Conversion from 20 Square millimeters to Square decameters

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 Square decameter (dam²)

The square decametre is a unit of area equal to one hundred square metres. It is written dam², and it is the area of a square ten metres on a side, since a decametre is ten metres. It is exactly the same quantity as the are, and the choice between the two names is a choice between strict metric grammar and two centuries of habit.

The International System is built from coherent units, meaning every derived unit is a product or quotient of base units with no extra factors. Squaring a prefixed length gives a perfectly coherent area, so the square decametre needs no special permission to exist. The are, by contrast, is listed as a non-SI unit accepted for use with the SI, tolerated for land measurement rather than derived from anything.

Despite that formal advantage, the square decametre lost decisively. The are was already in every French land register when the modern system was codified, and a register does not change its columns for a matter of notation. The result is that the correct SI form is almost never written, and its accepted non-SI equivalent is what appears on deeds across half of Europe.

The quantity itself is a useful one to be able to picture, whichever name is used. A hundred square metres is a square ten metres on a side: a generous garden, the ground floor of a small house, a market stall pitch, or a large classroom. It is the last area a person can pace out accurately without instruments, and it is the natural size for describing a plot rather than a field.

Where the symbol does appear, the exponent must be read carefully. The string dam already means decametre, a length, so dam² and dam differ by an entire dimension, and a dropped superscript turns an area into a distance. Software that strips formatting from tables is a common cause of exactly that error.

For any practical purpose, converting is the right response. Multiply by a hundred for square metres, divide by a hundred for hectares, and the resulting figure will match whatever else is on the page. One square decametre is one are, and both are one hundred square metres, so nothing is lost in either direction.

One square decametre equals 100 square metres, 1 are, 0.01 hectares, or about 1076.4 square feet.