Foot How to Calculate Square Footage
Learn the formulas for common shapes with clear diagrams and worked examples.
Start with a sketch, then choose a formula
Square footage is the amount of flat surface inside a boundary. Measure lengths in feet before multiplying. If your tape reads 12 ft 6 in, use 12.5 ft—not 12.6 ft.
Rectangle: length × width
A 12 × 10 ft rectangle has 120 sq ft. Measure both dimensions along straight, perpendicular edges.
L-shape: outer rectangle − missing corner
A 12 × 10 ft outer rectangle with a 4 × 3 ft corner removed is 108 sq ft. You can also add two non-overlapping rectangles.
Circle: π × radius²
A radius of 5 ft produces 78.54 sq ft. Halve the diameter before using this formula.
Triangle and trapezoid
A triangle with a 12 ft base and an 8 ft perpendicular height has 48 sq ft. A trapezoid with parallel sides of 10 and 14 ft, separated by 6 ft, has 72 sq ft. Its formula is (side A + side B) × height ÷ 2.
Irregular boundaries
For a polygon, plot corners as ordered x,y coordinates using a shared origin and unit. The shoelace formula sums the signed cross-products around its boundary. A list of side lengths alone is insufficient: changing corner angles changes the area.
Use the square footage calculator to combine shapes. For geographic coordinates, use the acreage map instead of a flat-coordinate calculation.
Formulas and exact conversion constants: methodology and sources.