Area units explained
Area units are lengths squared, which is why the numbers move so fast: double a room's dimensions and you quadruple its floor. The metric side is tidy. The imperial side carries an acre that began as a day's ploughing.
Square millimetre mm²
1 mm² = 0.000001 m² Exact
A square one millimetre on a side. This is the scale of electrical cable ratings — a 2.5 mm2 cable is quoting the cross-section of the copper inside it, which is what decides how much current it can carry without overheating.
Square centimetre cm²
1 cm² = 0.0001 m² Exact
About the size of a fingernail. Used for wound measurements, small engineering cross-sections and the surface area of anything you can hold. A sheet of A4 paper is roughly 624 cm2.
Square metre m²
Base unit — every factor on this page is measured against it
The SI unit, and the one property is sold in almost everywhere outside North America. A generous double bedroom is around 14 m2 and a parking space about 12. Worth anchoring to something you know, because floor plans are easy to misjudge on paper.
Are a
1 a = 100 m² Exact
A hundred square metres — a square ten metres on a side. Largely obsolete by itself, but it is the root of the hectare, which is a hundred of them, and it still appears in land records in parts of Europe.
Hectare ha
1 ha = 10,000 m² Exact
Ten thousand square metres, or a square 100 m on a side. The standard unit for land worldwide: farms, forests, development sites. Roughly two and a half acres, and close to the area of a rugby pitch including the in-goal areas.
Square kilometre km²
1 km² = 1,000,000 m² Exact
A million square metres, and the unit for cities, lakes and countries. Manhattan is about 59 km2, Greater London about 1,570. Note it is a square kilometre: 2 km2 is two of them, not a square two kilometres on a side.
Square inch in²
1 in² = 0.00064516 m² Exact
About 6.45 cm2. It turns up inside pressure units — psi is pounds per square inch — and across US engineering drawings. There are 144 to a square foot rather than 12, because area scales with the square of length.
Square foot ft²
1 ft² = 0.09290304 m² Exact
The unit American and British property is advertised in, about 0.093 m2. A 1,000 sq ft apartment is roughly 93 m2. The quick mental version is to divide by ten and add a little, which is close enough to judge whether a listing is worth a viewing.
Square yard yd²
1 yd² = 0.83612736 m² Exact
Nine square feet. Mostly seen in carpet, turf and fabric, sold by the square yard in the US and by the square metre nearly everywhere else. A square metre is about 1.2 square yards, close enough that confusing them costs money quietly.
Acre ac
1 ac = 4,046.86 m² Exact
About 4,047 m2, defined as exactly 4,840 square yards. Traditionally one chain by one furlong: the strip a team of oxen could plough in a day, which is why it is a long thin shape rather than a round number. Still the unit of US and UK land deeds.
Square mile mi²
1 mi² = 2,589,988.11 m² Exact
Exactly 640 acres — the tidy figure that made the US Public Land Survey possible, dividing the country into square-mile sections. About 2.59 km2. Used for counties, national parks and weather systems.