Area units come from length units, squared. A square metre is the area of a square one metre on each side; a square foot is a square one foot on each side. Some traditional units have more colourful origins. The acre was historically the area a team of oxen could plough in a day, and was standardised as 4,840 square yards: a strip one furlong long and one chain wide. In metric terms that's about 4,046.86 square metres. The hectare, 10,000 square metres, is the metric unit used for land, and one hectare is about 2.47 acres.
Where you meet it in the UK
Area turns up in three places in British life. Estate agents and floorplans often give room and property sizes in both square feet and square metres. Farmers, landowners and planning documents use acres and hectares, often mixing the two. And DIY shops sell flooring, paint coverage and turf by the square metre. Moving between those worlds is where conversion errors creep in.
The squared-units trap
The most common mistake is converting a length factor and forgetting to square it. One metre is about 3.281 feet, so it's tempting to multiply square metres by 3.281. But a square metre is 3.281 feet by 3.281 feet, which is about 10.764 square feet. Using 3.281 instead of 10.764 undercounts the area by more than two-thirds. The same applies in reverse: to go from square feet to square metres, divide by 10.764, not 3.281.
A simpler safeguard is to convert each length first and multiply afterwards. If a room measures 4.2 metres by 3.6 metres, its area is 15.12 square metres. In feet, the sides are about 13.78 feet and 11.81 feet, giving about 162.7 square feet. Both routes give the same answer, but converting the lengths first makes it harder to forget the square.
Worked conversions
For that 15.12 square metre room, flooring is usually bought with around 10% extra for cutting and waste, so you'd order about 16.6 square metres, or about 179 square feet if the product is priced in feet. A 0.25-acre garden is about 1,012 square metres, roughly a tenth of a hectare. And a full-size football pitch, around 105 by 68 metres, covers about 7,140 square metres: roughly 1.76 acres or 0.71 hectares, which is why "football pitches" are a popular way to describe land areas in the news.
Planning permission and land listings
Area figures in planning documents and property listings can mean different things. Estate agents may quote "gross internal area", which measures inside the external walls, while a builder's floor area may include wall thicknesses. Garden and land areas are often estimated from maps rather than measured. When two figures for the same property don't match, the difference is usually what was included, not a conversion error. For anything that affects price or planning, ask how the area was measured.
Checking your answer
A quick test: a square metre is about the size of a small doormat pulled up to a metre across, and about ten and three-quarter square feet. A typical UK double bedroom is around 12 to 15 square metres, or roughly 130 to 160 square feet. An acre is a little smaller than a full-size football pitch, and a hectare is about two and a half acres. If a converted figure puts a bedroom at 1,500 square feet or a garden at 50 hectares, check whether a length factor was used instead of an area factor.
One more worked example
Paint coverage is quoted per square metre, often around 10 to 12 square metres per litre for one coat of emulsion. A bedroom 4 metres by 3 metres with 2.4-metre ceilings has about 33.6 square metres of wall, before subtracting a door and window of perhaps 3.5 square metres together. That leaves about 30 square metres. Two coats need around 60 square metres of coverage, so a 5-litre tin at 12 square metres per litre should just be enough, though most decorators would buy a little extra for touching up.