Area units and their exact factors
Area is length squared, so the exact 1959 length definitions carry straight through. One square inch is 2.54 x 2.54 = 6.4516 sq cm. One square foot is 144 sq in = 0.09290304 sq m. A square yard is 9 sq ft = 0.83612736 sq m. An acre is 43,560 sq ft, which works out to 4,046.8564224 sq m, and a square mile is 640 acres = 27,878,400 sq ft = 2.589988 sq km. On the metric side, a hectare is 10,000 sq m (a 100 m square) and a square kilometer is 100 hectares.
The converter turns your value into square meters, then divides by the target unit. The useful cross-over numbers to remember are that 1 sq m is 10.764 sq ft, 1 hectare is 2.471 acres, and 1 acre is a little under half a hectare (0.4047 ha).
Worked examples
Real estate: a 1,000 sq ft apartment is 1,000 x 0.09290304 = 92.9 sq m. A 2,400 sq ft house is 223 sq m. A quarter-acre lot is 10,890 sq ft, roughly a 104 ft by 104 ft square, or 1,012 sq m. Land: a 40-acre parcel is 16.19 hectares; a 160-acre quarter section (the old homestead size) is 0.25 sq mi or 64.75 ha. Overseas listings quote square meters: an 80 sq m condo is 861 sq ft.
Home projects: flooring, tile and carpet are sold by the square foot in the US, while paint coverage is given in square feet per gallon (typically 350 to 400). A 12 ft by 15 ft room is 180 sq ft, which is 16.7 sq m or 20 sq yd if the carpet store still prices by the yard. Sod and mulch are often priced per pallet or cubic yard, so measure the bed in square feet first, then convert depth separately.
Tips and common mistakes
Do not convert the side lengths and then square the wrong number. A room 10 ft by 12 ft is 120 sq ft; converting each side to meters (3.048 by 3.658) and multiplying gives 11.15 sq m, the same as 120 x 0.0929. Either path works, but mixing them does not.
Watch the difference between sq ft and linear feet on fencing and trim quotes, and remember that the acre is a US and UK unit; most of the world measures land in hectares. Some older deeds and surveys use rods, chains and sections: a chain is 66 ft, an acre is 10 square chains, and a section is one square mile. The result table lists every unit at once so you can copy whichever the form or listing asks for.