| Square inches (sq in) | Square hectometers (hm²) |
|---|---|
| 1 Square inch | 0.000000064516 hm² |
| 2 Square inches | 0.000000129032 hm² |
| 3 Square inches | 0.000000193548 hm² |
| 4 Square inches | 0.000000258064 hm² |
| 5 Square inches | 0.00000032258 hm² |
| 10 Square inches | 0.00000064516 hm² |
| 20 Square inches | 0.00000129032 hm² |
| 25 Square inches | 0.0000016129 hm² |
| 50 Square inches | 0.0000032258 hm² |
| 100 Square inches | 0.0000064516 hm² |
| Reference | Square inches (sq in) | Square hectometers (hm²) |
|---|---|---|
| A sheet of A4 paper | 96.6737 sq in | 0.000006237 hm² |
| A tennis court | 404241 sq in | 0.02608 hm² |
| A football pitch | 11067022 sq in | 0.714 hm² |
| One hectare | 15500031 sq in | 1 hm² |
| Central Park, New York | 5.28551 × 109 sq in | 341 hm² |
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.
The square hectometre is a unit of area equal to ten thousand square metres. It is written hm², and it is the area of a square one hundred metres on a side, since a hectometre is one hundred metres. It is exactly one hectare, and the two names describe the same quantity by two different routes.
The routes are worth distinguishing. The hectare is built by putting the prefix hecto in front of the are, so it is a hundred ares. The square hectometre is built by squaring a prefixed length, which is how the International System forms every derived area. The second construction is the strictly coherent one; the first is the one written on every land title in the world.
That difference explains where each name is found. Hectare appears in agriculture, forestry, planning, taxation and international statistics. Square hectometre appears mainly in technical and cartographic work in Spanish-speaking countries, where the hectometre is a familiar prefixed length for other reasons, and in documents where an author has chosen to keep every unit inside strict SI form.
The related cubic form is genuinely common in one field. Spanish and Latin American hydrology reports reservoir capacity and river flow in cubic hectometres, a cubic hectometre being one million cubic metres, so a reservoir holding 500 hm³ is holding half a cubic kilometre of water. Readers used to seeing hm in that context find hm² unremarkable, which is much of why the form survives there and almost nowhere else.
For scale, ten thousand square metres is a square one hundred metres on a side. That is about one and a half association football pitches, a large city block, or a good-sized field. It is the largest area that can still be walked around comfortably in a couple of minutes, and it is where land measurement stops being architectural and starts being agricultural.
When converting, the two names need no arithmetic between them: one square hectometre is one hectare exactly. Going further, multiply by ten thousand for square metres or divide by a hundred for square kilometres, and remember that the imperial comparison, 2.471 acres, is the same for both names.
One square hectometre equals 10,000 square metres, 1 hectare, 0.01 square kilometres, or about 2.471 acres.