How to measure square footage

Three formulas cover most rooms

Rectangles multiply length by width. Circles square half the diameter and multiply by pi. Triangles halve base times height.

Common room sizes, already worked out

A room described as 12 by 12 is 144 square feet, because square footage is simply the two side lengths multiplied together.

Reading a listing: a room written as “12x12” gives feet, so the answer is in square feet. A space measured in inches must be divided by 144 to reach square feet, and one measured in yards multiplied by 9.

Split irregular plans at inside corners

An L-shaped room becomes two rectangles; a bay window adds a triangle or part-circle. Sketch the plan, split it, label each measurement, and add the pieces.

Area is not the purchase quantity

Flooring, tile, and paint are sold in boxes, rolls, and cans with coverage ratings, and cutting always wastes some. Convert area to purchase units with coverage and a waste allowance.

Next step: the room finish estimator turns an area into purchase units with explicit coverage, coats, and waste assumptions.

Measuring accurately

Most area errors are measurement errors. Measure twice, write every figure down, and keep the units consistent.

• Measure wall to wall at floor level, where flooring will actually go.
• Record feet and inches, then convert inches to decimal feet by dividing by 12: 12 ft 9 in is 12.75 ft, not 12.9 ft.
• Measure opposite walls. If they differ, the room is not square; using the longer figure keeps you from running short.
• If you measured in inches, divide square inches by 144 to get square feet.

Square feet and square metres

One foot is exactly 0.3048 metres, so one square foot is 0.09290304 square metres.

Multiply square feet by 0.0929 for square metres, or divide square metres by 0.0929 for square feet. A 180 sq ft room is about 16.7 m², and a 20 m² room is about 215 sq ft.

Round only at the end. Converting each wall length to metres and rounding before multiplying adds error that a single conversion of the finished area avoids.