One acre is exactly 4,046.8564224 square metres. Every digit is definitional: an acre is exactly 43,560 square feet, a square foot is exactly 0.09290304 m², and the product terminates.
About the units
The acre is the oldest unit on this page and the only one whose definition encodes a shape. It was one chain by one furlong — 66 feet by 660 feet — which was the area a team of oxen could plough in a day, given that a furlong was the length of a furrow before the team needed rest and a chain was a practical width to work. That rectangular origin means an acre has no defined side length, and a square acre is about 63.6 metres on a side. The square metre, by contrast, is a pure geometric derivation from the metre with no agricultural history whatsoever.
The exact factor
The chain of exact steps: 1 acre = 43,560 ft²; 1 ft² = 0.09290304 m²; 43,560 × 0.09290304 = 4,046.8564224 m². For estimation, an acre is roughly 4,000 m², which is low by 1.2 per cent, or roughly 0.4 hectares. Two anchors that help: an acre is about 40 per cent of a hectare, and a hectare is about two and a half acres. An American football field including both end zones is 1.32 acres, or 5,351 m².
Where you meet this conversion
Property and agricultural transactions in the United States, the United Kingdom and much of the Commonwealth. Land is listed in acres while surveys and site plans within it use metres, so a plot advertised at 2.5 acres becomes 10,117 m² on the drawings. Development calculations — units per acre, floor area ratios, setback requirements — mix the two constantly. Conservation and rewilding projects report in hectares internationally and acres inside the United States, which makes comparing figures across sources a routine conversion task.