Acres and Hectares: Definitions and Calculated Land Areas

Start with the area and unit stated by the source. The area calculator and worked examples use international-foot definitions for acres, square feet and square miles. Conversion re-expresses that supplied quantity; it does not measure a property or determine its legal boundaries.

Identify the definition

NIST Chapter 5, Table 6 lists the hectare (ha) as a non-SI unit accepted for use with the SI: 1 ha = 10000 m². A hypothetical 100 m × 100 m rectangle has that area.

The NIST revised land-unit table distinguishes international-foot and U.S. survey-foot definitions. Its international foot is exactly 0.3048 m, and its international-foot acre is exactly 43560 ft² = 4046.8564224 m². A square mile is 640 acres under the same convention.

Historic source documents may specify U.S. survey feet. Read that stated definition rather than inferring a unit from a country or document title. The calculator uses the international foot; it does not provide a separate survey-foot acreage conversion.

Convert acres to hectares

The definition-derived calculation is:

hectares = acres × 4046.8564224 / 10000

Thus 1 acre = 0.40468564224 ha and 100 acres = 40.468564224 ha under the international-foot definition. The calculation uses the full definitions before display rounding.

For a hypothetical stated area of 5000 acres:

5000 × 4046.8564224 / 10000 = 2023.4282112 ha, or 2023.4282 ha displayed to four decimal places.

Use acres to hectares for another stated amount.

Convert hectares to acres

The reciprocal calculation is:

acres = hectares × 10000 / 4046.8564224

One hectare is approximately 2.471053815 acres when displayed to ten significant digits. The commonly displayed 2.47105 is rounded; do not treat it as the exact factor. Exact finite decimal definitions give a rational ratio: 10000 / 4046.8564224 can be expressed as a fraction, even though its decimal expansion does not terminate.

For a hypothetical report of 15000 ha:

15000 × 10000 / 4046.8564224 ≈ 37065.8072200748 acres (calculated decimal, rounded here), or 37065.8072 acres displayed to four decimal places. Divide the unrounded acreage by 640 to obtain approximately 57.9153 international square miles, displayed to four decimal places.

Use hectares to acres for another stated amount.

Work from dimensions when no area is supplied

For a rectangle, express both side lengths in the same length unit, then multiply them. A hypothetical 10 ft × 12 ft rectangle is 120 ft². Using the international foot:

120 × (0.3048)² = 11.1483648 m² before rounding.

Converting each dimension first gives the same result: (10 × 0.3048 m) × (12 × 0.3048 m). The squared factor comes from converting both length dimensions. NIST Appendix B.8 also lists 1 ft² = 0.09290304 m². An irregular shape requires its actual geometry and measurements; this rectangle calculation does not apply to every parcel.

Preserve the source’s precision

The conversion definitions are exact, but the supplied area may be rounded or uncertain. More displayed digits do not improve the original measurement. Preserve its stated unit and convention, calculate before rounding, and choose an appropriate display precision.

The area reference table is generated from the calculator’s unit definitions. For land and floor-area comparisons, check the quantity described by each source as well as its units. For dimensional calculations, see the construction measurement workflow; unit conversion alone does not establish material allowances or structural requirements.

Reviewed October 6, 2026 against the linked NIST sources. Numerical examples above are arithmetic derivations and hypothetical quantities, rather than claims about typical properties, regional practices or actual reports.