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.

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.