How this instrument works
An acre began as a day's ploughing, not as a shape. Medieval English open fields were worked in strips one furlong long — one furrow-long, 660 feet — and a yoke of oxen could turn about four rods of width before dusk. Edmund Gunter's surveying chain of 1620 pinned those customary lengths to hard numbers: 66 feet per chain, ten chains per furlong. One chain by one furlong is 66 × 660 = 43,560 square feet, and that rectangle has served ever since.
Because an acre is defined in feet rather than in metres, 43,560 carries no uncertainty whatsoever. It is arithmetic, not measurement, and it survives even that messiest corner of US surveying: 43,560 holds for international feet of 0.3048 m and equally for US survey feet, which NIST and NOAA retired on 31 December 2022. Only metric equivalents part company — 4,046.8564224 m² against 4,046.8726098 m², roughly one part in 250,000.
This unit stayed wherever English land law went. American deeds, farm auctions and Public Land Survey System plats still run on it; one survey section is exactly 640 acres, or one square mile, and that quarter-quarter section generations nicknamed the back forty is 40 of them. Britain metricated its shops but not its fields, so HM Land Registry keeps recording acreage, as do Canadian, Australian and Indian listings. Square feet, meanwhile, is how buildings get described — which is why anyone moving from site plan to floor plan crosses this factor daily.
- Type your parcel size into the Acres (ac) field; it opens at 1 acre as reference.
- Read the Square feet (ft2) line beneath it — that figure recomputes as you type, to six decimals.
- For part-acre lots, enter your decimal directly: 0.25, one quarter-acre suburban plot, returns 10,890.
- Working backwards from survey figures? Divide your square footage by 43,560 — 87,120 sq ft is exactly 2 acres.
Worked example — one acre of building land
A rural lot is advertised at 1 acre and you want to know how much house will fit on it. Leave the Acres (ac) field at 1 and your Square feet (ft2) line reads 43,560.000000 — that whole parcel, before any setback or easement comes off.
That total is easier to judge as shape. Squared up, an acre runs 208.71 feet per side; laid out old-style, chain by furlong, it becomes one ribbon 66 by 660 feet. Some 2,400 sq ft of house footprint covers about 5.5 per cent of it, while an American football field with its end zones, 57,600 sq ft, would overhang your boundary by roughly one third.
Questions
Is 43,560 square feet exact, or has it been rounded?
Exact. An acre is defined as ten square chains, and Gunter's chain is exactly 66 feet, so 66 × 660 = 43,560 square feet with nothing left over. No approximation enters this conversion at any point. Rounding appears nowhere except in displayed decimals, and only then for an awkwardly sized parcel.
Do US survey acres convert to a different number of square feet?
No — both give 43,560, because each is defined in feet, so their ratio never changes. What differs is that foot itself: US survey feet ran 1200⁄3937 m, about two parts per million longer than international feet, making one survey acre 4,046.8726098 m² against 4,046.8564224 m². NIST and NOAA deprecated survey feet in 2022, and new US work uses international definitions. This distinction only bites on state plane coordinates spanning tens of miles.
How wide is an acre if you lay it out as a square?
About 208.71 feet on each side — √43,560 is 208.7103…, an irrational number, which is itself a hint that this unit was never conceived as square. Its historical shape is furlong by chain, 660 by 66 feet, because that is how one ploughed strip ran. Any rectangle enclosing 43,560 square feet qualifies: 100 by 435.6, 200 by 217.8, or that classic ribbon.
How many acres make one square mile or one hectare?
One square mile holds exactly 640 acres, which is why Public Land Survey System plats cut each township into 36 numbered sections of 640. One hectare — 10,000 m², metric farming's field unit — comes to about 2.4711 acres, or 107,639 square feet. That second figure genuinely is rounded, since hectares are not whole numbers of anything imperial, whereas this acre-to-square-foot step never loses a digit.
Why do some listings quote a commercial acre of 36,000 square feet?
Commercial acres are US real estate convention rather than legal units: 36,000 square feet, reached by deducting roughly 17 per cent from full measure as an allowance for streets, sidewalks and alleys. Subdivision developers price by it so buyers can compare usable ground. Where listings quote commercial acres, raw geometric area on that plat runs larger than whatever number is shown.
What goes wrong most often when people convert acreage by hand?
Transposing 43,560 into 43,650 is one classic slip; it inflates every result by about 0.2 per cent, enough to survive casual checking and still misprice land. Next most common is remembering an acre as roughly 200 feet square, then squaring 200 to get 40,000 — some 3,560 square feet short. Old deeds hold one more trap: an Irish acre was 70,560 square feet and Scottish acres near 55,000, both abolished long ago yet still inked into pre-1830 conveyances.