How to calculate square footage

The basic formula

For any rectangular room, square footage is length times width, with both measurements in feet:

Square footage = length (ft) x width (ft).

A room 15 feet long and 12 feet wide is 15 x 12 = 180 square feet. That one line covers most bedrooms, offices, and living rooms, since they are close enough to true rectangles. The catch is getting both numbers into feet before you multiply, which is where most errors start.

Converting feet and inches to decimal feet

Tape measures read in feet and inches, but the formula wants feet. Divide the inches by 12 and add them to the whole feet:

decimal feet = feet + (inches / 12).

So 12 feet 6 inches becomes 12 + 6/12 = 12.5 feet, and 9 feet 3 inches becomes 9 + 3/12 = 9.25 feet. Do this for both the length and the width before multiplying. Do not just tack the inches on after the decimal point: 8 feet 4 inches is 8.33 feet, not 8.4 feet.

Worked example

Take a bedroom that measures 12 feet 6 inches by 10 feet.

Convert the length: 12 feet 6 inches = 12 + 6/12 = 12.5 feet. The width is already whole, 10 feet.

Multiply: 12.5 x 10 = 125 square feet.

That 125 square feet is the finished floor area, the figure you would start from for flooring or paint, usually before adding a waste allowance.

Adding multiple rooms together

For a whole floor or home, calculate each room on its own and add the areas at the end. Keep the individual figures rather than combining measurements as you go, so a stray number is easier to catch.

A 125 square foot bedroom, a 180 square foot living room, and a 90 square foot kitchen add up to 125 + 180 + 90 = 395 square feet. Hallways, landings, and closets count too if you want the full usable area, so walk the space in a consistent order and list every section as you measure it.

L-shaped and irregular rooms

When a room is not a neat rectangle, split it into rectangles you can measure, find each area, and add them up.

For an L-shaped room, draw an imaginary line to divide it into two rectangles: one 10 feet by 8 feet, and a smaller one 6 feet by 4 feet. The first is 10 x 8 = 80 square feet and the second is 6 x 4 = 24 square feet, so the whole room is 80 + 24 = 104 square feet.

The same method handles alcoves, bay windows, and bump-outs: treat each as its own small rectangle and add it in. For curved or angled walls, this gives a close estimate rather than an exact figure, so measure the largest rectangle that fits and add smaller pieces for the leftover areas.

• Break the shape into non-overlapping rectangles so no area is counted twice.
• Measure the length and width of each rectangle in feet.
• Calculate each rectangle's area with length x width.
• Add all the areas together for the total square footage.

Common mistakes

A handful of slip-ups cause most wrong answers:

• Mixing units. Multiplying feet by inches without converting first throws off the whole result. Convert every measurement to decimal feet using feet + (inches / 12) first.
• Forgetting closets, alcoves, and bump-outs. They sit off to the side and are easy to skip, but they add real area. Treat each as its own rectangle and add it in.
• Rounding too early. Round each measurement to the nearest foot and the errors pile up, especially across many rooms. Keep the decimals through the whole calculation and round only the final total.
• Mixing wall-to-wall and usable measurements. Decide up front whether you want gross floor area or usable area, and measure every room the same way.
• Transposing length and width between rooms. Write each measurement down as you take it, so a 12 does not become the width of the wrong room.

Turning square footage into materials

Once you have the area, it feeds other estimates. Flooring is sold by the square foot, so your total is the starting point, plus a waste allowance for cuts and offcuts. Paint is figured from wall area, not floor area, though the same measure-and-multiply approach applies. The Flooring Calculator and Paint Calculator on this site take room dimensions and turn them into rough quantities.

These are estimates only. Real rooms have out-of-square walls, thick baseboards, and doorways, so measure carefully, and if a purchase depends on the figure, add a margin and check the supplier's guidance.

Triangles, circles, and half-rounds

Not every shape breaks cleanly into rectangles. Three other formulas cover almost everything you will meet in a house or yard. A triangle is half of its base times its height. A gable end that is 24 feet across the base and 8 feet tall at the peak has 0.5 x 24 x 8 = 96 square feet.

A circle is pi x radius squared, with the radius being half the diameter. A round patio 12 feet across has a 6-foot radius, so its area is 3.1416 x 6 x 6 = 113.1 square feet. Do not square the diameter by mistake, because that gives 144, which overstates the area by more than a quarter.

A bay window or curved alcove is often a half circle. If the opening across the wall is 6 feet, the radius is 3, and the area is half of 3.1416 x 3 x 3 = 14.1 square feet. Add that to the main rectangle. For a trapezoid, average the two parallel sides and multiply by the distance between them.

Metric rooms and other units

Outside the United States, areas come in square meters. A room 4.2 m by 3.5 m is 4.2 x 3.5 = 14.7 square meters. To convert to square feet, multiply by 10.764, which gives 158.2 square feet. The Square Footage Calculator, switched to metric, shows both figures. Going the other way, multiply square feet by 0.0929 to get square meters.

Two other units come up often in the American trades. A square yard is 3 ft x 3 ft, or 9 square feet, so the 15 by 12 foot room from the earlier example, at 180 square feet, is 20 square yards. Carpet is commonly priced by the square yard, so convert before you compare prices to a flooring quote in square feet.

Land is another scale. An acre is 43,560 square feet, which is why a quarter-acre lot is a little under 11,000 square feet. Always check which unit a price or listing is using before comparing two numbers.

Wall and ceiling area

Floor area is not the only figure people need. For paint or wallpaper you want wall area: the perimeter of the room times the ceiling height, then minus the openings. A room 12 feet by 15 feet has a perimeter of 2 x (12 + 15) = 54 feet. With an 8-foot ceiling, the gross wall area is 54 x 8 = 432 square feet.

Subtract doors and windows. A standard interior door is about 3 ft x 7 ft = 21 square feet. Two windows of 3 ft x 5 ft are 15 square feet each. The paintable wall area is 432 - 21 - 30 = 381 square feet. The ceiling in a flat-ceilinged room equals the floor area, 180 square feet here, and is measured separately because it is usually painted with different paint.

Small openings are often ignored in practice, since the paint saved by subtracting a tiny window is less than the margin you would add anyway. Large openings, such as a sliding glass door or picture window, are worth subtracting.

Why the listed size of a house differs from your tape

If you measure the inside of a home, the total will usually come in under the figure in the listing. Many appraisers and agents in the United States follow the ANSI Z765 measurement standard, which measures finished space from the exterior walls. Interior measurements exclude the thickness of the walls themselves.

A simple example shows the gap. A house that measures 30 ft by 40 ft on the outside is 1,200 square feet. With walls about 6 inches thick on every side, the interior is 29 ft by 39 ft, which is 1,131 square feet. That is about 6 percent less, and it is the right number for flooring or paint even though it is lower than the advertised size.

Definitions of what counts also differ. Finished space below ground level, garages, and unheated porches are commonly treated differently from main living space, and local practice varies. When a listed figure matters to a decision, ask how it was measured. For materials, always trust your own interior measurements.

Checking a room that is not quite square

Older houses and DIY additions are rarely perfect rectangles. A quick test is to measure both diagonals. If they match, the room is square. A 12 ft by 16 ft room should have diagonals of exactly 20 feet, since 12 squared plus 16 squared is 400, and the square root of 400 is 20. A difference of an inch or more points to a skewed wall.

When two opposite walls are parallel but the side walls are not, average the widths. Say a room is 15 feet long, with the width measured at 11.5 feet at one end and 12.5 feet at the other. The area is the average width times the length: 12 x 15 = 180 square feet. For walls that bow or angle in more than one place, break the room into several pieces.

Measure each dimension twice, preferably in different directions, and sketch the room with every number written next to its wall. A laser distance meter makes measuring faster, but the habit of double-checking matters more than the tool.

Using the area to compare prices

Square footage turns messy quotes into comparable ones. Flooring priced at 4.25 dollars per square foot for the 395 square feet in the earlier whole-floor example comes to 395 x 4.25 = 1,678.75 dollars before any waste allowance. Price per square foot lets you compare a box of laminate with a roll of vinyl, provided you work out the price on the area actually purchased, not the area covered.

Rent and real estate are quoted the same way. A 1,200 square foot unit leasing for 2,400 dollars a month costs 2 dollars per square foot per month, or 24 dollars per square foot per year. Commercial listings are often stated on an annual basis, so confirm whether a figure is monthly or yearly before comparing.

Keep the exact figure and the purchase figure apart. Your measured area is the floor you need to cover. The amount you buy is that number plus a waste allowance, rounded up to whole boxes or rolls.