| Square meters (m²) | Decares (daa) |
|---|---|
| 1 Square meter | 0.001 daa |
| 2 Square meters | 0.002 daa |
| 3 Square meters | 0.003 daa |
| 4 Square meters | 0.004 daa |
| 5 Square meters | 0.005 daa |
| 10 Square meters | 0.01 daa |
| 20 Square meters | 0.02 daa |
| 25 Square meters | 0.025 daa |
| 50 Square meters | 0.05 daa |
| 100 Square meters | 0.1 daa |
| Reference | Square meters (m²) | Decares (daa) |
|---|---|---|
| A sheet of A4 paper | 0.06237 m² | 0.00006237 daa |
| A tennis court | 260.8 m² | 0.2608 daa |
| A football pitch | 7140 m² | 7.14 daa |
| One hectare | 10000 m² | 10 daa |
| Central Park, New York | 3410000 m² | 3410 daa |
The square metre is the SI unit of area, written m². It is the area of a square whose sides each measure one metre, and because the metre is a base unit, every other area unit in the system is derived from it by squaring a length. It is the unit in which most of the world buys homes, specifies materials and describes land.
Property is its largest everyday use. Outside the United States and, in part, the United Kingdom, dwellings are advertised, taxed and compared by their floor area in square metres, and price per square metre is the standard way of comparing one district with another. A modest one-bedroom flat runs to about 45 square metres, a family house between 100 and 150, and a single parking space takes up roughly 12.5.
Materials are sold and specified by the square metre because coverage, not volume, is what a builder needs to know. A litre of emulsion paint covers about ten square metres, floor tiles and laminate are priced by the square metre, and insulation is rated by its thermal resistance per square metre. Paper carries its own version of the same idea: the weight of a sheet stock is given in grams per square metre, so 80 g/m² office paper and 300 g/m² card are compared on one scale.
Physics uses the square metre as the denominator of almost every flux. Solar irradiance at the earth's surface peaks near 1000 watts per square metre, which is the figure solar panels are rated against. Pressure is force per square metre, and one newton per square metre is one pascal by definition. Heat loss through a wall is quoted in watts per square metre per kelvin, and sound intensity in watts per square metre.
Medicine uses it for the human body. An adult's skin covers between 1.6 and 2.0 square metres, and that figure is calculated for each patient before chemotherapy, because many drug doses are prescribed in milligrams per square metre rather than per kilogram. Burn severity is assessed as the percentage of body surface affected, using the same underlying measurement.
Where imperial units survive, the square foot competes directly with it. American and British commercial property is still let by the square foot, and one square metre is 10.76 square feet, which is why converted figures rarely come out round. Land at a larger scale moves to the hectare, equal to 10,000 square metres, and then to the square kilometre.
One square metre equals 10,000 square centimetres, about 10.764 square feet, about 1.196 square yards, or 0.0001 hectares.
The decare is a unit of area equal to ten ares, or one thousand square metres. Its symbol is daa. Ten decares make one hectare. Unlike the other members of the are family it is in daily commercial use, though under several different names, because it happened to match land measures that a number of countries were already using when they went metric.
Norway is the clearest case. The Norwegian mål was redefined as exactly one thousand square metres in the metrication of the late nineteenth century, and dekar has been the working unit of Norwegian farming and forestry ever since. Agricultural statistics, subsidy applications and farm sale advertisements are all written in dekar, and a typical Norwegian holding runs to a few hundred of them.
The eastern Mediterranean tells a similar story with different words. The Greek stremma became one thousand square metres on metrication in 1899, the Turkish dönüm was fixed at the same figure, and the metric dunam of Israel, Jordan and Iraq matches both. Bulgaria uses the decare directly, and its farm census is published in it. In each of these places a traditional unit of roughly this size was rounded to the metric value rather than abolished.
The reason it works is that a thousand square metres suits smallholdings. A hectare is a large field, and a hundred square metres is a garden; the decare falls between them, at about the size of an orchard plot, a vineyard parcel or a village building lot. It is also close to a quarter of an acre, which is why it reads naturally to people brought up with either system.
Reporting to international bodies converts it away. European Union agricultural returns, the statistics of the Food and Agriculture Organization and most academic work are all in hectares, so national figures in dekar or stremmata are divided by ten before they travel. This is a frequent source of order-of-magnitude errors in translated agricultural reporting, since a farm of 300 dekar is 30 hectares and not 300.
The symbol daa is awkward but unambiguous, being the deca prefix applied to the are. It should not be confused with da, which is the deciare and one hundred times smaller, nor with the prefix symbol da used before other units.
One decare equals 1000 square metres, 10 ares, 0.1 hectares, or about 0.247 acres.