.

Every scale in the world—even those that measure in pounds—is ultimately based on the IPK, which was commissioned by the General Conference on Weights and Measures. But the IPK’s uniqueness may be its downfall. “The problem with the kilogram in Paris is that it’s so precious that people don’t want to use it,” said Stephan Schlamminger, a physicist with the National Institute of Standards and Technology, in an interview with the American Institute of Physics. Even handling the model kilogram with your fingers will leave oil, changing its weight ever so slightly. It’s rarely removed from its enclosure, and never transported to other areas. Most people who care about exactly how much a kilogram weighs (chemists and physicists, mostly) calibrate their most precise instruments using replicas of the IPK, not the real thing. Problematically, these replicas vary slightly in weight when compared to one another. That’s why in 2005, the International Committee for Weights and Measures proposed that the kilogram be slightly redefined, anchored not to a physical object but to some fundamental property of nature that could be easily replicated in labs across the world.