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 a square footage figure and turn it 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.