A hectare is 10,000 square meters, the area of a square 100 m on each side. NIST's conversion table lists it as exactly 1.0 × 10⁴ m². It is the standard metric unit for land, farms and forests, much as the acre is in US customary units.
Formula
acres = hectares × 2.4710538
hectares = acres × 0.40468564
square meters = hectares × 10,000
The acre figure follows from the acre being 4,046.8564224 m² on the international foot: 10,000 ÷ 4,046.8564 = 2.4710538.
Example
A 40-acre field is 40 × 0.40468564 = 16.19 ha. A 10 ha plot is 10 × 2.4710538 = 24.71 acres, or 100,000 m².
Common mistakes
A hectare is not a length: 1 ha is 100 m × 100 m, not 100 m. One square kilometer is 100 hectares.
Convert with hectares to acres or acres to hectares, and see how many square feet in an acre.