How to calculate square footage
Square footage is the area of a flat surface measured in square feet — the number of 1 ft × 1 ft squares that would cover it. Measure each dimension in feet (convert inches by dividing by 12), then apply the formula for the shape.
Circle: π × (diameter ÷ 2)² ≈ 0.7854 × diameter²
Triangle: base × height ÷ 2
Trapezoid: (side a + side b) ÷ 2 × height
Example: a 12 × 10 ft bedroom
12 × 10 = 120 square feet. That's 13.3 square yards (÷ 9) or 11.15 square meters (× 0.0929).
Example: measurements in feet and inches
A room that measures 11 ft 6 in × 9 ft 3 in is 11.5 × 9.25 = 106.38 square feet. Convert the inches to a decimal first: 6 in ÷ 12 = 0.5 ft and 3 in ÷ 12 = 0.25 ft. (In the calculator you can also type 11'6" directly.)
Example: a round patio (circle)
Measure straight across the widest point through the center to get the diameter. A 14 ft round patio has a 7 ft radius, so its area is 3.1416 × 7 × 7 = 153.94 square feet (14.30 m²). If you can only reach the edge, measure the circumference with a tape and divide by 3.1416 to get the diameter.
Example: a corner garden bed (triangle)
A triangular bed tucked into a fence corner has a 12 ft base along one fence. The height is measured square to that base — from the base to the opposite point — and comes to 9 ft. Area = 12 × 9 ÷ 2 = 54 square feet. Measuring the sloping side instead of the true height is the most common mistake and overstates the area.
Example: a tapered lot or yard (trapezoid)
A side yard is 18 ft wide at the street and 26 ft wide at the back, with the two ends parallel and 15 ft apart. Average the parallel sides and multiply by the distance between them: (18 + 26) ÷ 2 × 15 = 22 × 15 = 330 square feet. If the ends aren't parallel, split the area into a rectangle and a triangle instead.
Measuring irregular rooms
Most rooms that aren't rectangles can be split into shapes that are. For a bay window, add a trapezoid. For a room with a closet or chimney you won't be flooring, calculate the full rectangle and subtract the smaller one.
Example: an L-shaped room
An L-shaped living area measures 20 ft along its longest wall and 18 ft along the other outside wall. Draw a line across the inside corner to split it into two rectangles:
- Main section: 20 × 12 ft = 240 sq ft
- Wing: 8 × 6 ft = 48 sq ft
- Total: 240 + 48 = 288 square feet
Check it the other way: the full 20 × 18 ft outline is 360 sq ft, and the missing notch is 12 × 6 ft = 72 sq ft, so 360 − 72 = 288 sq ft. When both methods agree, your measurements are consistent. Enter each rectangle separately in the calculator and add the results.
Measuring wall area and deducting openings
For paint, drywall, paneling or wallpaper you need the area of the walls, not the floor. Add up the length of every wall (the room's perimeter) and multiply by the wall height. A 14 × 12 ft room has a perimeter of 14 + 12 + 14 + 12 = 52 ft; with 8 ft ceilings that's 52 × 8 = 416 square feet of wall — enter 52 by 8 as a rectangle.
Then subtract the openings you won't cover. A standard 3 ft × 6 ft 8 in door is 20 sq ft, and a 3 × 5 ft window is 15 sq ft. With one door and two windows: 416 − 20 − 15 − 15 = 366 square feet. Small openings such as outlets and switches aren't worth deducting. The paint calculator and drywall calculator do this subtraction for you and turn the result into gallons or sheets; measure gable ends and stairwell walls separately, as triangles or trapezoids.
Square footage conversions
| From | To | Multiply by |
|---|---|---|
| Square feet | Square yards | ÷ 9 |
| Square feet | Square meters | × 0.092903 |
| Square meters | Square feet | × 10.7639 |
| Square feet | Acres | ÷ 43,560 |
| Acres | Hectares | × 0.404686 |
Square footage of a house
When a listing quotes a home's square footage, it usually means finished, heated living area, measured to the exterior walls. Garages, unfinished basements and open porches are normally excluded. Exact rules vary by region and by who is measuring (appraisers in the US commonly follow ANSI Z765), so treat a listing figure as a guide rather than a precise number for ordering materials.
For material estimates, always measure the actual surface you're covering — use the flooring, tile or paint calculators to turn the area into boxes, tiles or gallons with waste included.
Frequently asked questions
How do I calculate square feet?
Multiply the length by the width, both in feet. A 15 ft × 12 ft room is 15 × 12 = 180 square feet.
How do I convert inches to square feet?
Multiply length × width in inches, then divide by 144 (12 × 12). For example, 36 in × 48 in = 1,728 sq in ÷ 144 = 12 sq ft.
How many square feet are in a square yard?
Nine. A square yard is 3 ft × 3 ft. Carpet and some sod and fabric products are still priced per square yard.
How many square feet are in an acre?
An acre is 43,560 square feet — for example, a plot about 208.7 ft × 208.7 ft.
How do I find the square footage of a circle?
Multiply the radius by itself and then by π (3.1416). A 10 ft diameter circle has a 5 ft radius, so 5 × 5 × 3.1416 ≈ 78.5 square feet.