Land units are wildly inconsistent
An acre is 43,560 square feet — a figure with agricultural origins, being roughly what one person could plough in a day with an ox. A hectare is 10,000 square metres, about 2.47 acres, and is the metric standard.
Regional units vary far more. A bigha ranges from about 1,600 to 27,000 square feet depending on the Indian state; a cent is 435.6 square feet in the south. Always confirm which local definition a listing uses before comparing plots.
Carpet area, built-up area and super built-up
Property listings in several markets quote three different areas for the same flat. Carpet area is the usable floor inside the walls. Built-up area adds the walls themselves and balconies, typically 10–15% more. Super built-up area adds a share of lobbies, stairwells and amenities, often 25–40% above carpet area.
Price per square foot quoted on super built-up area is therefore not comparable to the same figure quoted on carpet area. Several jurisdictions now require carpet area to be disclosed for exactly this reason.
Measuring a plot accurately
Measure each side at ground level and check the diagonals. In a true rectangle both diagonals are equal; if they differ, the plot is a parallelogram or trapezium and the length-times-width figure will overstate the area.
For anything irregular, split the plot into triangles, measure all three sides of each, and use Heron's formula. It is more work but it is the method surveyors use, and it does not require any angle to be a right angle.
Frequently asked questions
Does this work for non-rectangular plots? +
No — it assumes a rectangle. Divide an irregular plot into rectangles and triangles, calculate each, and add them up.
How many square feet in an acre? +
43,560. It also equals 4,047 square metres, or about 0.405 hectares.
What is a guntha, cent or bigha? +
Regional land units in South Asia. A guntha is 1,089 sq ft and a cent is 435.6 sq ft, but a bigha varies enormously by state — always confirm the local definition before comparing plots.