| Square kilometers (km²) | Square centimeters (cm²) |
|---|---|
| 1 Square kilometer | 10000000000 cm² |
| 2 Square kilometers | 20000000000 cm² |
| 3 Square kilometers | 30000000000 cm² |
| 4 Square kilometers | 40000000000 cm² |
| 5 Square kilometers | 50000000000 cm² |
| 10 Square kilometers | 100000000000 cm² |
| 20 Square kilometers | 200000000000 cm² |
| 25 Square kilometers | 250000000000 cm² |
| 50 Square kilometers | 500000000000 cm² |
| 100 Square kilometers | 1000000000000 cm² |
| Reference | Square kilometers (km²) | Square centimeters (cm²) |
|---|---|---|
| A sheet of A4 paper | 0.00000006237 km² | 623.7 cm² |
| A tennis court | 0.0002608 km² | 2608000 cm² |
| A football pitch | 0.00714 km² | 71400000 cm² |
| One hectare | 0.01 km² | 100000000 cm² |
| Central Park, New York | 3.41 km² | 3.41 × 1010 cm² |
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 centimetre is a unit of area equal to one ten-thousandth of a square metre, written cm². It is the area of a square one centimetre on a side, and it contains one hundred square millimetres. It is the working unit for anything the size of a hand or smaller: wounds, printed circuits, laboratory cultures, sensor chips and postage stamps.
The arithmetic catches people out. Because area scales with the square of length, one hundred square centimetres make a square decimetre and ten thousand make a square metre, not one hundred. A square ten centimetres on a side is 100 cm², and doubling its side to twenty centimetres gives 400 cm², not 200. Nearly every mistake made with this unit comes from applying the linear factor instead of the squared one.
Medicine measures skin in it. Wounds, ulcers, burns and skin lesions are recorded in square centimetres so that healing can be tracked as a shrinking number over weeks, and dermatological treatments are dosed by the area treated. A typical adult palm covers roughly 100 square centimetres, which is why the palm is used as a rough field estimate of burn extent.
Laboratories rely on it for cell culture. Flasks are named by their growth surface, so a T25 offers 25 square centimetres and a T75 offers 75, and cell densities are counted per square centimetre. Photovoltaic research reports efficiency on cells of exactly one square centimetre, precisely so that results from different laboratories describe the same thing.
Engineering uses it for pressure in some traditions. The kilogram-force per square centimetre, close to one atmosphere, is still printed on tyre gauges, pressure vessels and hydraulic equipment in Japan, Korea and parts of eastern Europe, and it survives because the number it produces is conveniently near the values people already knew. Printed circuit boards are quoted and priced by the square centimetre for the same practical reason.
Against the imperial system it faces the square inch, which is 6.4516 square centimetres exactly. That factor comes straight from the inch being defined as exactly 2.54 centimetres, so unlike most imperial conversions this one is not an approximation, merely an awkward number.
One square centimetre equals 100 square millimetres, 0.0001 square metres, or about 0.155 square inches.