| Square kilometers (km²) | Square inches (sq in) |
|---|---|
| 1 Square kilometer | 1550003100.01 sq in |
| 2 Square kilometers | 3100006200.01 sq in |
| 3 Square kilometers | 4650009300.02 sq in |
| 4 Square kilometers | 6200012400.02 sq in |
| 5 Square kilometers | 7750015500.03 sq in |
| 10 Square kilometers | 15500031000.1 sq in |
| 20 Square kilometers | 31000062000.1 sq in |
| 25 Square kilometers | 38750077500.2 sq in |
| 50 Square kilometers | 77500155000.3 sq in |
| 100 Square kilometers | 155000310001 sq in |
| Reference | Square kilometers (km²) | Square inches (sq in) |
|---|---|---|
| A sheet of A4 paper | 0.00000006237 km² | 96.6737 sq in |
| A tennis court | 0.0002608 km² | 404241 sq in |
| A football pitch | 0.00714 km² | 11067022 sq in |
| One hectare | 0.01 km² | 15500031 sq in |
| Central Park, New York | 3.41 km² | 5.28551 × 109 sq in |
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.
The square inch is a unit of area equal to 6.4516 square centimetres exactly. Its symbol is sq in, sometimes written in². It is the area of a square one inch on a side, and one hundred and forty-four of them make a square foot. Its most important use is not as an area at all but as the denominator of the pound per square inch, the pressure unit that governs tyres, hydraulics and gas supply across the English-speaking world.
That pressure unit does most of the work. A car tyre is inflated to around 32 pounds per square inch, a bicycle tyre to 80 or more, a domestic water supply arrives at 40 to 60, and hydraulic machinery runs at several thousand. Because a hydraulic cylinder produces a force equal to pressure times piston area, the square inch is what turns a pressure gauge reading into a load, which is why the unit is printed on machinery even in workshops that measure everything else in millimetres.
Materials strength uses a multiple of it. Steel and aluminium are rated in thousands of pounds per square inch, written ksi, and a common structural steel yields near 50 ksi. Concrete in the United States is specified in pounds per square inch, with 3000 to 5000 typical for building work. The corresponding metric figures in megapascals are smaller by a factor of about 6.9, and mixing the two produces some of the most dangerous errors in engineering.
As a plain area it describes small flat things. A postage stamp is about one square inch, a large coin rather less, a credit card about 3.3, and the die inside a large processor around one. Gaskets, contact patches, wound dressings, printed labels and machined faces are all quoted in square inches in American workshops.
The conversion to metric is exact rather than approximate. Because the inch is defined as exactly 2.54 centimetres, one square inch is exactly 6.4516 square centimetres, or 645.16 square millimetres. The exactness is worth knowing, because it means a converted figure can be checked rather than merely trusted.
Its awkwardness lies in the ladder above it. A square foot is 144 square inches and a square yard is 1296, so imperial area arithmetic never reduces to shifting a decimal point. That is the practical reason engineering drawing in Britain moved to millimetres while American practice stayed with the inch and simply accepted the multiplication.
One square inch equals 6.4516 square centimetres, 645.16 square millimetres, or about 0.006944 square feet.