Square footage comes up constantly in real estate, home improvement, and landscaping. Knowing how to calculate it accurately — and how professional appraisers handle irregularly shaped rooms — saves money on materials and gives you accurate numbers for listings and permits.
The Basic Formula
For rectangular rooms: Length × Width = Square Footage. A room that is 12 feet long and 10 feet wide is 120 square feet. Always measure to the wall, not to furniture or trim. Measure in feet; if you measure in inches, divide by 144 to convert square inches to square feet.
Measuring Irregularly Shaped Rooms
Most rooms aren't perfect rectangles. The solution: break the space into rectangles, calculate each, and add them together. An L-shaped room can be divided into two rectangles at the corner. A room with a bay window adds a small rectangle at the projection. Complex rooms with many angles can be broken into as many rectangles as needed.
💡 Professional appraisers use exterior measurements for total home square footage, measuring the outside perimeter of the house. Interior measurements (which exclude wall thickness) will give you a slightly smaller number — typically 3–5% less. Know which method your purpose requires.
Formulas for Other Shapes
- Circle: π × radius² (a circular room with 7-foot radius = 3.14159 × 49 = 154 sq ft)
- Triangle: 0.5 × base × height
- Trapezoid: 0.5 × (base1 + base2) × height
Square Footage vs Square Yards
Carpet and some flooring materials are priced per square yard. To convert: divide square feet by 9 (since 1 square yard = 9 square feet). A 150 square foot bedroom = 16.7 square yards. This is a frequent calculation when purchasing carpet — get the square footage right and the conversion is trivial.
Adding a Waste Factor
For flooring, tile, and wallpaper, always purchase 10–15% more than your calculated square footage. Cutting, fitting around obstacles, and inevitable mistakes consume material. For tile with complex patterns, waste can be 20% or higher. Buying extra upfront is far cheaper than a return trip to find the same dye lot is gone.