Can you guess — what is 1 kilogram?

Inside the table in front of you, five different masses are hidden.
Pull each string and try to estimate which one has a mass of approximately 1 kilogram.

 

🔍 Check your answer

 

Lift the panel below this text.

Underneath, you will see which mass is attached to each string.

 

(In everyday life, people often overestimate or underestimate weight by as much as 50%!)

 

 

 

 

What is one kilogram, really?

 

Today we take the kilogram for granted — but humanity searched for its precise definition for more than two centuries.

 

When the first kilogram prototype was made in Paris in 1799, crafted from platinum, its mass represented the mass of one litre of water.

 

Later, in 1889, the international prototype kilogram made of platinum–iridium was produced. It was kept in Paris for more than a century and served as the global reference.

 

But even this perfectly crafted weight was not unchanging — its mass shifted over time due to microscopic deposits of dust, moisture, and chemical changes on the surface, as well as tiny losses during cleaning and handling. Although these changes were measurable only in millionths of a kilogram, they were significant enough for scientists to conclude that the kilogram must not depend on a physical object.

 

That is why, in 2019, the kilogram was finally freed from its material form and redefined using the Planck constant (h)— a fundamental constant of the universe.

 

Today, the kilogram is no longer measured by comparing it with a weight, but is realised through precise physical instruments, such as the Kibble balance.