Conversion from 20 Square chains to Deciares

Information about the Square chain (sq ch)

The square chain is a unit of area equal to 484 square yards, or 404.686 square metres. Its symbol is sq ch. It is the area of a square one chain on a side, a chain being 22 yards, and ten square chains make exactly one acre. That last relationship is the entire reason the unit exists.

The chain was invented by the English mathematician Edmund Gunter around 1620, and it was a physical object before it was a measurement: a steel chain of one hundred links, 22 yards long, that two people carried across a field. Its length was chosen so that ten square chains would be an acre and eighty chains would be a mile, which let a surveyor work in whole numbers and reach acres by shifting a decimal point.

That was a genuine achievement in an age before decimal calculation. Measurements taken in chains and links could be multiplied together and divided by ten to give acres directly, without the awkward factors of 4840 or 43,560 that the yard and the foot force on the same problem. For three centuries almost every land survey in Britain, Ireland, North America and the colonies was recorded this way.

The consequences outlived the tool. American public land records are written in chains, so deeds and plats still describe boundaries as so many chains and links, and anyone tracing an old title has to convert. Wildland firefighting in the United States measures fire line and fire perimeter in chains to this day, because the unit was already embedded in forestry practice when the agencies were formed.

One familiar object preserves the length exactly. A cricket pitch is one chain from wicket to wicket, twenty-two yards, which is why that number appears in the laws of the game and nowhere else in modern sport. Anyone who can picture a cricket pitch can picture a chain, and a square chain is that length squared, roughly a tenth of an acre or a large suburban garden.

For conversion, the useful facts are that ten square chains make an acre and that one square chain is about 405 square metres, so a hectare is nearly twenty-five square chains. Old surveys quoting areas in chains convert cleanly to acres and only awkwardly to metric, which is a good reason to do the acre step first.

One square chain equals 484 square yards, 0.1 acres, about 404.686 square metres, or about 0.0405 hectares.

Information about the Deciare (da)

The deciare is a unit of area equal to one tenth of an are, or ten square metres. Its symbol is da. It completes the decimal ladder that the French metric law of 1795 built around the are, alongside the centiare below it and the hectare above, but unlike either of those it never found a job and is today the least used member of the family.

Ten square metres is an easily pictured quantity. It is a small bathroom, a single bedroom in a modest flat, or four fifths of a car parking space. It is the floor of a garden shed, the area a market trader is allotted at a fair, or the footprint of a large dining table with chairs pushed back. Nothing about the size is obscure; only the name is.

The reason it failed is structural. Land registration adopted a three-column system running hectare, are and centiare, each step a factor of one hundred, which mirrors how areas actually behave: squaring a tenfold change in length produces a hundredfold change in area. A tenth of an are sits between two columns and belongs to neither, so no register ever needed a place for it.

Its symbol makes matters worse. In the International System, da is the prefix symbol for deca, so a reader meeting da in a technical document is far more likely to take it as the start of a compound like dam or daN than as a unit in its own right. The decare, ten times larger, is written daa, which does nothing to help.

Where the quantity does need naming, ordinary metric notation handles it without effort. Ten square metres is written 10 m², which is shorter than one deciare, needs no explanation, and sorts correctly beside every other figure on a plan or in a contract.

The unit therefore survives almost entirely in conversion tables and in teaching material that runs through the complete prefix ladder. Encountering it in a real document usually means the document is old, translated from an older source, or reproducing a table rather than making a measurement, and converting it to square metres at once is the safe response.

One deciare equals 10 square metres, 0.1 ares, 0.001 hectares, or about 107.6 square feet.