One hectare is 2.4710538147 acres. The two are the working units of land measurement on either side of the metric divide, and the ratio of roughly two and a half is worth committing to memory for anyone reading land data from both.
About the units
The hectare is 10,000 square metres — a square 100 metres on a side — and it is the metric land unit despite not being an SI unit itself. It is built from the are, 100 m², with the prefix hecto meaning a hundred. The acre is far older and stranger. It was originally the area a yoke of oxen could plough in one day, formalised as one chain by one furlong: 66 feet by 660 feet, giving 43,560 square feet. That rectangular origin is why the acre has no natural side length, and why land described in acres says nothing about its shape.
The exact factor
An acre is exactly 4,046.8564224 m², derived from 43,560 square feet at 0.09290304 m² each. A hectare is exactly 10,000 m². The ratio is therefore 10,000 ÷ 4,046.8564224 = 2.47105381467... In the other direction, an acre is 0.404686 hectares, close enough to 0.4 that the approximation is often used. Two useful anchors: 100 hectares is one square kilometre, and 640 acres is one square mile — the latter being the section of the American public land survey, which is why so much American farmland comes in multiples of 640, 320 and 160 acres.
Where you meet this conversion
Agriculture, forestry and property. European farm subsidies, crop yields and forest inventories are all reported per hectare, while American and British agricultural data uses acres. Yield comparisons across the Atlantic — bushels per acre against tonnes per hectare — require both this conversion and a mass conversion, and getting either wrong produces figures that look plausible. Land sales, conservation areas and deforestation statistics all cross the same boundary in international reporting.