| Reference | Meters (m) | Decameters (dam) |
|---|---|---|
| A sheet of A4 paper (long side) | 0.297 m | 0.0297 dam |
| Average adult human height | 1.7 m | 0.17 dam |
| A football pitch (length) | 105 m | 10.5 dam |
| A marathon | 42195 m | 4219.5 dam |
| Height of Mount Everest | 8849 m | 884.9 dam |
The metre is the base unit of length in the International System of Units, written m. Every other SI length unit is defined as a multiple or fraction of it, and units of area, volume, speed and many others are built from it in turn.
Its definition has been revised four times, each revision replacing a physical object with something more reproducible. The French Academy of Sciences proposed in 1791 that the metre should be one ten-millionth of the distance from the North Pole to the equator along the meridian through Paris. Surveying that arc took six years and produced a small error, since the Earth is not a perfect sphere. In 1889 the definition moved to a platinum-iridium bar held at Sèvres, near Paris, with copies distributed to signatory nations. In 1960 it was redefined as 1,650,763.73 wavelengths of orange-red light emitted by krypton-86.
The current definition dates from 1983 and is unusual in that it fixes a different quantity. The speed of light in vacuum is defined as exactly 299,792,458 metres per second, and the metre follows as the distance light travels in 1/299,792,458 of a second. Light speed is therefore no longer measured; it is a defined constant, and improvements in measurement now refine the metre rather than the speed.
The unit is used worldwide for building dimensions, athletics tracks, swimming pools, water depth, fabric and rope. Only a small number of countries retain imperial units for general purposes, and even there the metre dominates science and medicine.
Everyday reference points help fix the scale. A standard interior door is roughly two metres tall. A single stride for an adult is close to three quarters of a metre, which is why pacing is a workable rough measure of distance. The width of a single traffic lane is between three and three and a half metres in most countries.
One metre equals 100 centimetres, 1000 millimetres, or 0.001 kilometres. It is approximately 3.28084 feet, or 39.3701 inches.
The decametre is ten metres, written dam. It is a valid SI unit formed with the prefix deca, but like the decimetre it is rarely used. The prefix deca is the only SI multiplier whose symbol is two letters, which makes it slightly awkward to write and may have contributed to its neglect.
Spelling varies. Decametre is the international form; dekameter appears in some American texts, and the symbol dkm is occasionally seen in older material, though dam is the form recognised by the BIPM. The similarity between dam and the abbreviation for decimetre has caused confusion, and some standards bodies discourage the unit for that reason alone.
Where it does appear, it is usually in surveying, forestry and agronomy. Field dimensions, plot spacing and tree heights sit conveniently in the range of a few decametres. Some European cadastral records use the unit. It also occurs in specifications for cable and hose supplied in ten-metre lengths.
The derived area unit is more familiar than the length unit. A square decametre is 100 square metres, which is exactly one are. The are is itself little used, but its hundredfold multiple, the hectare, is the standard measure of land area across most of the world. A hectare is 100 ares, or 10,000 square metres, and can equally be described as a square hectometre.
In practice most people express ten metres simply as ten metres, or as 0.01 kilometres when working at larger scales. The decametre is best understood as a formally correct unit that everyday usage has passed over.
The unit does appear in one everyday context without being named. Swimming pool lane lengths, running track segments and the spacing of street lighting are frequently set in multiples of ten metres. Planning documents that lay out plot frontages or building setbacks often work to the same grid. The quantity is useful; it is only the word that has fallen out of use.
Its square has fared better than the length itself. The are, a hundred square metres, is the square decametre under another name, and although the are is now rare its hundredfold the hectare governs land measurement across most of the world. The same pattern shows in the decare of a thousand square metres, which is the standard unit of farmland in Turkey, Greece, Norway and parts of the Balkans and is simply ten ares. So the decametre survives chiefly by inheritance: nobody quotes a field boundary in decametres, but a great deal of the world's land is bought and sold in units built from it.
One decametre equals 10 metres, 1000 centimetres, or 0.01 kilometres. It is about 32.8084 feet.