How to calculate square footage

Square footage is just length times width, but real rooms are rarely neat rectangles. Here is how to measure any space, handle L-shapes and circles, and add the right amount for flooring waste.

By the Calculator Den team · Updated 2026-10-19

Whether you are buying flooring, ordering paint, pricing a rental or just settling an argument about how big a room is, it all starts with one number: square footage. The good news is the core maths is as simple as it gets. The part that trips people up is real rooms, which are rarely perfect rectangles. Here is how to calculate square footage for any space, including the odd shapes, plus how much extra to buy so you are not short halfway through a job. A free square footage calculator does the arithmetic for you.

In short: square footage = length in feet x width in feet. For a space that is not a simple rectangle, split it into rectangles (and the odd triangle or circle), work out each piece, and add them up. If you measured in inches, divide the final total by 144 to get square feet.

The basic formula

For any rectangular area, square footage is simply length times width, both measured in feet:

Area (sq ft) = length (ft) x width (ft).

A room that is 12 feet by 10 feet is 12 x 10 = 120 square feet. That is the whole idea. The unit "square feet" is literally how many one-foot squares would tile the floor. Everything else is just applying this to shapes that are not tidy.

Measuring in inches? Divide by 144

Tape measures give you feet and inches, and a common mistake is mixing them. Two clean ways to handle it:

• Convert to feet first. There are 12 inches in a foot, so 10 feet 6 inches is 10.5 feet. Then multiply as normal.
• Work in inches, then divide by 144. If a space is 126 inches by 120 inches, that is 15,120 square inches. Since one square foot is 12 x 12 = 144 square inches, divide by 144 to get 105 square feet.

Pick one method and stick to it for the whole job. Mixing feet and inches mid-calculation is the number-one source of ordering errors.

Odd shapes: split the room into rectangles

Almost no real room is a perfect box. The trick the pros use is to divide the space into simple shapes, measure each, and add them together.

• L-shaped room: draw a line to split it into two rectangles. Say one part is 12 x 10 (120 sq ft) and the other is 6 x 8 (48 sq ft). Total = 168 square feet.
• A triangular corner or bay: the area of a triangle is half the base times the height. A triangle with a 6 ft base and 4 ft height is (6 x 4) / 2 = 12 sq ft. (Our Pythagorean theorem finds a missing side of a right triangle when you only know the other two.)
• A circular area or rounded bay: the area of a circle is pi times the radius squared. A round patio 10 ft across has a radius of 5 ft, so the area is 3.14159 x 5 x 5 = about 78.5 sq ft.

Add every piece together for the total. A quick sketch on paper with each section labelled makes this almost foolproof. Another way to handle a notch is to measure the full bounding rectangle and subtract the missing corner: an 18 x 10 room with a 6 x 2 bite taken out is 180 minus 12 = 168 square feet, the same answer.

Square footage of a wall (for paint or drywall)

A wall is just a rectangle standing up, so the formula is the same: width x height. A wall 12 ft wide and 8 ft tall is 12 x 8 = 96 square feet. To find how much to paint in a whole room, add up the walls, then subtract the openings. For a 12 x 10 ft room with 8 ft ceilings, the perimeter is 2 x (12 + 10) = 44 ft, so the wall area is 44 x 8 = 352 sq ft. Take off one door (about 21 sq ft) and one 3 x 4 ft window (12 sq ft) and you have 352 - 21 - 12 = 319 sq ft to paint. A gallon of paint typically covers about 350 to 400 sq ft per coat, so check the tin and plan for two coats.

Area vs perimeter: do not mix them up

This is the most common beginner slip. Area is the space inside, in square feet, and tells you how much flooring, tile or carpet to buy. Perimeter is the distance around the edge, in plain feet, and tells you how much skirting board, trim, fencing or edging you need. Our 12 x 10 room has an area of 120 sq ft but a perimeter of 44 ft. Two rooms can share a perimeter and have completely different areas, so never use one number to do the other's job.

Square footage of a whole house vs a room

For a single room, measure wall to wall at floor level. You can leave out the space under permanent fixtures like built-in cabinets or a fixed kitchen island, but include it if the fixture might be moved or removed later. For a whole house, the "square footage" quoted in listings normally means finished, livable floor area, measured to the inside of exterior walls and typically excluding garages, unfinished basements and open-to-below spaces. In the US the common reference is the ANSI Z765 standard for measuring homes, but appraisers and agents can still differ on things like stairwells and ceiling height. Rules also vary by country, so if an exact figure matters (a sale, a mortgage, a tax assessment), ask which standard is being used and get the floor plan rather than assuming.

How much extra to buy: the waste factor

Here is the step that saves a second trip to the store. Never order exactly your square footage, because cuts, offcuts, breakages and pattern-matching all eat into material. Add a waste factor on top:

So for a 168 sq ft floor in hardwood at 10 percent waste, order 168 x 1.10 = about 185 sq ft. Flooring is sold in boxes, so round up to whole boxes: if a box covers 20 sq ft, you need 185 / 20 = 9.25, which means 10 boxes (200 sq ft). The spare box also leaves you matching planks for future repairs, which matters because dye lots differ between batches. These are rough industry rules of thumb; your installer or the product's own guidance may differ, so check before you buy.

Turning square footage into a cost

Once you have the area, costing a job is one more multiplication: square footage x price per square foot. For 185 sq ft of flooring at 4 per square foot (in your own currency), that is 185 x 4 = 740 for materials, before fitting. Price the waste-adjusted area (185), not the bare room area (168): costing 168 would give 672, leaving you 68 short. The same logic prices paint (by coverage per tin), rent (per square foot or metre), and cleaning quotes. Our percentage calculator helps if you want to add the waste percentage or a tax on top.

Square metres instead of feet?

Outside the US, areas are usually in square metres. The method is identical, just measure in metres. To convert, 1 square metre = about 10.76 square feet, and to go the other way, multiply square feet by 0.0929. So a 168 sq ft room is about 15.6 square metres.

Common square-footage mistakes

Common questions

How many square feet is a 12x12 room? 12 x 12 = 144 square feet. A "12 by 12" room is 144 sq ft, which is also why 144 square inches make one square foot.

Do closets count? For flooring, yes, measure any floor you will cover, including closets and alcoves. For a house listing, built-in closets usually count as finished space; walk-in versus reach-in rules vary, so check the standard being used.

How do I calculate square footage for a ceiling? For a normal flat ceiling it is the same as the floor below it, so a 12 x 10 room has a 120 sq ft ceiling. Sloped or vaulted ceilings have more surface area and need to be split into shapes like any odd wall.