Hectares to acres (ha to acre)

One hectare is 2.471053815 acres. Both units are defined against the same SI base unit, so the factor is derived from two exact definitions rather than measured. It is exact in principle and does not land on a round number.

Worked through: 10 ha x 2.47105381 = 24.7105 acre.

Going the other way, one acre is 0.4046856422 hectares.

For mental arithmetic: Multiply by 2.5 and take off 1.2%.

The hectare is a square 100 meters on a side, and its name says how it was built: hecto-are, a hundred ares, where an are is a 10-meter square of 100 square meters. The are itself has almost vanished outside French land registers, and the hectare outlived it because a field-sized unit is what agriculture needs. A hectare is 2.47 acres, so a 'ten-hectare farm' is just under 25 acres. For a sense of scale, a full-size soccer field of 105 by 68 meters is about 0.71 hectares, or 1.76 acres.

Precision is worth matching to the input. A measurement given as 0.5 ha carries one or two significant figures, and reporting its conversion to six implies a precision the original never had.