National Physical Laboratory

Kilogram Redefining the kilogram

Stored in a basement at NPL is the UK's National Standard Kilogram, a copy of the international prototype and the standard for all mass measurements in the UK. But researchers at NPL and elsewhere are working hard to make the international prototype obsolete. Find out why below…

What is the kilogram?

The kilogram is the SI unit for measuring mass. It is the last remaining base unit to be defined as a physical object rather than in terms of a naturally occurring constant. 

For example, the metre’s definition comes from the speed of light, but the kilogram is defined by the mass of the prototype kilogram, the IPK. All mass measurements are made based on the mass of this one object.

See more about the SI unit of mass on our page about the kilogram

Why do we need to redefine the kilogram?

Currently, the kilogram is defined as the mass of the international prototype, the IPK. However, as it is a solid object, it could be changed physically due to environmental changes, accidental damage, or even sabotage.

To overcome such risks and also to improve our system of units, we need a definition that depends on nothing other than the value of a fundamental constant, e.g. the Planck constant h. This is what the current Kibble balance research is working towards.

The kilogram (kg) is equal to the mass of the international prototype of the kilogram, the IPK.

The IPK is made of an alloy of platinum (90 %) and iridium (10 %) and is kept at the International Bureau of Weights and Measures (BIPM) in France.

At present all standards of mass must ultimately be traceable to this one object. But as science and industry's requirement for a more accurate way to measure extreme weights increases, the search is on for a definition of the kilogram in terms of a fundamental constant so that it can be measured with greater accuracy and can be realised by many standards laboratories.

Read More

Weighing the options
How clean is our kilogram?
The origin of the name 'kilogram' - BIPM
Redefining the kilogram - one of the five experiments as hard as finding the Higgs - Nature

Introduction to Mass

The kilogram

Find out more about the kilogram on our SI units page.

Research at NPL

Find out more about the groundbreaking research that NPL scientists are undertaking to redefine mass.

Kibble balances (formerly known as the watt balance)
Redefining the kilogram


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