| Square inches (sq in) | Square feet (sq ft) |
|---|---|
| 1 Square inch | 0.00694444444444 sq ft |
| 2 Square inches | 0.0138888888889 sq ft |
| 3 Square inches | 0.0208333333333 sq ft |
| 4 Square inches | 0.0277777777778 sq ft |
| 5 Square inches | 0.0347222222222 sq ft |
| 10 Square inches | 0.0694444444444 sq ft |
| 20 Square inches | 0.138888888889 sq ft |
| 25 Square inches | 0.173611111111 sq ft |
| 50 Square inches | 0.347222222222 sq ft |
| 100 Square inches | 0.694444444444 sq ft |
| Reference | Square inches (sq in) | Square feet (sq ft) |
|---|---|---|
| A sheet of A4 paper | 96.6737 sq in | 0.671345 sq ft |
| A tennis court | 404241 sq in | 2807.23 sq ft |
| A football pitch | 11067022 sq in | 76854.3 sq ft |
| One hectare | 15500031 sq in | 107639 sq ft |
| Central Park, New York | 5.28551 × 109 sq in | 36704935 sq ft |
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 foot is a unit of area equal to 144 square inches, or 0.092903 square metres. Its symbol is sq ft, sometimes written ft². It is the area of a square one foot on a side, and it is the unit in which property is bought, sold and rented across the English-speaking world and in several places that are otherwise entirely metric.
Real estate is its home. American and British homes are advertised by their floor area in square feet, and commercial space is let at a rent per square foot per year, which is the standard way of comparing one building or one city with another. A one-bedroom flat runs to about 500 square feet, a family house between 1500 and 2500, and an office desk with its share of circulation is reckoned at 150 to 200.
Its reach goes beyond countries that use imperial units generally. Apartments in India are sold by the square foot despite the country being metric in every other respect, and Hong Kong quotes flat sizes and prices the same way, which produces some of the highest per-square-foot figures on earth. In both cases the unit survived because the property market adopted it before metrication and never had a reason to change.
Building trades use it for materials and for capacity. Flooring, carpet, tile, plasterboard and insulation are priced by the square foot in North America; roofing has its own multiple, the square, equal to one hundred square feet; and air conditioning is sized in British thermal units per square foot of conditioned space. Leather is sold by the square foot in the United States where European tanneries use square decimetres.
The conversion to metric is exact but inconvenient. Since the foot is defined as exactly 0.3048 metres, one square foot is exactly 0.09290304 square metres, and one square metre is 10.7639 square feet. Neither figure is memorable, so converted property listings often show suspiciously round numbers in one system and awkward ones in the other, which is a reliable sign of which unit the original was written in.
Above it, land moves to the acre at 43,560 square feet, and below it the square inch takes over at one hundred and forty-fourth of the value. That factor of 144 rather than 100 is what makes mental arithmetic in imperial areas harder than in metric ones.
One square foot equals 144 square inches, about 0.0929 square metres, or about 0.1111 square yards.