Rectangle Area Calculator

The rectangle is the shape behind almost every square footage question. Rooms, slabs, decks, lots, walls, countertops: measure the two sides, multiply, and you have the area. The calculator below does it in feet, inches, yards, or meters, and gives you the perimeter and the diagonal at the same time, because those are the two numbers you usually need next.

The formula

Area = length x width

A 12 by 16 foot room is 12 x 16 = 192 square feet. A square is the same formula with both sides equal, so a 15 by 15 room is 225 square feet.

Three things decide whether the answer is right:

Both measurements in the same unit. Feet times feet gives square feet. Feet times inches gives nothing useful. Convert first.

Inches turned into decimals, not left as fractions. A wall that measures 12 feet 7 inches is 12.58 feet, because 7 divided by 12 is 0.583. Multiplying 12.7 by anything gives you a wrong answer that looks plausible, which is the worst kind. The feet and inches calculator takes both boxes and does it for you.

The right two sides. On a room, that is the inside face of the walls for flooring, and the outside face for living area under ANSI Z765. Those are different numbers on the same room.

Common rectangles and their square footage

Dimensions	Square feet	Typical use
8 x 10 ft	80 sq ft	Small bedroom, home office
10 x 10 ft	100 sq ft	Bedroom, storage unit
10 x 12 ft	120 sq ft	Bedroom
12 x 12 ft	144 sq ft	Bedroom, small patio
12 x 16 ft	192 sq ft	Living room, deck
14 x 16 ft	224 sq ft	Master bedroom
16 x 20 ft	320 sq ft	Large living room
20 x 20 ft	400 sq ft	Two-car garage
24 x 24 ft	576 sq ft	Larger two-car garage
50 x 120 ft	6,000 sq ft	City lot, about 0.14 acres

The full version of this, including metric equivalents and building sizes, is on the square footage chart.

Perimeter and diagonal

Area answers "how much surface." The other two measurements answer questions that come up just as often.

Perimeter = 2 x (length + width). This is what you buy baseboard, crown molding, trim, fencing, and edging by. A 12 by 16 room has a perimeter of 2 x 28, which is 56 feet. It is also the starting point for painting walls, since perimeter times ceiling height gives you the wall area, which the wall calculator turns into sheets and gallons.

Diagonal = the square root of (length squared plus width squared). That is the Pythagorean theorem, and it is how you check whether something is actually square. A 12 by 16 rectangle has a diagonal of exactly 20 feet. Measure both diagonals of a deck frame or a wall layout, and if they match, the corners are true.

Squaring a layout with 3-4-5

For setting out a corner rather than checking one, the 3-4-5 method is the field version of the same math. This Old House describes it for deck framing: mark 3 feet along one side, 4 feet along the other, and adjust until the diagonal between the marks measures exactly 5 feet. The corner is then a true 90 degrees.

Scale it up for accuracy on larger work: 6-8-10 or 9-12-15 do the same job with less error over a long run.

Worked examples

A room for flooring. 13 feet 6 inches by 11 feet 3 inches. Convert to decimals first: 13.5 x 11.25 = 151.875, so 152 square feet. Add 10 percent for hardwood waste and order about 167 square feet.

A concrete slab. A 12 by 12 shed pad at 4 inches thick is 144 square feet of surface, and 144 x (4/12) = 48 cubic feet of concrete, which is 1.78 cubic yards. The concrete calculator runs the depth conversion.

A lot. 50 by 120 feet is 6,000 square feet. Divide by 43,560 and it's 0.138 acres. The acres converter covers the land side.

A wall. 20 feet wide by 9 feet tall is 180 square feet, minus a 21 square foot door, so 159 square feet to paint.

When length times width is the wrong tool

The formula only holds for a true rectangle, with four right angles and opposite sides equal. Two situations break it:

The shape isn't rectangular. An L-shaped room, a five-sided lot, a bay window. Split it into pieces and add them, or use the irregular shape calculator, which handles trapezoids, cut-outs, and curved edges.

The corners aren't square. Old houses settle and walls bow. If the diagonals don't match, multiplying two sides gives an approximation. For a tolerable difference, measure both length and both width, average each, and multiply. For anything serious, treat it as an irregular shape.

Common mistakes

Multiplying feet by inches. Convert everything to one unit before you multiply.

Reading 12 feet 7 inches as 12.7. It's 12.58. The error grows with every multiplication.

Using perimeter where you need area. Baseboard is a perimeter buy. Flooring is an area buy. They are not interchangeable.

Measuring at the widest point of a non-square room. That inflates the area. Measure both directions in two places and average if they differ.

Ordering exactly what you measured. Every material has a waste factor, and cuts eat more of it than people expect.

Frequently asked questions

How do you calculate the area of a rectangle?

Multiply the length by the width, with both in the same unit. A 12 by 16 foot room is 192 square feet. If you measured in inches, multiply and then divide by 144 to get square feet.

How do I find the square footage of a rectangle?

Measure the two sides in feet and multiply them. For a measurement in feet and inches, convert the inches to a decimal first by dividing by 12, so 14 feet 6 inches becomes 14.5 feet.

What is the formula for the area of a rectangle?

Area equals length times width, written A = l x w. For a square, where every side is the same, it simplifies to side squared.

How do I find the perimeter of a rectangle?

Add the length and the width, then double it. A 12 by 16 rectangle has a perimeter of 2 x (12 + 16), which is 56 feet. Use this for baseboard, trim, and fencing.

How do I calculate the diagonal of a rectangle?

Square the length, square the width, add them, and take the square root. A 12 by 16 rectangle has a diagonal of 20 feet, since 144 plus 256 is 400.

How do I know if a rectangle is actually square?

Measure both diagonals. In a true rectangle they're identical. If they differ, the corners aren't at 90 degrees, and the area from length times width is an estimate rather than an exact figure.

How many square feet is a 12x12 room?

144 square feet. Any room quoted as 12 by 12 in feet gives you 12 x 12 = 144. A 10 by 10 room is 100 square feet and a 15 by 15 is 225.

Is a square a rectangle?

Yes. A square is a rectangle whose sides happen to all be equal, so the same area formula works. Multiplying side by side is the same as squaring it.

Sources

How to square joists for a deck, This Old House

Rectangle Area Calculator