SI units

kilogram (kg)

The kilogram is the SI base unit of mass

Accurately measuring the mass of an object in kilograms is essential in many applications, from administering the optimum dose of a drug to correctly manufacturing materials with the desired properties.

For more than a hundred years we compared the gravitational force on an object with the gravitational force on a reference piece of metal known as a 'standard weight'. The standard weight was in turn compared with the International Prototype of the Kilogram (IPK), held in the International Bureau of Weights and Measures in France. However, this may have changed since it was produced in 1884; contamination, cleaning or just time may have increased or decreased it.

Now we compare the gravitational force on an object with a magnetic force using a Kibble balance. A kilogram is therefore defined using the Planck constant, a fixed numerical constant which will not change over time.


The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m2 s−1, where the metre and the second are defined in terms of c and ∆ν.

This was a new definition in May 2019.

More about the redefinition in May 2019


NPL provides traceability for measurement of active components of drugs that can weigh from less than a millionth of a kilogram to oil sea platforms that can weigh over 200 million kilograms.

Did you know?

  • A 10cm cube of water at 4°C has a mass of a kilogram and a metre cube has a mass of approximate a tonne
  • NPL is home to the 18th copy of the International Prototype of the Kilogram
  • The Kibble balance was developed at NPL in the UK
  • A balance comparing the weight of a tonne of feathers and a tonne of lead will change according to temperature and atmospheric pressure, due to the different buoyancy effects of the air displaced by the objects

The science behind the unit

The kilogram was the final base unit to be defined by a physical object. Science and industry required a more accurate way to measure extreme weights, so the new definition of the kilogram is in terms of a fundamental constant of nature, which improves its long-term stability and eliminates the necessity for traceability to a single physical artefact and improves scalability.

NPL has developed the Kibble balance, which balances the gravitational force with an electromagnetic force. The electromagnetic force can be calculated in terms of h, the Planck constant. The ultimate target is to measure a kilogram with an accuracy of a millionth of one percent, every time.

The weight of a 1 kg mass is balanced against the electromagnetic force generated by a current-carrying coil hung in a magnetic field. The ratio of the force generated by the coil to the current passing through it is calibrated in a second phase of the experiment, which measures the voltage generated by the coil as it is moved at a measured velocity through the magnetic field. As the voltage and the current are measured using quantum electrical standards, the kilogram can be defined in terms of a fixed value of the Planck constant plus the existing definitions of the metre and the second.