Go into a supermarket today and you’ll find milk in litres and flour in kilograms. But how did ancient Egyptians measure their groceries, thousands of years before kilograms were invented?

Egyptologists have been piecing together this puzzle for many years. Some of the earliest clues found were measuring rods, made of stone. These rods were used to measure distances. The most important distance was the royal cubit, about 52 centimetres long.

Archaeologists have also uncovered documents that might be ancient maths textbooks. The Rhind papyrus contains several problems and worked solutions. Several of the questions are about storing grain, and the answers are given in hekats. From these questions, mathematicians calculated that one hekat was about 4.8 litres. But it didn’t answer the question: how did an Ancient Egyptian measure a hekat?

Recently, researchers from Israel noticed a surprising fact – a sphere that measured one royal cubit around its equator had a volume of half a hekat. Half a hekat is about 2.4 litres, which would be convenient for measuring grains or liquids for a few people.

The researchers took measurements of hundreds of ancient Egyptian jars, to see if they had a volume of half a hekat. Although the jars were not completely spherical, they had roughly the expected volume and measured about one royal cubit around their widest part. Jars from nearby Phonecia were spherical, and even more accurate.

Only jars from Egypt and nearby countries had these measurements. Jars from further away were different sizes and shapes. At least we now know how shoppers likely measured their groceries in ancient Egypt.