Acres to Square Metres

How the acres to square metres conversion works

An acre is an imperial unit of area; a square metre is the SI unit. The two are linked by a fixed, defined factor: one international acre equals 4,046.8564224 square metres. Enter an area in acres and the calculator multiplies it by that factor, then derives the equivalent in hectares and square feet. Nothing here is an estimate — the conversion is exact, and the only figure that carries uncertainty is the acreage you start with.

The exact conversion factor

The acre is defined as one chain by one furlong, or 66 ft × 660 ft, which is 43,560 square feet. Because a foot is defined as exactly 0.3048 m, a square foot is 0.09290304 m², so 43,560 ft² works out to 4,046.8564224 m². The calculator displays this rounded to two decimal places (4,046.86 m² for one acre), while the underlying multiplication uses the full factor. The reverse direction — square metres to acres — divides by 4,046.8564224.

What an acre actually measures

Historically an acre was the area a yoke of oxen could plough in a day: a strip one furlong long (660 ft, the length of a furrow) and one chain wide (66 ft). That origin is why an acre is a long, narrow rectangle by definition rather than a square — a square of the same area is about 63.6 m on each side. The modern international acre fixes the figure at exactly 4,046.8564224 m² regardless of shape, so a square, a strip, or an irregular field of the same area all convert identically.

Acres, hectares and square metres compared

A hectare is 10,000 m², so one acre is 0.404686 ha and one hectare is 2.47105 acres. For land area, hectares are the metric unit seen most often on Ordnance Survey mapping and agricultural records, while square metres suit smaller plots, gardens, and building footprints. The calculator shows all three, so a figure quoted in any one unit reads across to the others without a second lookup.

Worked examples

A quarter-acre building plot is 0.25 × 4,046.8564224 = 1,011.71 m² (0.1012 ha). A 2.5-acre paddock is 10,117.14 m² (1.0117 ha, 108,900 ft²). A 0.1-acre garden is 404.69 m². Each result follows directly from the single conversion factor, with no intermediate rounding step that would compound error.

Where acres still appear in UK practice

Although the UK adopted metric units for most official purposes, acres persist in everyday property and land dealings: estate-agent particulars, auction catalogues, farm tenancy agreements, and older title deeds frequently state plot size in acres. Land Registry title plans and planning-application site areas, by contrast, are given in square metres or hectares. Converting between the two is a routine step when comparing a marketing figure against an official record.

Acres on planning and Building Control submissions

Building Regulations and planning documents in the UK are drawn up in metric units. Site areas, plot ratios, and density figures are expressed in square metres or hectares, not acres, so an acreage taken from a sales listing converts to square metres before it lines up with what a local planning authority or Building Control body works with.

Keeping the units straight

Two errors crop up most often. The first is mixing up the international acre used here with the US survey acre, which is larger by about four parts per million — a difference of roughly 0.0083 m² across a full acre, irrelevant for UK land though it matters if a figure originates from older US survey data. The second is confusing area units with their linear roots: an acre is an area, not "so many metres", and 43,560 ft² does not correspond to 43,560 m². The factor above keeps the units straight.

How precise are the results?

The conversion factor is exact to as many digits as needed, so precision is limited only by the acreage entered and by sensible display rounding. The calculator shows square metres to two decimal places, hectares to four, and square feet to the nearest whole number — each chosen to suit the scale of the unit, since a hundredth of a square metre is a meaningful resolution whereas a hundredth of a hectare is a full square metre. For survey-grade work, the area on a measured site plan is more reliable than a converted marketing figure, because the input measurement, not the arithmetic, sets the real accuracy.

Using this alongside other BuildMetricLab tools

Once a plot size is in square metres it feeds straight into area-based estimators — turf and artificial grass, fencing around a boundary, paving, or a concrete slab. The hectares-to-acres calculator handles the reverse direction, and the linear-to-square-metres tool converts a length-and-width measurement into the same area unit. Every BuildMetricLab tool runs entirely in your browser — no sign-up, no data sent anywhere, and the formula is shown on the page so the maths can be audited.