| Square decimeters (dm²) | Square furlongs (sq fur) |
|---|---|
| 1 Square decimeter | 0.000000247105381467 sq fur |
| 2 Square decimeters | 0.000000494210762934 sq fur |
| 3 Square decimeters | 0.000000741316144401 sq fur |
| 4 Square decimeters | 0.000000988421525869 sq fur |
| 5 Square decimeters | 0.00000123552690734 sq fur |
| 10 Square decimeters | 0.00000247105381467 sq fur |
| 20 Square decimeters | 0.00000494210762934 sq fur |
| 25 Square decimeters | 0.00000617763453668 sq fur |
| 50 Square decimeters | 0.0000123552690734 sq fur |
| 100 Square decimeters | 0.0000247105381467 sq fur |
| Reference | Square decimeters (dm²) | Square furlongs (sq fur) |
|---|---|---|
| A sheet of A4 paper | 6.237 dm² | 0.0000015412 sq fur |
| A tennis court | 26080 dm² | 0.00644451 sq fur |
| A football pitch | 714000 dm² | 0.176433 sq fur |
| One hectare | 1000000 dm² | 0.247105 sq fur |
| Central Park, New York | 341000000 dm² | 84.2629 sq fur |
The square decimetre is a unit of area equal to one hundredth of a square metre, or one hundred square centimetres. It is written dm². It is the area of a square ten centimetres on a side, which is about the size of a palm or a drinks coaster, and it sits in the gap between the square centimetre of small objects and the square metre of rooms.
The decimetre owes its survival to volume rather than to area. A cubic decimetre is exactly one litre, which quietly builds the prefix into the most used volume unit in the world, and a square decimetre is the cross-section of that litre cube. Anyone who can picture a litre carton already knows what a square decimetre looks like: it is the face of the box.
Its one substantial commercial home is the leather trade. Hides are irregular, so they are measured by area rather than by length, and European tanneries, shoe factories and leather goods makers buy and sell in square decimetres. A whole cowhide runs to something like 200 to 250 square decimetres, a lambskin to about 50, and prices, cutting yields and waste allowances are all calculated on that basis. British and American buyers use square feet for the same purpose, and one square foot is 9.29 square decimetres.
Everyday objects land conveniently on it. A sheet of A4 paper is 6.24 square decimetres, a standard floor tile of thirty centimetres is nine, and a large postage stamp is a small fraction of one. Schools use the unit for exactly this reason: it lets a class measure real objects without either very large or very small numbers, and it makes the squaring rule visible, since ten centimetres on a side gives a hundred square centimetres.
Outside those uses it is rare, and for the usual reason. Engineering standardised on prefixes that step by a factor of a thousand, so square millimetres, square metres and square kilometres cover everything in practice. Ten square decimetres is more naturally written as 0.1 square metres, which sorts correctly beside every other figure in a specification.
Converting is straightforward once the squaring is kept in mind. Multiply by a hundred for square centimetres, divide by a hundred for square metres, and remember that the factor between neighbouring units is the square of the factor between the lengths, not the factor itself.
One square decimetre equals 100 square centimetres, 0.01 square metres, or about 15.5 square inches.
The square furlong is a unit of area equal to ten acres, or 40,468.6 square metres. Its symbol is sq fur. A furlong is 220 yards, an eighth of a mile, so a square furlong is a hundred square chains and sixty-four of them make a square mile. It is one of the tidiest relationships in the imperial system and one of the least used.
The furlong is older than the acre and gave it its shape. The name means furrow long, and it was the distance an ox team was expected to plough before resting and turning. In the open fields of medieval England a furlong was a block of cultivated strips as well as a length, so the word described a place and a measurement at the same time, and the acre was defined as one furlong by one chain because that was what a day's ploughing produced.
That history explains why the square furlong is arithmetically neat but practically absent. Land was allotted in strips and acres, not in blocks a furlong on a side, so the square form never had a use even when the linear one was in daily service. A surveyor who needed ten acres wrote ten acres, and the square furlong was left as an entry in tables.
The length itself has survived by a peculiar route. Horse racing kept it, and races in Britain, Ireland, the United States, Australia and much of Asia are still described in furlongs: a six-furlong sprint, a mile and a half meaning twelve furlongs. Nothing else in modern life uses the unit, but racing uses it constantly, which keeps the word familiar to people who would struggle to say how long it is.
As an area it is a useful size to picture even so. Ten acres is a small farm, a large public park, about fourteen football pitches, or a city block of generous proportions. New York's Central Park covers roughly eighty-four square furlongs. The unit is large enough that anything measured in it is being described as a place rather than a property.
Converting is cleanest through the acre. Multiply by ten for acres, then by 0.4047 for hectares, or use the direct factor of 4.047 hectares to the square furlong. Going the other way, a hectare is about a quarter of a square furlong, which is close enough for rough comparison of old and new land records.
One square furlong equals 10 acres, 100 square chains, about 40,469 square metres, or about 4.047 hectares.