Conversion from 100 Square millimeters to Ares

Information about the Square millimeter (mm²)

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.

Information about the Are (a)

The are is a unit of area equal to one hundred square metres, written a. It is a square ten metres on a side. It was created by the French metric legislation of 1795 as the basic decimal unit of land, taking its name from the Latin word for a level open space, and it is the parent of the hectare that eventually overshadowed it.

The original scheme was tidy. The are would measure a garden or a plot, the hectare a field, and the centiare a single square metre, giving three decimal steps that covered everything from a doorway to a farm. In practice the middle rung carried almost all the weight: fields were described in hectares and buildings in square metres, and the are was left with the narrow band between.

It nevertheless survives in land registration, and there it is entirely alive. French, Belgian and Luxembourgish deeds record parcels as hectares, ares and centiares in three columns, so a plot might be written as 1 ha 25 a 30 ca, meaning 12,530 square metres. In Poland the ar is the ordinary way of describing a building plot, and in Indonesia and parts of central Europe it fills the same role.

For scale, one are is a square ten metres on a side: a generous garden, a small yard, or the footprint of a modest house with a little room to spare. A badminton court is a little over eight tenths of an are, a single-car garage about one fifth, and a tennis court about two and a half. It is the largest unit that still describes something a person can pace out in a few seconds.

In strict SI terms the are is exactly the square decametre, since a decametre is ten metres. The two are the same quantity written two ways, and the SI form is essentially never used, which makes the are one of the few cases where the older unit is the readable one.

Its main practical trap is the symbol. A single lower-case a is easily lost in running text or mistaken for an abbreviation, which is why deeds usually write it in a fixed column and why anything ambiguous is safer converted to square metres before it is passed on.

One are equals 100 square metres, 0.01 hectares, or about 1076.4 square feet.