Where 43,560 comes from
An acre is exactly 43,560 square feet. That number looks like somebody made it up, and in a sense nobody did: it is what falls out of much older units that were never designed to be tidy.
A chain is 66 feet, a surveyor's unit that is still why American streets are laid out the way they are. A square chain is 66 by 66, or 4,356 square feet. An acre is ten of those. Multiply and you get 43,560, with nobody ever having chosen it.
The formula
square feet = acres × 43,560
The figure is exact rather than rounded. An acre is defined as 4,840 square yards, and there are 9 square feet in a square yard, so 4,840 times 9 gives 43,560 precisely.
Worked example
A suburban lot, 100 feet by 150 feet.
100 × 150 = 15,000 square feet
15,000 ÷ 43,560 = 0.344 acres
Which is why listings talk about third-acre and quarter-acre lots so often: those are the sizes ordinary suburban parcels land on. A quarter acre is 10,890 square feet, near enough to 100 by 109.
An acre is not a square, and never was
If you made an acre square it would be about 208.7 feet on a side. Almost no acre is shaped that way.
The traditional acre was a strip one furlong long by one chain wide, which is 660 feet by 66 feet. That is a long thin ribbon, ten times as long as it is wide, and the shape is not decorative: it is how far a team of oxen could plough before needing to be turned around. The word furlong is literally a furrow long.
| Shape | Dimensions | Area |
|---|---|---|
| Traditional strip | 660 ft × 66 ft | 1 acre |
| If it were square | 208.7 ft × 208.7 ft | 1 acre |
| A deep narrow lot | 435.6 ft × 100 ft | 1 acre |
| A wide shallow lot | 217.8 ft × 200 ft | 1 acre |
All four are exactly one acre. This matters when you are looking at land, because "an acre" tells you how much there is and nothing at all about whether you can fit what you want on it. A 660 by 66 acre and a 208 by 208 acre are very different propositions for a house.
Things an acre is about the size of
- An American football field including both end zones is about 1.32 acres, so an acre is roughly three quarters of one.
- A soccer pitch is usually 1.5 to 2 acres, depending on which of the many permitted sizes it was built to.
- A city block in much of the American Midwest is around 2 to 3 acres, because the grid was laid out in chains.
- A tennis court is about a sixteenth of an acre, so sixteen of them fit with a little room to spare.
If you are measuring a room or a roof rather than a parcel, the square footage calculator handles awkward shapes, and if you are about to cover the ground with something, the mulch calculator and concrete calculator take it from area to volume.