Where the acre came from, and why land is still measured in it

43,560 square feet. That's an acre, and if you've ever wondered why it's that number instead of something rounder, the answer has nothing to do with geometry. It comes from farming, and once you know the backstory, the number stops looking random.

A day of plowing, turned into a unit

An acre was originally understood as roughly the amount of land a farmer could plow with a team of oxen in a single day, a practical, work-based definition rather than a geometric one. To actually survey and record land that way, English surveyors in the medieval period standardized it as a rectangle: one furlong long by one chain wide. A furlong, literally "a furrow's length," was set at 660 feet, the distance oxen were expected to plow before a rest. A chain, popularized as a surveying tool in the 1600s, measured 66 feet. Multiply those together, 660 by 66, and you land on 43,560 square feet. It's not an arbitrary number, it's two older, equally practical units multiplied together.

1 furlong  = 660 ft
1 chain    = 66 ft
1 acre     = 1 furlong × 1 chain = 43,560 sq ft

Why 640 acres makes a square mile exactly

This is the part that actually made the old system useful for large-scale land division: a square mile works out to exactly 640 acres, no rounding needed. That's why so much of the historical land-grant surveying in the US, the Jefferson-era grid that carved the Midwest into square-mile "sections," divides so cleanly into half sections (320 acres), quarter sections (160 acres, the standard Homestead Act allotment), and further down from there. The whole system was designed to nest evenly, which is more than you can say for most historical units.

Where the hectare comes from instead

The hectare skips all of that history and just does the metric thing: 100 meters by 100 meters, exactly 10,000 square meters. Because it's built on round metric numbers rather than farming logistics, converting between hectares and square meters is trivial, while converting between hectares and acres (about 2.47 acres per hectare) will never come out to a clean number, since the two units were built on completely unrelated logic.

Where each one actually gets used today

The acre is still the standard for land area in the US and a handful of other countries, showing up in real estate listings, agriculture, and zoning. Building interiors and smaller lots, though, usually switch to square feet even in acre-using countries, since an acre (close to the size of a football field's playing area, minus the end zones) is a fairly large chunk of land for describing something like a house footprint. Most of the rest of the world uses hectares for land and square meters for buildings, with no acre involved anywhere in the conversation.