The dunam is a measurement of area and corresponds to one thousand square metres.
This is how it came to be used in modern-day Israel.
The metric system originated in revolutionary France, when reformers wanted a universal and rational system of measurement to replace the hundreds of inconsistent local units used across the country.
In 1790 the French government commissioned the French Academy of Sciences to design a new universal system of measurement.
A year later they defined the metre as a fraction of the Earth’s meridian. Looking back 250 years it seems the oddest and most incapable of being described as rational when It was settled on one ten-millionth of the distance from the equator to the North Pole along a meridian through Paris.
In 1795 France adopted the metric system by law and in 1799 they created physical platinum standards for the metre and kilogram.
Israel
Israel uses the metric system as its official system but there are a few remnants of other systems.
One remnant is that land is measured in dunams, a legacy from the Ottoman Empire.
So now comes the question of why Israel uses the metric system given that it was part of the British mandate before its establishment as a country in 1948.
The answer is that the British administration in Palestine kept the existing Ottoman land-measurement system out of convenience.
When the modern state of Israel was created in 1948, it simply continued using the system that had already been embedded in land law and cadastral records.
Under the Ottoman Empire, land was measured using the dunam after the empire introduced the metric system in the late 19th century,
After the Dissolution of the Ottoman Empire following World War I, Britain governed the region as Mandatory Palestine.
Although Britain itself used the imperial system, the colonial administration kept Ottoman legal and administrative frameworks unless there was a strong reason to change them. Land ownership records were already based on the dunam.
Replacing them with acres would have required rewriting land registries and recalculating millions of property records.
So the British continued to use the dunam in land administration, and when Israel became independent, the country adopted the metric system for general measurement and kept the Dunham for records, property law, and agricultural planning.
Since the dunam is exactly 1,000 m², it fitted within the metric framework, making it practical to retain, so it did and continues to do so today.
So a building without land might be measured in square metres, land is measured in dunams.