How to Calculate Square Footage: Rooms, Walls, Roofs and Circles
Part of the project planner: Planning new floors and tile.
To find the square footage of a space, multiply the length by the width in feet. A room 20 feet long and 15 feet wide is 300 square feet. Other shapes have their own formulas, and any odd shape can be split into rectangles and triangles. The area calculator does rectangles, triangles, circles, trapezoids and ellipses.
Formulas for common shapes
| Shape | Formula | Example | Area |
|---|---|---|---|
| Rectangle or square | length × width | 20 × 15 ft | 300 sq ft |
| Triangle | base × height ÷ 2 | base 20 ft, height 12 ft | 120 sq ft |
| Circle | π × radius² | diameter 20 ft (radius 10 ft) | 314.2 sq ft |
| Trapezoid | (top + bottom) ÷ 2 × height | sides 20 and 12 ft, height 10 ft | 160 sq ft |
| Ellipse | π × half the long axis × half the short axis | axes 20 and 12 ft | 188.5 sq ft |
Common room sizes
Square footage, with square yards (for carpet and sod) and square meters:
| Room | Square feet | Square yards | Square meters |
|---|---|---|---|
| 10 × 10 ft | 100 sq ft | 11.1 sq yd | 9.3 m² |
| 10 × 12 ft | 120 sq ft | 13.3 sq yd | 11.1 m² |
| 12 × 12 ft | 144 sq ft | 16.0 sq yd | 13.4 m² |
| 12 × 14 ft | 168 sq ft | 18.7 sq yd | 15.6 m² |
| 14 × 16 ft | 224 sq ft | 24.9 sq yd | 20.8 m² |
| 16 × 20 ft | 320 sq ft | 35.6 sq yd | 29.7 m² |
| 20 × 20 ft | 400 sq ft | 44.4 sq yd | 37.2 m² |
| 24 × 30 ft | 720 sq ft | 80.0 sq yd | 66.9 m² |
An L-shaped or irregular room
Draw the room on paper, split it into rectangles, measure each one, and add them. For example, a main room 20 × 15 ft (300 sq ft) with an alcove 10 × 8 ft (80 sq ft) is 380 sq ft. If the shape cuts a corner off, take that corner off a bigger rectangle instead. Use the triangle calculator for a diagonal corner.
Walls and ceilings
A wall is its length times its height. For the four walls of a room, multiply the distance around the room (the perimeter) by the ceiling height, and take away the doors and windows. A room 14 × 12 ft has a perimeter of 52 ft, so with 8 ft ceilings the walls are 416 sq ft. Take off a 20 sq ft door and two 15 sq ft windows and you paint 366 sq ft. The ceiling is the floor area, 168 sq ft. See the paint calculator for gallons.
A roof
A pitched roof is bigger than the floor under it. Multiply the footprint by the pitch factor, the square root of 1 + (rise ÷ 12)². For a 1,500 sq ft footprint:
| Pitch (rise in 12) | Pitch factor | Roof area |
|---|---|---|
| 3/12 | 1.031 | 1,546 sq ft |
| 4/12 | 1.054 | 1,581 sq ft |
| 5/12 | 1.083 | 1,625 sq ft |
| 6/12 | 1.118 | 1,677 sq ft |
| 7/12 | 1.158 | 1,737 sq ft |
| 8/12 | 1.202 | 1,803 sq ft |
| 9/12 | 1.250 | 1,875 sq ft |
| 10/12 | 1.302 | 1,953 sq ft |
| 12/12 | 1.414 | 2,121 sq ft |
The roofing calculator adds waste and counts the bundles of shingles.
Unit conversions
- 1 square yard = 9 square feet (3 ft × 3 ft)
- 1 square meter = 10.7639 square feet
- 1 acre = 43,560 square feet
- 1 square foot = 144 square inches
Tips for accurate measuring
- Measure each wall at the base and take the longest length, since few rooms are perfectly square. Check the diagonals to see if a room is square (see how to square a foundation or deck).
- Measure to the finished surface for flooring and to the framing for materials that cover the structure.
- Round up for things that you buy by the area, and add a waste allowance for cuts. See material waste factors.
- For a house, real estate square footage usually counts only finished living area (the ANSI standard Z765), so garages and unfinished basements are not included.
Frequently asked questions
How do you calculate square footage?
Multiply the length by the width, both in feet. A room 20 feet long and 15 feet wide is 20 × 15 = 300 square feet. For other shapes, use the formula for the shape, or split the space into rectangles and add them.
How do you calculate the square footage of an L-shaped room?
Split it into two rectangles and add them. A 20 × 15 ft main room plus a 10 × 8 ft extension is 300 + 80 = 380 square feet.
How do you calculate the square footage of a circle?
Square the radius and multiply by 3.14159 (π). A circle 20 feet across has a radius of 10 feet, so its area is 314.2 square feet.
How do you convert square feet to square yards?
Divide by 9, because a square yard is 3 feet by 3 feet. 300 square feet is 33.3 square yards.
How do I measure a room that is not square?
Measure each wall and check the diagonals. If the walls differ, split the room into rectangles and triangles, measure each one, and add the areas.
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Estimates only. Quantities depend on your materials, site and supplier. Confirm important numbers before you order, and follow local building codes for anything structural.