Hectare (ha)
Modern equivalent
10,000 square metres = 2.4711 acres
Ten thousand square metres, being a square of one hundred metres on each side, and the metric standard for agricultural land.
The hectare is not strictly an SI unit but was retained for land measurement because the square kilometre is too large for a farm and the square metre too small. It sits awkwardly against the acre, being neither a round multiple nor an obvious fraction: two and a half acres is the working approximation, though the true figure is 2.4711. British farm subsidies and Ordnance Survey figures are now given in hectares, while farmers themselves frequently still think in acres.