How to use the square footage calculator
Start with Plan your area. Pick how you measure, then add a piece for each simple part of the space: a rectangle for the main room, another for an alcove, a triangle for a gable end, a circle for a round bed. Type each length the way you read it off the tape, such as 12'6", 12 ft 6 in or 3.5 m. The page draws every piece to scale and adds them up. Use Take an area away for anything you do not count, like a stairwell or a pool.
One shape writes out the formula with your numbers. Walls and openings finds paint or drywall area with doors and windows taken out. Convert units turns square feet into yards, meters and acres.
How area is figured
Area counts how many unit squares cover a shape. A rectangle 12 feet by 10 feet holds 10 rows of 12 squares, so 120 square feet. Every other formula is a way of counting squares without drawing them.
| Shape | Area |
|---|---|
| Rectangle | length × width |
| Triangle | ½ × base × height |
| Triangle, three sides | √(s(s−a)(s−b)(s−c)), where s is half the perimeter |
| Circle | π × radius² |
| Trapezoid | ½ × (top + bottom) × height |
| Parallelogram | base × height |
| Oval | π × half the long width × half the short width |
| Regular polygon | sides × side² ÷ (4 × tan(180° ÷ sides)) |
| L-shape | whole length × whole width − notch length × notch width |
L-shaped room: 20 × 15 − 8 × 6 = 300 − 48 = 252 sq ft
Press Imperial or Metric to work in feet and square feet or in meters and square meters. The switch changes what you type, what the results lead with, the examples and the shared picture, and it converts what you have already entered. The page keeps every length in feet, so 6 inches is 0.5 feet and 3.5 meters is about 11.48 feet. Imperial results lead with square feet, then square yards (divide by 9), square meters (multiply by 0.09290304) and acres (divide by 43,560). Metric results lead with square meters, then square feet and hectares (10,000 square meters).
Measuring an odd-shaped room
Break the floor into rectangles first. An L-shaped room is two rectangles, or one big rectangle minus the missing corner; both give the same answer, so use whichever gives easier measurements and check one against the other. A bay window, closet or alcove is one more rectangle. A triangle or half circle covers an angled wall or a curved patio edge.
Measure wall to wall at the floor and measure each piece twice. Read the tape to the nearest sixteenth of an inch; the tape reader above shows how.
What counts as square footage
For flooring, paint or a patio, square footage is simply the area you will cover. For a house, the number is a convention. The voluntary standard appraisers follow for single-family homes, ANSI Z765-2021, measures the outside of the house and counts finished, heated, above-grade space with a ceiling at least 7 feet high. Under a sloped ceiling, at least half of the room must reach 7 feet, and no floor under 5 feet counts. A garage, an unfinished basement or a porch is not living area, though you can measure it here as its own piece. Two listings can show different sizes for the same house because one measured inside the walls and another outside, so ask which method a number used.
Walls, ceilings and openings
A room's wall area is its perimeter times its ceiling height. A 12 by 10 foot room with 8 foot ceilings has 44 feet of wall, so 352 square feet before openings. Take away each door and window (width × height × how many). The ceiling has the same area as the floor. Walls and openings does this for you and draws it.
Where the numbers come from
- Unit factors: the foot is exactly 0.3048 meters (the 1959 international agreement), so 1 square foot is exactly 0.09290304 square meters and 1 acre (43,560 square feet) is exactly 4,046.8564224 square meters. The U.S. National Institute of Standards and Technology publishes the same factors in its SP 811 conversion tables.
- Area formulas: standard plane geometry, including Heron's formula and the regular polygon formula, as listed in the CRC Concise Encyclopedia of Mathematics (MathWorld).
- Measuring homes: ANSI Z765-2021, Square Footage, Method for Calculating, as adopted by Fannie Mae for appraisals. The standard is voluntary and your appraiser or agent decides what applies.
- Room sizes: no code decides how you measure a floor. Where a code sets a minimum, the International Residential Code (IRC R304) asks habitable rooms to have at least 70 square feet and at least 7 feet in every horizontal direction, with kitchens excepted. Your building department decides which edition applies.
This page does not size structure and is not an appraisal. Round up when you order materials. The Add extra box starts at zero because the right waste allowance depends on the material and the layout.
Uses
Flooring, carpet, tile and paint all start with area. So do sod, seed, mulch and gravel for a yard, and shingles for a roof.
Tips and common mistakes
- Do not mix units. Feet and inches typed as 12.6 means 12.6 feet, not 12 feet 6 inches. Use 12'6" or 12 ft 6 in. In metric, 3.5 means 3.5 meters.
- For a triangle, the height is measured straight up from the base at a right angle, not along the slanted side.
- A circle needs the radius, which is half the distance across. Using the full width as the radius makes the area four times too big.
Questions
How do I calculate square footage?
Multiply length by width, both in feet. A room 12 feet by 10 feet is 120 square feet. For other shapes, use the formula in the table or let the page do it.
How do I find the square footage of a room that is not a rectangle?
Split it into rectangles, triangles and circles, find each area, and add them. Subtract any part you do not count. The Plan your area tab does exactly that.
How many square feet are in a square yard, a square meter or an acre?
A square yard is 9 square feet, a square meter is about 10.7639 square feet, and an acre is 43,560 square feet.
Are my measurements saved?
No. Everything is figured in your browser. If you press Share on Facebook or Copy the link, the pieces, the sizes and the total are saved on AerialQuote for 90 days so the link can show them.