Conversion from Square decimeters to Square kilometers

Information about the Square decimeter (dm²)

The square decimetre is a unit of area equal to one hundredth of a square metre, or one hundred square centimetres. It is written dm². It is the area of a square ten centimetres on a side, which is about the size of a palm or a drinks coaster, and it sits in the gap between the square centimetre of small objects and the square metre of rooms.

The decimetre owes its survival to volume rather than to area. A cubic decimetre is exactly one litre, which quietly builds the prefix into the most used volume unit in the world, and a square decimetre is the cross-section of that litre cube. Anyone who can picture a litre carton already knows what a square decimetre looks like: it is the face of the box.

Its one substantial commercial home is the leather trade. Hides are irregular, so they are measured by area rather than by length, and European tanneries, shoe factories and leather goods makers buy and sell in square decimetres. A whole cowhide runs to something like 200 to 250 square decimetres, a lambskin to about 50, and prices, cutting yields and waste allowances are all calculated on that basis. British and American buyers use square feet for the same purpose, and one square foot is 9.29 square decimetres.

Everyday objects land conveniently on it. A sheet of A4 paper is 6.24 square decimetres, a standard floor tile of thirty centimetres is nine, and a large postage stamp is a small fraction of one. Schools use the unit for exactly this reason: it lets a class measure real objects without either very large or very small numbers, and it makes the squaring rule visible, since ten centimetres on a side gives a hundred square centimetres.

Outside those uses it is rare, and for the usual reason. Engineering standardised on prefixes that step by a factor of a thousand, so square millimetres, square metres and square kilometres cover everything in practice. Ten square decimetres is more naturally written as 0.1 square metres, which sorts correctly beside every other figure in a specification.

Converting is straightforward once the squaring is kept in mind. Multiply by a hundred for square centimetres, divide by a hundred for square metres, and remember that the factor between neighbouring units is the square of the factor between the lengths, not the factor itself.

One square decimetre equals 100 square centimetres, 0.01 square metres, or about 15.5 square inches.

Information about the Square kilometer (km²)

The square kilometre is a unit of area equal to one million square metres, or one hundred hectares. It is written km². It is the area of a square one kilometre on a side, and it is the unit in which countries, cities, lakes, forests and administrative districts are measured almost everywhere in the world.

Its main work is geography. Monaco covers about 2 square kilometres, Manhattan about 59, Greater London around 1572, and France some 551,500. Because the unit spans that whole range without changing, it makes very different places directly comparable, which is why atlases, censuses and statistical yearbooks settle on it rather than on hectares above and square metres below.

Population density is its most quoted derived figure. Dividing inhabitants by square kilometres produces a number that is meaningful at every scale: a few per square kilometre across Mongolia or the Australian interior, a few hundred across most of western Europe, and eight thousand or more in Singapore and central Paris. The same arithmetic done in square miles gives figures 2.6 times larger, which is a common source of confusion in translated sources.

It is worth being careful about the phrasing. A square kilometre is one kilometre by one kilometre. An area described as two kilometres square is two kilometres on each side and therefore four square kilometres, not two. The distinction sounds pedantic until a fire, a flood or a proposed development is being described, at which point the difference is a factor of the side length itself.

Environmental and infrastructure figures live here. Rainfall converts neatly: one millimetre falling on one square kilometre is a thousand cubic metres, or a million litres of water. A utility-scale solar farm producing a gigawatt at peak needs roughly twenty square kilometres of ground. Wildfire extent, glacier retreat, deforestation and sea-ice cover are all reported in square kilometres because that is the resolution at which satellites usefully see them.

Against imperial units it faces the square mile, which is 2.59 square kilometres. Because a square mile also equals 640 acres, the three units chain together awkwardly, and any figure that has passed through more than one of them is worth checking. Below the square kilometre, land measurement passes to the hectare, and one square kilometre is exactly one hundred of them.

One square kilometre equals 1,000,000 square metres, 100 hectares, about 0.386 square miles, or about 247.1 acres.