| Square inches (sq in) | Square yards (sq yd) |
|---|---|
| 1 Square inch | 0.000771604938272 sq yd |
| 2 Square inches | 0.00154320987654 sq yd |
| 3 Square inches | 0.00231481481481 sq yd |
| 4 Square inches | 0.00308641975309 sq yd |
| 5 Square inches | 0.00385802469136 sq yd |
| 10 Square inches | 0.00771604938272 sq yd |
| 20 Square inches | 0.0154320987654 sq yd |
| 25 Square inches | 0.0192901234568 sq yd |
| 50 Square inches | 0.0385802469136 sq yd |
| 100 Square inches | 0.0771604938272 sq yd |
| Reference | Square inches (sq in) | Square yards (sq yd) |
|---|---|---|
| A sheet of A4 paper | 96.6737 sq in | 0.0745939 sq yd |
| A tennis court | 404241 sq in | 311.914 sq yd |
| A football pitch | 11067022 sq in | 8539.37 sq yd |
| One hectare | 15500031 sq in | 11959.9 sq yd |
| Central Park, New York | 5.28551 × 109 sq in | 4078326 sq yd |
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 yard is a unit of area equal to nine square feet, or 0.83612736 square metres exactly. Its symbol is sq yd, sometimes written yd². It is the area of a square one yard on a side, and 4840 of them make an acre. It occupies the awkward middle of the imperial ladder: too large for a workshop, too small for a field, and used mainly for surfaces that are laid rather than built.
Ground coverings are its main business. Turf is sold and laid by the square yard, asphalt paving and road resurfacing are measured in square yards in the United States, and carpet was priced that way for most of the twentieth century before American retailers moved to the square foot. Anything that arrives on a roll or a pallet and is spread out tends to be counted in this unit.
Textiles use it in an indirect but important way. Cloth is bought by the linear yard off a roll of known width, but its weight is specified in ounces per square yard, which is what allows two fabrics of different widths to be compared. A shirting might be four ounces per square yard and a heavy canvas eighteen, and those numbers describe the cloth more usefully than any measurement of the bolt does.
It also carries a large part of the South Asian property market. In Pakistan and northern India, plots are commonly sold in gaz, which is the square yard under a local name, so a house plot might be advertised at 240 gaz. Larger holdings switch to the marla and the kanal, both of which are defined as multiples of the square yard, so the whole regional system rests on this unit even where the English name is never used.
Its relationship to the rest of the imperial ladder is unusually clean at one step and unusually messy at the next. Nine square feet is easy; 1296 square inches and 4840 to the acre are not. That mixture is characteristic of a system built from body measures and ploughing practice rather than from a single decimal idea.
Conversion to metric is exact but not memorable: one square yard is 0.83612736 square metres and one square metre is 1.19599 square yards. A rough rule that a square metre is about a fifth larger than a square yard is close enough for judging a room, though not for ordering material.
One square yard equals 9 square feet, 1296 square inches, about 0.8361 square metres, or about 0.000207 acres.