The U.S. customary units – that is, the units of measure commonly used in the United States – have four options for length: inches, feet, yards and miles. If you want to measure the area (or two-dimensional space) taken up by something, you can gauge it in square inches, square feet, square yards and square miles. But there's another way of measuring area: The acre.

If you're converting from feet to acres (or back again), one acre equals 43,560 square feet. But it's important to realize that the acre isn't a strictly defined unit. For example, an acre could be 180 ft × 242 ft, or it could be 90 ft × 484 ft, or it could even be a long, skinny area of 10 ft × 4,356 ft. All of those are equal to 43,560 square feet, or 1 acre.

Because of that flexibility, and because acres are so large, they're often used to measure everything from home lots to public lands. For example, Denali National Park and Preserve in Alaska encompasses more than 6 million acres. Just imagine trying to express that amount of area in square feet. The result would be an impossibly unwieldy number: 261,360,000,000 square feet, to be exact.

But there's another reason to use acres instead of square feet (or the square mile, which at first seems like a more sensible unit for measuring large areas). As a unit of land measure, acres are understood to be an artificially flat, totally horizontal measure. In other words, if you're measuring an area of land that has a hill in it, when you use acres, the measurements are given as if the hill were cut down to nothing and the land were level – as opposed to measuring all the ups and downs, as if you'd draped a blanket over the hills. You can't necessarily assume that when dealing in square feet and square miles.

Although measuring an area as big as Denali National Park and Preserve in square feet doesn't make sense, when you deal with smaller areas it makes sense to be able to convert between square feet and acres – if only because the square foot measurement is more intuitive to many people. For example, imagine that you have a home lot that is 0.1 acre, a common size in residential neighborhoods of many cities. To convert the measurement in acres to square feet, multiply it by 43,560:

​Example 1:​ Convert 0.1 acre to square feet.

So the 0.1-acre lot is equivalent to 4,356 square feet.

​Example 2:​ In some suburban areas, you might see much larger lots of 0.5 acre or more. How much is that in square feet?

Of course, any time you convert units in one direction, it pays to be able to convert the other way as well. Imagine that you were looking at a house lot and knew it was 10,890 square feet, but wanted to know its area in acres without having to dig up a lot size calculator. To convert from square feet to acres, divide by 43,560.

​Example 3:​ Convert 10,890 square feet to acres.

So the lot you're considering measures 0.25 acre.