National Physical Laboratory

Kilogram 125th kilogram anniversary

The creation of the international prototype kilogram (IPK), a global standard for all mass measurements, was approved in September 1889 and 2014 marks its 125th anniversary. Although it has served us well for all these years, this may be its last major birthday.

Future of the kilogram

The kilogram is the last base SI unit that is linked to a physical object; the others are all defined in terms of naturally occurring constants, like the speed of light. By linking the definitions in this way, these units are guaranteed to be stable in the long term thus ensuring that all research and manufacturing will have a consistent standard to work from and all results will be compatible.

Scientists are close to completing two experiments, the Watt balance and the International Avogadro coordination, both of which define the value of the kilogram with relation to a fundamental constant.

Watt balances

Avogadro Project

The kilogram

A video taking a look at the kilogram, the SI base unit of mass

Micro Video ThumbThe Watt balance and redefining the kilogram
With Dr Bryan Kibble, Dr Tony Hartland and Dr Ian Robinson...

The SI unit of mass, the kilogram (kg), is defined as being equal to the mass of the international prototype of the kilogram, the IPK.

The origins of the kilogram date from 1799, when a cylinder of pure platinum was manufactured to have the same mass as one cubic decimetre (equivalent to a litre in today's terms) of water at 4 ºC. The current kilogram replaced this original weight in September 1889 when the first General Conference on Weights and Measures (CGPM) sanctioned the IPK for use as the global standard for the mass. The IPK is a cylinder of 39 mm in height and diameter and is made from an alloy of platinum (90 %) and iridium (10 %).

The IPK is kept at the International Bureau of Weights and Measures (BIPM) in Sèvres just outside Paris. National Measurement Institutes, such as the National Physical Laboratory, possess copies that they use as their national standards. These national standard kilograms are tested against the IPK about every 40 years to see whether their mass has deviated from that of the IPK.

Image gallery

  • Scientists James Berry and Simon Reilly with the UK national standard kilogram - the 18th copy of the International Prototype.
  • The UK national standard kilogram - the 18th copy of the International Prototype.
  • The British metre next to the UK national standard kilogram.
  • The UK national standard kilogram removed from its protective casing.
  • Simon Reilly opening the National Physical Laboratory's kilogram safe, located in the vault of Bushy House, Teddington.
  • The NPL mass comparator reading the mass of the silicon sphere in the Avogadro Project.
  • A close-up of the NPL mass comparator.
  • The silicon sphere used in the Avogadro Project.
  • Dr Ian Robinson looking at the Watt balance, another experiment that is trying to redefine the kilogram.

Fact File

  • The IPK would feel 27 times heavier on the Sun than on Earth, 6 times lighter on the Moon, and 12 times lighter on Pluto
  • The IPK has a fixed mass but its weight can vary by 1 milligram or more depending on the density of the air in which it is weighed
  • New experiments to define the kilogram aim to have a target uncertainty of 2 parts in 108 (0.000 002%)
  • By definition, the mass of the IPK is constant; however, at the periodic verifications the mass of the national copies were seen to have changed by up to 50 micrograms. These differences led to the press stories about the 'kilogram losing weight'.


Please note that the information will not be divulged to third parties, or used without your permission