Square Footage Calculator

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

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 are easier if you break them into rectangles, triangles and circles, calculate each piece, and add them up. Closets, alcoves and bay windows count as their own rectangles. For a whole house, measure the outside dimensions of each floor and subtract garages, porches and unfinished areas.

Using the number

Most materials are sold by the square foot, but ordering exactly the measured area leaves you short. Add 5 to 10 percent for straight-lay flooring, tile and laminate, and 15 percent or more for diagonal patterns and rooms with many cuts. Carpet is sold in 12 ft wide rolls and priced per square yard, so seam layout matters as much as area.

Paint coverage is also quoted per square foot, typically 350 to 400 sq ft per gallon, and land is quoted in acres. The calculator gives all three conversions so one measurement works for the whole project.

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

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)