Brought to you by Newscorp Australia

The world is about to agree to update the definition of a kilogram, changing maths forever

John Leicester, November 15, 2018 6:40AM AP

Print Article

The international prototype of a kilogram. Picture: AFP media_cameraThe international prototype of a kilogram. Picture: AFP


Reading level: red

The kilogram is getting an update.

A kilogram of fruit will still weigh a kilogram. But the way scientists define the exact mass* of a kilogram is about to change.

Until now, its mass has been defined by a golf ball-sized metal cylinder locked in a vault in France. For more than 100 years, it has been the one true kilogram upon which all others were based.

No longer.

Gathering in Versailles, west of Paris, France, governments are expected on Friday to approve plans to instead use a scientific formulation* to define the exact mass of a kilogram, or kilo for short.

It will mean redundancy for the so-called Grand K, the kilo that has defined the kilo for the whole world since it was made in 1889.

Made of a corrosion-resistant* alloy of 90 per cent platinum and 10 per cent iridium, the international prototype* kilo has rarely seen the light of day. Yet its role has been crucial, as the foundation for the globally accepted system for measuring mass upon which things like international trade depend.

Three different keys, kept in separate locations, are required to unlock the vault where the Grand K and six official copies — collectively known as “the heir and the spares” — are entombed together under glass bell-jars at the International Bureau of Weights and Measures (BIPM), in Sevres on the western outskirts of Paris.

Founded by 17 nations in 1875, BIPM is the guardian of the seven main units the whole world uses to measure the world: the metre for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for the amount of a substance and the candela for luminous intensity.

Of the seven, the kilo is the last still based on a physical object, the Grand K. The metre, for example, used to be a metre-long metal bar but is now defined as the length that light travels in a vacuum in 1/299,792,458th of a second.

media_cameraA replica of the International Meter Prototype is pictured at the International Bureau of Weights and Measures, in Sevres, near Paris. The meter used to be a meter-long metal bar but is now defined as the length that light travels in a vacuum in 1/299,792,458th of a second. Picture: AP

“This, if you like, is a moment of celebration because it’s like the last standard remaining from 1875 that will finally be replaced by new innovation,” Martin Milton, the BIPM director, said in an interview with news agency Associated Press. “Everything else has been recycled and replaced and improved. This is the last improvement that dates back to the original conception in 1875. So that’s a tribute to what was done in 1875, that it’s lasted this long.”

Martin J.T. Milton media_cameraHead of BIPM (International Bureau of Weights and Measures) Martin J.T. Milton holds a replica of the International Prototype Kilogram in Sevres, near Paris. The golf ball-sized metal cylinder at the heart of the world’s system for measuring mass is heading into retirement. Picture: AP

Only rarely, and very carefully, have the BIPM’s master kilos been taken out so that other kilos sent back to Sevres from around the world could be compared against them, to be sure they were still properly calibrated*, give or take the mass of a dust particle or two.

Kilograms from around three dozen other countries were measured in Sevres against the BIPM’s master kilos in a painstaking calibration exercise from 1988 to 1992.

The kilo is “a tribute to man’s ability to collaborate*,” Mr Milton said. “It’s been called a great work of peace, actually, because it’s one of the areas where all of the states of the world come together with absolutely the same objective.”

The metal kilo is being replaced by a definition based on a very complex physics equation. In the future, scientists should be able to accurately calculate an exact kilo, without having to measure one precious lump of metal against another.

media_cameraSetting up a weight at the International Bureau of Weights and Measures. Picture: AP

Mr Milton said the change will have applications in computing, manufacturing, pharmaceuticals*, the study of climate change and other sciences where precise measurements are required.

“The system will be intrinsically* correct by reference to the laws of science, the laws of nature,” he said. “We won’t have to depend on just assuming that one particular object never changes.”

media_cameraExecutive Secretary of the Consultative Committee for Mass and related quantities (CCM) Dr Hao Fang at the International Bureau of Weights and Measures, in Sevres, near Paris. Picture: AP


How does the Australian dollar get its value?

How Aussie coins are made

World 1886 penny farthing record still stands

Good at maths equals good with money


  • mass: what you measure when you weigh something
  • formulation: a maths or science description
  • corrosion resistant: won’t rust
  • prototype: model, standard, first of its kind
  • calibrated: measured against
  • collaborate: work together
  • pharmaceuticals: medicines
  • intrinsically: an essential part of it



  • What has a kilogram been defined as until now?
  • What is Grand K?
  • When was it made and what is it made of?
  • What are the seven units of measurement BIPM holds?
  • Why has the kilo been called a great act of peace?

1. How many methods?
List all of the different methods that we can use to measure how big, how long or how heavy things are. How many can you think of?

Time: Allow 15 minutes
Curriculum Links: Mathematics

2. Extension
Why is it important for the world to have exactly the same way of knowing how heavy a kilogram should be? Plan and create a poster that will help other students understand this.

Time: Allow 30 minutes
Curriculum Links: Mathematics, Visual Communication Design, English

After reading the article, with a partner, highlight as many connectives as you can find in pink. Discuss if these are being used as conjunctions, or to join ideas and create flow.

HAVE YOUR SAY: The universe is full of amazing numbers and other maths facts. What is the most amazing number or maths fact you know? A big distance? A fast speed? A world record?
Share your favourite and what it means.

Extra Reading in mathematics