| Square millimeters (mm²) | Square nautical miles (sq nmi) |
|---|---|
| 1 Square millimeter | 2.91553349598 × 10-13 sq nmi |
| 2 Square millimeters | 5.83106699196 × 10-13 sq nmi |
| 3 Square millimeters | 8.74660048794 × 10-13 sq nmi |
| 4 Square millimeters | 1.16621339839 × 10-12 sq nmi |
| 5 Square millimeters | 1.45776674799 × 10-12 sq nmi |
| 10 Square millimeters | 2.91553349598 × 10-12 sq nmi |
| 20 Square millimeters | 5.83106699196 × 10-12 sq nmi |
| 25 Square millimeters | 7.28883373995 × 10-12 sq nmi |
| 50 Square millimeters | 1.45776674799 × 10-11 sq nmi |
| 100 Square millimeters | 2.91553349598 × 10-11 sq nmi |
| Reference | Square millimeters (mm²) | Square nautical miles (sq nmi) |
|---|---|---|
| A sheet of A4 paper | 62370 mm² | 0.0000000181842 sq nmi |
| A tennis court | 260800000 mm² | 0.0000760371 sq nmi |
| A football pitch | 7.14 × 109 mm² | 0.00208169 sq nmi |
| One hectare | 1 × 1010 mm² | 0.00291553 sq nmi |
| Central Park, New York | 3.41 × 1012 mm² | 0.994197 sq nmi |
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.
The square nautical mile is a unit of area equal to 3,429,904 square metres, or about 3.43 square kilometres. Its symbol is sq nmi. It is the area of a square one nautical mile on a side, the nautical mile being defined as exactly 1852 metres. It is the working unit of everything that happens at sea and is measured across a surface rather than along a line.
Its usefulness comes from the definition of the nautical mile itself. One nautical mile is very nearly one minute of latitude, so a square nautical mile is close to a one-minute cell on a chart. That makes it the natural grid square for anything plotted on a nautical chart, because the unit and the graticule agree without any conversion, and a navigator can read distance and area off the same scale.
Maritime law is written in nautical miles and therefore accounted in square ones. Under the international law of the sea, a coastal state has a territorial sea of twelve nautical miles, a contiguous zone of twenty-four and an exclusive economic zone reaching two hundred. The resulting areas are vast: a state with scattered island possessions can control several million square kilometres of ocean, and comparisons between such claims are usually made after converting these figures to metric.
Search and rescue planning depends on the unit directly. A search area is defined in square nautical miles, and the time to cover it follows from the search vessel's speed in knots and the sweep width of its sensors, both of which are already in nautical units. Keeping the whole calculation in one system is what allows a coordinator to estimate coverage without stopping to convert anything.
Fisheries science and naval operations use it the same way. Fish stock density is reported per square nautical mile, survey transects are spaced in nautical miles, exercise and restricted areas are drawn as blocks of them, and aviation control zones over water are described in the same terms because aircraft also navigate in knots.
For conversion, one square nautical mile is 3.43 square kilometres, 1.32 square statute miles, or about 848 acres. The ratio to the statute mile is not intuitive, since the nautical mile is only about 15 per cent longer but the square is a third larger, which is the usual consequence of squaring a length ratio.
One square nautical mile equals 3,429,904 square metres, about 3.43 square kilometres, about 1.32 square miles, or about 848 acres.