Square Footage Calculator

How to use this calculator

1. Pick the shape closest to your space. Split L-shapes into rectangles and add them up.
2. Enter measurements in feet or meters (switch with the unit toggle).
3. Use the result in the flooring, tile, paint or mulch calculators.

Method & assumptions

Rectangle: Length × Width
Triangle: Base × Height ÷ 2
Circle: π × (Diameter ÷ 2)²
Trapezoid: (Side a + Side c) ÷ 2 × Height

Height (triangle/trapezoid)

perpendicular distance, not the slanted side

Assumptions & limits

• Measurements are horizontal (plan) dimensions.
• For sloped surfaces such as roofs, use the roofing calculator.

Rounding: purchase quantities are always rounded up to whole bags, boxes, sheets or bundles. See our methodology.

Worked examples

In depth: square footage planning notes

Measuring accurately

Area is the foundation of almost every estimate on this site, so it pays to measure carefully. Use a tape that is long enough to run wall to wall without joining measurements, or a laser measure for large rooms. Measure each side at least twice and in two places, because walls are rarely perfectly parallel; use the larger figure. For irregular rooms, sketch the outline, split it into rectangles, triangles and circles, and add the parts.

Feet and inches are a common trap. 12 ft 6 in is 12.5 ft, not 12.6 ft. Use the feet-and-inches calculator to convert tape readings, or switch to metric, where 3.81 m is simply 3.81 m. The calculator accepts decimals, so you can type 12.5 directly.

From area to materials

Once you know the area, most materials follow directly. Flooring and tile are bought by area plus a waste allowance. Paint needs wall area rather than floor area — perimeter times height, minus doors and windows. Mulch, topsoil, gravel and concrete need area times depth, which is where the cubic-yard calculator comes in. Roofing needs the sloped area, which is the footprint multiplied by a pitch factor.

For rooms with sloped ceilings or stairs, floor area and wall area can be very different; measure what you are actually covering. In real estate listings, square footage follows local measurement standards and may include or exclude walls, stairs and unfinished space, so do not rely on a listing figure for material orders.

If you are pricing a job, multiply the area by the cost per square foot or square meter and remember that contractors often quote installed prices that include preparation, trim and disposal, while store prices are materials only.

Triangles, circles and odd shapes

Not every area is a rectangle. A triangle's area is half the base times the height, measured at right angles to the base — useful for gable walls and corner beds. A circle is π × radius², and a half-circle bay is half of that. For a trapezoid, average the two parallel sides and multiply by the distance between them. Garden beds with curved edges can be approximated by measuring the width every few feet along a center line and averaging.

For walls, the area is the perimeter times the height, minus large openings. For gable ends, add the triangle above the eaves line. Always note whether a figure is floor area or wall area — confusing the two is a common reason for buying twice the paint or half the drywall.

Tips & common mistakes

• Convert inches to decimal feet before multiplying: 6 in = 0.5 ft, 3 in = 0.25 ft.
• Measure twice at different points — walls are rarely perfectly parallel; use the larger figure.
• Sketch the room and write each measurement on it before you calculate.

Sources

1. NIST – SI unit conversion (SP 811) — checked 2026-10; verify before launch
2. UK National Physical Laboratory – units of measurement — checked 2026-10; verify before launch
3. Khan Academy – area of shapes — checked 2026-10; verify before launch

Default values (bag yields, coverage rates, densities) are typical figures from manufacturer literature. Always check the label on the product you buy.

Cite or embed this calculator

DIYCalcLab. “Square Footage Calculator.” Oct 18, 2026. https://diycalclab.com/conversions/square-footage-calculator/

An embeddable version for blogs and contractor sites is planned.