| Square feet (sq ft) | Square millimeters (mm²) |
|---|---|
| 1 Square foot | 92903.04 mm² |
| 2 Square feet | 185806.08 mm² |
| 3 Square feet | 278709.12 mm² |
| 4 Square feet | 371612.16 mm² |
| 5 Square feet | 464515.2 mm² |
| 10 Square feet | 929030.4 mm² |
| 20 Square feet | 1858060.8 mm² |
| 25 Square feet | 2322576 mm² |
| 50 Square feet | 4645152 mm² |
| 100 Square feet | 9290304 mm² |
| Reference | Square feet (sq ft) | Square millimeters (mm²) |
|---|---|---|
| A sheet of A4 paper | 0.671345 sq ft | 62370 mm² |
| A tennis court | 2807.23 sq ft | 260800000 mm² |
| A football pitch | 76854.3 sq ft | 7.14 × 109 mm² |
| One hectare | 107639 sq ft | 1 × 1010 mm² |
| Central Park, New York | 36704935 sq ft | 3.41 × 1012 mm² |
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.
The square millimetre is a unit of area equal to one millionth of a square metre, written mm². It is the area of a square one millimetre on a side. It is the standard unit of engineering cross-sections, and two of the most consequential numbers in construction and electrical work are expressed in it.
Electrical cable is the first of those. Outside North America, conductors are specified by their cross-sectional area in square millimetres: 1.5 mm² for lighting circuits, 2.5 mm² for socket outlets, 6 mm² for a cooker, and 16 or 25 mm² for a house supply. Current-carrying capacity follows the area closely, so a 2.5 mm² copper conductor safely carries about twenty amperes in ordinary domestic installation, and the whole of wiring regulation is built on that relationship.
North America uses American Wire Gauge instead, where the numbers run backwards and the steps are not proportional: gauge 14 is about 2.08 mm² and gauge 12 about 3.31. Converting between the two systems is one of the more error-prone tasks in electrical work, because a gauge number carries no information about area until it is looked up.
The second consequential use is stress. One newton per square millimetre is exactly one megapascal, which is why structural drawings state steel and concrete strengths in N/mm². A common structural steel is designated S355 because it yields at 355 newtons per square millimetre, and a concrete grade of C30/37 reaches 30 newtons per square millimetre in cylinder testing. Bolts are rated the same way, using their tensile stress area: an M10 bolt has 58 square millimetres of it.
Electronics measures its products here too. A large processor die covers between 300 and 600 square millimetres, and the cost of a chip is driven directly by that number because a wafer holds a fixed area. A full-frame camera sensor is 864 square millimetres against roughly 30 for a phone camera, which is most of the reason the two behave so differently in low light.
Below the square millimetre the metric ladder continues to the square micrometre, used for transistors and cell biology, but almost nothing in ordinary manufacturing needs to go further. Above it, the square centimetre takes over at a hundred square millimetres.
One square millimetre equals 0.01 square centimetres, 0.000001 square metres, or about 0.00155 square inches.