How to Measure Square Footage
Almost every material estimate starts with square feet. Get the area right and the rest of the math is simple. This guide covers the shapes you will meet in a yard or a house and how to split anything irregular into pieces you can measure.
What you need
A 25 ft tape measure handles rooms and small beds. For lawns, driveways and fences, a 100 ft reel tape or a measuring wheel is faster. Take a notepad, sketch the area roughly from above, and write each measurement on the sketch as you take it. A helper holding the other end of the tape makes long measurements more accurate.
Measure in feet and convert inches to decimal feet by dividing by 12. Eight inches is 0.67 ft, and 6 inches is 0.5 ft. Rounding each measurement to the nearest few inches is fine for materials that come with a waste allowance.
Rectangles and squares
Area = length x width. A 20 ft by 12 ft patio is 240 square feet. Most rooms, decks, slabs and lawns can be treated as a rectangle or a few rectangles added together.
For an L-shaped area, split it into two rectangles along one of the inside edges. Measure each rectangle separately and add the two areas. Do not measure the full outside length of both legs and multiply, which counts the corner twice.
Triangles
Area = base x height / 2, where height is measured at a right angle to the base. A triangular corner bed with two 10 ft sides meeting at a square corner is 10 x 10 / 2 = 50 square feet.
Triangles come up in gable ends of a house when painting or siding, in corner garden beds and in angled patios. For a gable end 24 ft wide and 6 ft from the top of the wall to the peak, the triangle is 24 x 6 / 2 = 72 square feet.
Circles and curves
Area = 3.14 x radius x radius. The radius is half the distance across the circle. A round bed around a tree 8 ft across has a radius of 4 ft and an area of 3.14 x 4 x 4 = 50 square feet. A half circle is half that.
For a curved bed edge or a kidney-shaped lawn, the easiest method is to measure the length of the area and its width at several evenly spaced points. Average the widths and multiply by the length. Five or six width measurements give a close answer for most garden shapes.
Irregular yards
For a whole yard, draw the outline and divide it into rectangles, triangles and partial circles. Measure each piece and add them up. Then subtract anything inside the area that will not get material: the house footprint, a patio inside a lawn, or a shed.
Online satellite map tools can measure area by tracing an outline, which is handy for a rough check of a large lawn or roof. Use tape measurements for the final order, since images can be off by a few feet at the edges.
Sloped surfaces
A roof or a hillside is larger than its outline on a map. For roofs, measure the footprint from the ground and multiply by the pitch factor, as the roofing calculator does. For a sloped lawn, measure along the ground rather than across the level, since sod and seed cover the actual surface.
From square feet to materials
Sod, tile, flooring and pavers are sold by area, so square feet go straight into the calculators. Mulch, topsoil, gravel and concrete are sold by volume, so you also need a depth. Our guide to cubic yards, bags and tons explains that step, and how much extra to buy covers waste allowances.
Frequently asked questions
How do I calculate square feet?
Multiply length by width in feet. For other shapes use base x height / 2 for triangles and 3.14 x radius squared for circles.
How do I convert inches to feet for area?
Divide inches by 12 before multiplying. A 10 ft 6 in wall is 10.5 ft.
How do I measure an irregular shaped yard?
Split it into rectangles, triangles and circles, find each area and add them, then subtract buildings and hardscape inside the outline.
Calculators for this guide
Results are estimates, not a contractor quote. Check quantities with your supplier before you order, and follow local building codes. Last reviewed 2026-10-12.