How to Calculate Square Footage of Any Room

Most rooms are rectangles, so the area is length times width. A room 12 feet by 10 feet is 120 square feet. Keep both measurements in the same unit before multiplying, since mixing feet and inches is the most common mistake.

To convert inches to feet, divide by 12. A wall measuring 10 feet 6 inches is 10.5 feet, not 10.6. Getting this conversion right matters because a small per-side error is multiplied across the whole area.

A circular area, such as a round patio, uses pi times the radius squared. The radius is half the diameter, so a circle 8 feet across has a 4 foot radius and an area of about 50.3 square feet.

A triangle is one half of base times height, where the height is the perpendicular distance from the base to the opposite point. These formulas let you handle bay windows, angled corners, and wedge-shaped rooms that are not simple rectangles.

For an L-shaped room, split it into two rectangles, calculate each area, and add them together. A room that is 15 by 10 feet with a 5 by 4 foot alcove is 150 plus 20, or 170 square feet.

Handle features that are not floor by subtracting. For a room with a kitchen island footprint or a stair opening, calculate the full rectangle and subtract the area of the cutout. The same subtraction logic works for paint when you deduct doors and windows from a wall.

The calculator supports multiple shapes and can add or subtract areas for irregular spaces.

Once you have square footage, multiply by a price per square foot for flooring, tile, or carpet, or by coverage for paint and mulch. If flooring costs 4 dollars per square foot, a 170 square foot room is about 680 dollars in material before labor.

Always add a waste allowance. Flooring and tile installers typically add 5 to 10 percent for cuts, offcuts, and pattern matching, and more for diagonal layouts. Treat the raw square footage as the minimum and round your order up so you are not short mid-project.