Square Footage Calculator

What square footage measures

Square footage is area: the amount of surface inside a boundary. It is what materials are sold by, what contractors quote from and what most home improvement estimates start with. Get the area wrong and every number downstream, from boxes of flooring to gallons of paint, is wrong with it.

Two habits prevent most errors. Measure in one consistent unit before doing any arithmetic, and break complicated shapes into simple ones instead of trying to handle them in a single step.

Formulas for basic shapes

Rectangles and squares

Circles

Triangles

Trapezoids

L-shaped and irregular rooms

Almost no real room is a clean rectangle. The reliable method is to divide the floor into rectangles that do not overlap, calculate each one, and add the results. For a shape with a single notch, the subtraction method works as a cross check: take the area of the full bounding rectangle and remove the missing piece.

For a genuinely irregular outline, such as a curved bay or an angled corner, approximate it with the closest simple shape. A bay is roughly a half circle or a trapezoid, and an angled corner is a right triangle. Rounding the estimate up slightly is the safer direction when ordering material.

Worked example: an L-shaped room

A living room fits inside an 18 foot by 12 foot rectangle, with a 6 foot by 5 foot corner taken out where a closet and stair bulkhead intrude.

Both routes give 186.00 square feet, which is the point of running the check. Rectangle A is the full 12 foot depth across the first 12 feet of width. Rectangle B is the remaining 6 feet of width at the reduced depth of 7 feet, because 5 feet of that section is taken by the notch.

In metric that is 186 divided by 10.764 = 17.28 square metres. In square yards, used for carpet, it is 186 divided by 9 = 20.67 square yards.

Measuring a whole house room by room

There is no shortcut for a whole house. Measure each room individually and add the results, because exterior dimensions include wall thickness and give a figure that no flooring order can use.

1. Sketch a rough floor plan, one page per level, and label every room.
2. Measure the length and width of each room at floor level, along the baseboard, not at hand height where furniture and trim distort the line.
3. Take a second measurement on the opposite side of each room. Older houses are rarely square, and a 3 inch difference across 15 feet is common.
4. Note closets, alcoves and bay windows separately, then add or exclude them deliberately.
5. Record measurements in feet and inches exactly as taken, then convert everything at once.
6. Add the room totals for a floor, and the floors for a house.

Be aware that gross living area, the figure used by appraisers and real estate listings, follows its own standards. It typically counts finished, heated space measured from the exterior and excludes garages and most below grade areas, so it will not match a flooring take-off.

The inches problem

Rooms rarely measure a whole number of feet, and this is where the most damaging arithmetic error happens. There are 12 inches in a foot, but there are 144 square inches in a square foot, because 12 x 12 = 144.

The alternative is to convert each measurement to decimal feet first, by dividing the inches by 12, and then multiply. 148 inches is 12.333 feet and 92 inches is 7.667 feet, and 12.333 x 7.667 = 94.55 square feet. The small difference is rounding, so keep three decimal places if using this route.

Common inch to decimal foot conversions

Unit conversions

Two of these come up constantly. One square metre is 10.764 square feet, so a 60 square metre apartment is about 646 square feet. One square yard is exactly 9 square feet, because a yard is 3 feet and 3 x 3 = 9, which is why carpet priced per square yard looks so much cheaper per unit than the same carpet priced per square foot.

Gross area versus net area

Gross area is the full surface. Net area is what is left after openings are deducted. For flooring the distinction barely matters, since a doorway threshold is a small fraction of a floor. For wall painting and wall tile it matters a great deal.

• A standard interior door is about 3 feet by 7 feet, or 21 square feet.
• A typical bedroom window is about 3 feet by 4 feet, or 12 square feet.
• A large picture window can easily be 30 to 40 square feet.

A common trade shortcut is to ignore small openings on the grounds that the deduction and the waste factor cancel out. That is fine on one door in a big room. In a room with three windows and two doors it can overstate the wall area by more than 100 square feet.

Wall and ceiling area for painting

Ceiling area equals the floor area, so the same 186 square feet applies. Paint coverage is normally quoted at 350 to 400 square feet per gallon for one coat on a smooth, previously painted surface. Two coats over 495 square feet of wall is 990 square feet of coverage, which at 350 square feet per gallon is 2.83 gallons, so buy 3 gallons. Rough, porous or heavily textured surfaces absorb noticeably more, and a strong colour change may need a primer coat as well.

Waste factors by material

Applying that to the 186 square foot room: at a 10 percent flooring waste factor, order 205 square feet. If the product comes in 24 square foot boxes, 205 divided by 24 is 8.54, so 9 boxes are needed. For diagonal tile at 15 percent the requirement is 214 square feet, which at 2 square feet per 12 inch by 24 inch tile is 107 tiles.

Buy the whole order at once so it comes from the same production run. Dye lots and batch shading vary, and a single extra box bought three weeks later frequently does not match. Keeping one spare box after the job is finished is cheap insurance against future repairs.