Square Footage Calculator

Calculate the square footage of a room, floor, wall or lot. Handles rectangles, circles, triangles and L-shaped rooms and converts to square yards and acres.

By CalcBuilt Editorial TeamUpdated September 13, 2026Formulas checked against manufacturer specs, see how we calculate.

Quick answer: To calculate square footage, measure the length and width in feet and multiply them. A 20 x 15 ft room is 300 sq ft, which is 33.33 square yards or 0.0069 acres, with a perimeter of 70 ft.

Results

Square footage: 300 sq ft
Square yards: 33.33 sq yd; 1 sq yd = 9 sq ft
Acres: 0.0069 acres; 1 acre = 43,560 sq ft
Perimeter: 70 ft; 2 x (Length + Width)

Square Footage formula

Rectangle = Length x Width;  Triangle = Base x Height / 2
Circle = 3.1416 x (Diameter / 2)²;  L-shape = (Length 1 x Width 1) + (Length 2 x Width 2)
Square yards = Square feet / 9
Acres = Square feet / 43,560

Worked example

An L-shaped living room made of an 18 x 12 ft main area and an 8 x 6 ft alcove: 18 x 12 = 216 sq ft, and 8 x 6 = 48 sq ft. Together 264 sq ft, or 264 / 9 = 29.3 square yards of carpet before waste.

Two round patios 20 ft across: 3.1416 x 10² = 314.2 sq ft each, or 628.3 sq ft in total. A triangular garden bed with a 14 ft base and 9 ft height is 14 x 9 / 2 = 63 sq ft.

Quick reference

Square footage of common room sizes
Room size	Square feet	Square yards	Square meters
10 x 10 ft	100	11.1	9.3
10 x 12 ft	120	13.3	11.1
12 x 12 ft	144	16.0	13.4
12 x 15 ft	180	20.0	16.7
15 x 20 ft	300	33.3	27.9
20 x 20 ft	400	44.4	37.2
20 x 30 ft	600	66.7	55.7
30 x 40 ft	1,200	133.3	111.5

How to measure square footage

For a rectangular room, measure the length and width in feet at floor level and multiply. Measure in inches and divide by 12 if the room is not a whole number of feet: a room 14 ft 6 in by 11 ft 3 in is 14.5 x 11.25 = 163.1 sq ft. Measure wall to wall for flooring; for paint or wallpaper measure each wall separately.

Odd shapes break into rectangles, triangles and circles; a bay window is a rectangle plus two triangles, and a room out of square gets the longest length and widest width for flooring. Check the diagonals: if they differ by more than 1 inch, snap a straight starting line off the longest wall. For a house, ANSI Z765 measures from the exterior face of the walls. It counts only finished space with a 7 foot ceiling (5 feet under a slope if half the room clears 7). Garages, porches and any level below grade are excluded; below grade is reported separately.

Using the number

Ordering exactly the measured area leaves you short, and the waste depends on the material. Plank and tile lose 5 to 7 percent to cuts at the walls, 10 in a room with jogs, and 15 or more on a diagonal; boxes only come whole. Carpet is sold in 12 foot rolls (some 13 ft 6 in and 15 ft) and priced per square yard. A 10 by 14 foot room takes a 12 by 14 cut, and any room wider than 12 feet takes a seam.

Paint runs 350 to 400 sq ft per gallon. Real estate square footage is a different number: Fannie Mae appraisals have used ANSI Z765 since 2022, and a listing that folds the finished basement into the total gets corrected at appraisal.

What this calculator leaves out

The result is measured area. Not included: waste and box rounding, carpet roll width and seams, pattern repeat, transitions and underlayment, and the above-grade versus below-grade split under ANSI Z765.

Mistakes that cost money

Carpet comes on 12 ft rolls; a 10 x 14 ft room takes a 12 x 14 cut, 168 sq ft, not 140.
Listing a house with the garage or finished basement in the total: ANSI Z765 counts only finished above-grade space, and the appraiser corrects it.
A room 12 ft at one end and 11 ft 8 in at the other is out of square; buy flooring on the larger dimension.
Sloped attic ceilings: ANSI Z765 counts area under 7 ft only if half the room clears 7 ft, and nothing under 5 ft.
Flooring sells by the box: 264 sq ft at 22.4 sq ft per box is 11.8 boxes; buy 13 with waste, one lot number.
A basement counts as below-grade under ANSI Z765 even with a walkout; it is listed separately and appraised lower than above-grade space.

Key facts

One square yard is 9 square feet, and one acre is 43,560 square feet (about 209 x 209 ft).
One square meter is 10.764 square feet, so 100 sq ft is roughly 9.3 m².
The median new single-family house sold in the US is roughly 2,200 to 2,400 square feet.
Flooring, tile and paint are priced per square foot; carpet is often quoted per square yard.
Real estate square footage usually counts only finished, heated living space, not garages or unfinished basements.

Frequently asked questions

How do I calculate square feet of a room?

Multiply the length by the width in feet. A room 12 ft long and 10 ft wide is 120 square feet.

Does a finished basement count in the square footage of a house?

Not in the above-grade total. ANSI Z765 reports any level with part of it below grade separately, even a walkout, and Fannie Mae appraisals have used that standard since 2022. Quote it as 1,800 sq ft above grade plus 800 finished below grade; a listing that adds them together gets corrected at appraisal.

How many square feet are in an acre?

43,560 square feet. A quarter-acre lot is 10,890 square feet and a half-acre is 21,780. For a lot rather than a room, use the acreage calculator, which takes irregular boundaries and returns both acres and square feet.

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

Divide it into rectangles, triangles and half-circles, calculate each one, and add them together. The L-shape option handles the most common case of two rectangles.

Sources and references

ANSI Z765 Square Footage - Method for Calculating (Home Innovation Research Labs)
NIST Handbook 44, Appendix C: General Tables of Units of Measurement (NIST)
Characteristics of New Housing (U.S. Census Bureau)

Next steps for this project

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