Square Feet Of Rectangle Calculator

Background.

A square foot is the area of a square one foot on each side, and a rectangle measured in feet holds exactly length × width of them. A 12-by-10 bedroom can be pictured as 12 rows of 10 one-foot floor tiles: 120 tiles, 120 square feet. That picture is worth keeping, because it explains the two things that most often trip people up.

The first is that area grows with both dimensions at once. Make the bedroom 2 feet longer and you add a strip of 2 × 10 = 20 square feet. Make it 2 feet longer and 2 feet wider and you add 48, not 40, because the corner where the two new strips meet counts too. Double both sides and the area quadruples.

The second is that ten square feet and ten feet square are different things. Ten square feet is an area, about the size of a rug 3 feet by 3⅓ feet. Ten feet square describes a square 10 feet on each side, which covers 100 square feet.

In practice the multiplication is the easy part and the measuring is where errors creep in. Tape measures read in feet and inches, and 12 feet 6 inches is 12.5 feet, not 12.6, because inches are twelfths of a foot rather than tenths. Rooms that look rectangular often are not quite: if the two diagonals of a floor measure noticeably differently, the corners are not square and length × width is only an approximation.

One length and one width, both in feet, are multiplied here. Nothing is added for waste or offcuts and nothing is taken off for doors, windows, columns or built-in cabinets; run those pieces separately and add or subtract them.

What is square feet of rectangle calculator?

Square footage is area expressed in square feet (ft²), the unit of floor and surface area used in US real estate, flooring and building work. One square foot is a square 12 inches by 12 inches, or 144 square inches. For a rectangle, where every corner is a right angle, the area is simply length multiplied by width with both sides measured in feet. Because the international foot is defined as exactly 0.3048 metres, one square foot is exactly 0.09290304 square metres.

How to use this calculator.

1. 01Measure the longer side wall to wall at floor level and enter it as Length in feet. Turn inches into feet by dividing by 12, so 12 ft 9 in is 12.75 ft.
2. 02Measure the shorter side the same way and enter it as Width. Which side you call length makes no difference to the area.
3. 03For an L-shaped or notched space, split it into rectangles that do not overlap, run each one and add the results; for a wall, run each door or window and subtract it.
4. 04Read the area in square feet, then add whatever waste allowance your supplier recommends before ordering flooring, tile or turf.

The formula.

Area = length × width, with both sides in feet and the answer in square feet. Multiplying feet by feet is what produces the square in the unit, which is why mixing units quietly breaks the answer: 12 feet times 120 inches gives 1,440, a number that is neither square feet nor square inches. For a true rectangle the result is exact. Because the formula is a product, a measuring error in one side passes straight through in proportion: a length that is 1% too long makes the area 1% too big. Other units follow from fixed factors — divide by 9 for square yards, multiply by 0.09290304 for square metres, divide by 43,560 for acres — although only square feet are reported here.

A worked example.

Frequently asked questions.

References& sources.

1. [1]National Institute of Standards and Technology. NIST SI Guide, 2008 edition. Appendix B unit relationships. Retrieved 2026-08-06.
2. [2]OpenStax, Rice University. Precalculus 2e, 2021. Trigonometry and analytic geometry chapters. Retrieved 2026-08-06.
3. [3]NIST/SEMATECH. e-Handbook of Statistical Methods, 2012. Chapter 1.3 and Chapter 4. Retrieved 2026-08-06.