How to calculate square footage: rooms, odd shapes and whole houses
Multiply the length by the width, both in feet: a 12 by 10 foot room is 12 × 10 = 120 square feet. Turn inches into feet first by dividing them by 12, split odd-shaped rooms into rectangles and triangles, work out each piece and add them up.
Anthony Hicks30+ years in construction. Updated November 5, 2026.
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The formula
Square feet = length in feet × width in feet.
Measure the two sides of the room that meet at a corner, multiply them, and that’s the floor’s area. A room 14 feet long and 12 feet wide is 14 × 12 = 168 square feet.
That’s all a rectangle needs. Everything below is about the rooms that aren’t rectangles, measurements that come off the tape in feet and inches, and the other surfaces a job prices: walls, ceilings, roofs and whole houses.
Measuring a room, step by step
- Measure along the floor, wall to wall, at the baseboard. Run the tape along the longest wall for the length, then along the wall that meets it for the width.
- Write down feet and inches as they read, for example 13’ 7”.
- Turn the inches into feet by dividing them by 12: 7 inches is 7 ÷ 12 = 0.58 feet, so 13’ 7” is 13.58 feet.
- Multiply the length by the width.
- Round at the end, not in the middle. Rounding each side first can throw the total off by several square feet.
The guide to measuring a room covers the ceiling height, doors and windows too.
Inches to feet
| Inches | Feet | Inches | Feet |
|---|---|---|---|
| 1” | 0.08 | 7” | 0.58 |
| 2” | 0.17 | 8” | 0.67 |
| 3” | 0.25 | 9” | 0.75 |
| 4” | 0.33 | 10” | 0.83 |
| 5” | 0.42 | 11” | 0.92 |
| 6” | 0.50 | 12” | 1.00 |
A worked one: a bedroom of 13’ 7” by 11’ 4” is 13.58 × 11.33 = 153.9 square feet, about 154.
Rooms that aren’t rectangles
Split the room into pieces that are simple shapes, work out each one, and add them.
- L-shaped room. Draw a line across where the L turns, making two rectangles. A room that’s 20 by 12 in one leg and 10 by 8 in the other is 240 + 80 = 320 square feet.
- Angled wall. The angle leaves a triangle. A triangle’s area is its base times its height, divided by 2: a corner cut 6 feet along one wall and 4 along the other is 6 × 4 ÷ 2 = 12 square feet. Subtract it from the full rectangle, or add it to the rest, depending on which way the angle goes.
- Bay or round wall. A half circle is 3.1416 × the radius × the radius ÷ 2. A bay 6 feet across has a radius of 3, so it’s 3.1416 × 3 × 3 ÷ 2 = 14.1 square feet.
- Closets and alcoves. Measure each one on its own and add it. For flooring, they count.
| Shape | Formula |
|---|---|
| Rectangle or square | length × width |
| Triangle | base × height ÷ 2 |
| Circle | 3.1416 × radius × radius |
| Half circle | 3.1416 × radius × radius ÷ 2 |
| Trapezoid | (top + bottom) ÷ 2 × height |
Common room sizes
| Room | Square feet |
|---|---|
| 10 × 10 | 100 |
| 10 × 12 | 120 |
| 11 × 12 | 132 |
| 12 × 12 | 144 |
| 12 × 14 | 168 |
| 12 × 15 | 180 |
| 14 × 16 | 224 |
| 15 × 20 | 300 |
| 20 × 20 | 400 |
Walls, ceilings and roofs
Different trades price different surfaces, and each has its own square footage.
- Floor and ceiling: length × width. A flat ceiling has the same area as the floor under it.
- Walls: the distance around the room times the ceiling height. A 14 by 12 room with 8 foot ceilings has 2 × (14 + 12) × 8 = 416 square feet of wall. Take out the doors and windows: a standard door is about 20 square feet and a window about 15.
- Roof: the footprint is not the roof. A sloped roof is bigger than the ground under it, and the steeper the pitch, the bigger. The guide to measuring a roof covers the pitch factor and squares.
A whole house
There are two different numbers, and they’re used for different things.
- For a job (flooring, paint, a water loss): add up the inside of each room, measured wall to wall. That’s the area the material actually covers.
- For a listing or an appraisal: US appraisers follow the ANSI Z765 standard, which Fannie Mae requires for a home’s gross living area. It reports finished square footage above grade and below grade separately, for the whole house, and doesn’t cover room dimensions.
The two numbers don’t match, so never use one for the other: a flooring order comes from the rooms, a listing comes from the appraisal.
Square feet, square yards and square meters
- 1 square yard = 9 square feet. Divide square feet by 9 for square yards; carpet is often sold that way.
- 1 square meter = 10.764 square feet.
- 1 square foot = 0.0929 square meters.
- 1 acre = 43,560 square feet.
Common questions
What is 12 by 8 feet in square feet?
96 square feet: 12 × 8 = 96.
How many square feet is a 12 by 12 room?
144 square feet.
What size room is 120 square feet?
A 10 by 12 room is 120 square feet. So is 8 by 15.
How do I work out the price per square foot?
Divide the price by the square feet. A $8,400 flooring job over 1,200 square feet is $8,400 ÷ 1,200 = $7 a square foot.
How much extra should I order?
For flooring and tile, about 10% over the measured area for a straight layout and about 15% for diagonal or herringbone patterns, which cut more at the walls. The square footage calculator adds the extra for you.
Is there a faster way than a tape?
Yes. A phone can measure the room: a Pro iPhone scans it with LiDAR, and any other phone can mark the corners. Both give the floor area, the wall area and the ceiling height of every room at once. The guides to which iPhones have LiDAR and floor plans on Android explain how.