Square Footage Calculator

How to calculate square footage from feet and inches

Square footage is length times width: measure the length and the width in feet, then multiply the length by the width for the area in square feet. Turn inches into feet first by dividing them by 12: 15 ft 6 in is 15.5 ft. For a room that is not one rectangle, split it into pieces, find the square footage of each one and add them for the total square footage. The L-shaped room, 15 ft 6 in × 12 ft plus 8 ft × 4 ft 6 in is 15.5 × 12 = 186 plus 8 × 4.5 = 36, so 222 sq ft. The steps below follow the sizes in the calculator above and change as you type.

A square is a rectangle with equal sides: side × side. A 12 × 12 ft room is 144 sq ft.

Other shapes, L-shaped rooms and cut-outs

Pick the shape of each piece in the calculator: rectangle, triangle, trapezoid, circle, half circle, ring, oval or border. A triangle takes its base and height; for a circle, the radius is half the diameter. The table gives each formula, worked on one example in feet.

Sizes in feet give square feet. 3.1416 is π rounded; the calculator uses π in full.

L-shaped and irregular rooms

Draw the room on paper and cut it into rectangles and the shapes above. An L-shaped room splits into two rectangles: work out each and add them; a bay window adds a trapezoid or a half circle. Measure each piece on its own and add it as a piece. The drawing in the calculator shows the pieces to scale, so a wrong size stands out.

Taking a piece away

Set a piece to Taken away to subtract it: a kitchen island that stays, a hearth, a stair opening. The total comes out net. A border is a band around a rectangle, such as a tile frame around a rug area; the ring is a band around a circle.

Square feet to square yards, square meters and acres

A square yard is 3 ft × 3 ft, so 1 square yard is 9 square feet: divide square feet by 9 for square yards. A square foot is exactly 0.09290304 m², so one square meter is 10.7639 square feet. An acre is 43,560 square feet. The calculator prints square yards, square meters and, for large areas, acres under the total.

Exact by definition: 1 ft = 0.3048 m, 1 sq yd = 9 sq ft, 1 acre = 43,560 sq ft (NIST Handbook 44, Appendix C).

An acre has no set shape. As a square it is about 208.71 ft on a side. Since 1 January 2023 every US foot is the international foot of 0.3048 m; the acre changed by about 0.17 sq ft, which makes no difference to a room, a house or a lot.

Square footage of common room sizes

Common room sizes, worked out with the extra most flooring makers advise for a straight install.

Worked out: length × width; ÷ 9 for square yards; × 1.10 and rounded up for the extra.

How to measure a room, and a house

For flooring, measure each room inside, from wall to wall at floor level, to the nearest inch. Measure both walls of each pair; if they differ, use the longer one.

A house is measured another way. The American standard for a home's square footage, ANSI Z765, measures each level at floor level to the outside face of the exterior walls, and states the result to the nearest whole square foot. Lenders follow it: Fannie Mae requires appraisers to use it. Under its public drafts:

• only finished space counts: walls, floor and ceiling like the rest of the house, suitable for year-round use;
• the ceiling must be at least 7 ft high, or 6 ft 4 in under beams and ducts; under a sloped ceiling, space lower than 5 ft is left out;
• garages, open porches and decks are left out, and so is an opening to the floor below; the stairs count on the floor they go down from;
• finished space below grade is stated apart from the space above grade.

So a house's square footage is larger than the sum of its rooms: the walls are inside it.

How much extra flooring to buy: the waste factor

Cutting boards and tiles to fit leaves offcuts, so flooring makers advise buying more than the area; the extra is often called the waste factor. For an ordinary straight install most of them say 10%, the calculator's default; diagonal and herringbone layouts need more. Round the total up to whole square feet, then to whole boxes.

Each figure from the maker's installation guide or FAQ: Bruce, Armstrong Flooring, MSI, Mercier Wood Flooring, AHF, Daltile.

Carpet is cut from rolls 12, 13.5 or 15 ft wide, laid out on a seaming diagram with at least 3 in to trim at each wall (CRI 105). The waste depends on the roll width and the room, so take the yardage from your installer's diagram.

Related calculators

• Cubic Yard Calculator
• Cubic Feet Calculator
• Concrete Calculator
• Flooring calculator
• Tile calculator
• Acres to square feet

Sources

Unit definitions, flooring extras, carpet widths and the house method on this page come from these documents, checked 9 November 2026.

1. NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement
2. NIST Special Publication 811, Guide for the Use of the International System of Units, 2008 edition
3. Federal Register 85 FR 62698, Deprecation of the United States Survey Foot, October 2020
4. Bruce, Engineered Hardwood Installation Instructions
5. Bruce, Hardwood Flooring FAQs
6. Armstrong Flooring, Rigid Core SPC Locking Installation System
7. AHF, Engineered Stone Tile Installation Instructions
8. Daltile, Tile and Stone FAQs
9. MSI, Best LVP Plank Size for Every Room, January 2025
10. Mercier Wood Flooring, Herringbone Installation Guide, January 2026
11. The Carpet and Rug Institute, Finding the Right Carpet
12. CRI 105, Standard for Installation of Residential Carpet
13. Home Innovation Research Labs, ANSI Z765 Square Footage – Method for Calculating, 2020 public drafts
14. Home Innovation Research Labs, ANSI Z765 FAQ, and Fannie Mae Selling Guide B4-1.3-05