It's one of those questions that can turn you in circles the longer you think about it: How much does a kilogram weigh? Because the kilogram is itself a unit of measurement, the question becomes a kind of notional Mobius strip, endlessly flipping back on itself.

But actually, the answer is surprisingly simple, and it's sitting in a basement outside Paris. Really. Jules Suzdaltsev has the twisty details in today's DNews dispatch.

First, some history: The kilogram is equal to 1,000 grams, so the gram is really the base metric here, and it was originally defined as the mass of one cubic centimeter of water at the melting point of ice. So the kilogram should ostensibly be the weight of 1,000 cubic centimeters of water -- except that it isn't. Instead, by virtue of international agreement, a kilogram is defined by the weight of one single object: a cylinder of platinum alloy called the International Prototype of the Kilogram, or IPK.

Also known as Le Grand K or the Big K, the IPK is kept in a climate-controlled safe in the basement of the International Bureau of Weights and Measures, on the outskirts of Paris, France. It's about the size of a golf ball and was put there all the way back in 1889. For all intents and purposes -- literally all intents and purposes in the scientific world -- the weight of this physical object in France defines the kilogram.

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As you might imagine, it's rather critical that scientists around the world agree on the specifics regarding the kilogram. Many of the fundamental equations of physics rely on this single metric. The newton, for instance, is defined by the amount of force necessary to move a kilogram at one meter per second squared. The pascal is defined as one newton per square meter, or one kilogram per meter per second squared -- and so on into the infinite abyss of mathematics.

Preserving the integrity of the IPK is therefore critical. The cylinder is made up of 90 percent platinum and 10 percent iridium, making it incredibly hard, resistant to oxidation, and nearly twice as dense as lead. It's pretty tough, but measurements taken in 1948 and 1989 suggest that the IPK is slowly losing weight -- about 50 micrograms in total.

No one knows why, but it's a problem. In 2011, The General Conference on Weights and Measures decided to figure out a better system for defining the kilogram. It was agreed to redefine the kilogram based on Planck's constant, a number representing the relationship between the energy of a particle and its frequency. However, the decision to officially implement the change was postposed to 2014, then again to 2018.

As such, the kilogram remains a chunk of metal in France. Check out Jules' report for more details, or click on over to our similarly themed mind bender: How Long is a Second?

**Nature: Kilogram conflict resolved at last**

**NPR: This Kilogram Has A Weight-Loss Problem**

**National Geographic: Elusive Quest for One True Kilogram Finally Pays Off**