Square Footage Calculator

About this calculator

A free square footage calculator for rooms, floors, walls, and yards. The area view covers the three shapes that handle almost every real space — rectangle (length × width), circle (π × (diameter ÷ 2)²), and triangle (½ × base × height) — and irregular rooms are just sums of those pieces: a 12 ft × 10 ft room with a 6 ft × 4 ft alcove is 120 + 24 = 144 square feet. Two more views turn the area into a shopping list. The flooring view adds a waste factor to the floor area and prices the material at your quoted cost per square foot; the paint view converts wall area and coat count into whole gallons at a disclosed coverage rate. Everything computes in your browser on the numbers you enter — a planning estimate, not a contractor quote.

How to measure a room — including L-shaped ones

Measure each dimension at floor level along the longest run of the wall, in feet (a tape measure reading of 12 ft 6 in is 12.5 ft). For a plain rectangular room that is all you need: a 12 ft × 10 ft bedroom is 120 square feet, full stop. Round dimensions to the nearest inch rather than the nearest foot — on a large room the difference compounds into real material dollars.

Irregular rooms are not a special formula; they are several rectangles wearing a trench coat. Break an L-shape at the inside corner into two rectangles, measure each, and add: a 12 ft × 10 ft main area plus a 6 ft × 4 ft alcove is 120 + 24 = 144 square feet. The same divide-and-add trick handles U-shapes, open-plan kitchens, and hallways that dogleg — if a corner cuts diagonally, that piece is a triangle and gets its own formula below.

The three shape formulas

Rectangle is length × width. Circle works from the diameter, the measurement you can actually take across a round rug, patio, or above-ground pool: area = π × (diameter ÷ 2)², so a 10 ft diameter circle is about 78.54 square feet. Triangle is ½ × base × height — an 8 ft base with a 5 ft height is 20 square feet — where the height is the perpendicular distance from the base to the opposite point, not the length of a slanted side.

Almost any floor plan decomposes into these three. A bay window is a rectangle plus a triangle or two; a room with one angled wall is a rectangle plus a triangle; a semicircular landing is half a circle. Compute each piece, then add them — the calculator's running total is exactly that sum.

Why flooring orders add about 10% waste

Flooring never installs at exactly the measured area. Boards get cut at walls and the offcut is often too short to reuse, a few pieces arrive damaged or break during install, and you want leftover material from the same dye lot for future repairs. The trade convention for a straight lay is to order about 10% over the measured area: for a 120 square foot room, that means buying 132 square feet.

Installation pattern moves that number. Running planks diagonally, or laying herringbone and other angled patterns, produces more unusable offcuts at every wall, so installers commonly step the factor up to 15% or more — on the same 120 square foot room that is 138 square feet of material. Small rooms with many doorways and closets also waste proportionally more than large open ones, because the ratio of cut edges to field area is higher.

Paint coverage and the 350 sq ft per gallon convention

A US gallon of typical interior latex covers roughly 350 square feet per coat on smooth, primed walls — major-brand product data sheets quote 350–400 square feet per gallon (checked 2026-06-10), and the low end is the safer planning number. Textured, porous, or previously unpainted surfaces drink more paint and cover less, so drop the coverage figure if your walls are rough.

Wall area comes from the room's perimeter times the ceiling height: a 12 ft × 10 ft room has 44 ft of perimeter, which at 8 ft ceilings is 352 square feet of wall. Doors and windows don't get painted, so the usual practice is to subtract about 20 square feet per door and about 15 square feet per average window before estimating. Two coats on the full 352 square feet is 704 square feet of coverage — 3 gallons at 350 per gallon — while subtracting one door and two windows (352 − 50 = 302 square feet) brings two coats down to 2 gallons.

By variant

• Flooring Cost Calculator
• Paint Coverage Calculator

Questions

Is the square footage calculator free?

Yes. It is free, needs no account, and calculates in your browser; nothing you enter is uploaded or stored.

How do I calculate the square footage of a room?

Multiply the length by the width, both in feet. A 12 ft × 10 ft room is 120 square feet. Convert inches to decimal feet first — 12 ft 6 in is 12.5 ft.

How do I find the square footage of an L-shaped room?

Split it at the inside corner into two rectangles, compute each, and add. A 12 ft × 10 ft main area plus a 6 ft × 4 ft alcove is 120 + 24 = 144 square feet.

How many square feet is a 10-foot circle?

About 78.54 square feet — π × (10 ÷ 2)². Measure the diameter straight across the widest point and the calculator does the rest.