How to calculate square footage

By Rohit GaurLast updated October 11, 2026Report a mistake

Square footage is length times width, measured in feet. A room 12 feet long and 10 feet wide is 120 square feet. Everything else on this page is about the rooms that are not that simple: the ones with an alcove, a kitchen island, a sloping wall, or a tape measure that reads 12 ft 6 in.

What you need

A tape measure, a laser measure, or a phone with a measuring app you trust enough to check twice.
Something to write on. Measure everything first, then do the maths — walking back to re-measure one wall is how mistakes get in.
The unit you will buy in. Flooring in the United States is sold by the square foot; in most of Europe, by the square metre. Measure in that unit and you skip a conversion.

Step 1: measure wall to wall, at floor level

Measure the floor you are standing on: inside of one wall to inside of the opposite wall, with the tape flat on the ground. The dashed line marks the skirting board — measure above it, not to the wall behind it, and the number matches the floor you will cover.

Two habits are worth building:

Measure at floor level. A tape held at hip height sags and drifts around furniture. Floor level is also where flooring actually goes.
Include the alcoves. A bay window or a recessed nook is floor, and it needs covering. The dashed rectangle in the drawing is the part people forget.

Step 2: multiply

Length × width. For a room 12 ft by 10 ft:

12 × 10 = 120 square feet

If your tape reads feet and inches, turn the inches into a fraction of a foot before multiplying: divide by 12. So 12 ft 6 in is 12 + 6 ÷ 12 = 12.5 ft, and a room 12 ft 6 in by 10 ft is:

12.5 × 10 = 125 square feet

The calculator on this site takes 12'6", 12 ft 6 in and 12.5 as the same measurement, so you can type whatever the tape says.

Step 3: split anything that is not a rectangle

Almost every awkward room is two or three rectangles standing next to each other.

An L-shaped room is one rectangle plus another. Draw the split where the wall steps in, measure each part separately, and add the two areas.

For an L-shaped room where part A is 13 ft × 13 ft and part B is 11 ft × 7 ft:

A: 13 × 13 = 169
B: 11 ×  7 =  77
total      = 246 square feet

The same idea covers a room with a chimney breast, a hallway that widens, or an open-plan space that turns a corner. Split, measure, add.

Two shapes are worth knowing by formula rather than splitting:

Triangle: base × height ÷ 2, where the height is measured square to the base, not along a wall.
Circle or half circle: π × radius², halved for a bay that is half a circle.

When the corners are not square

Old houses, extensions and anything built against a boundary often have four straight walls and not one right angle. Measuring such a room as a rectangle is the most expensive mistake on this page, because the four wall lengths do not tell you the area at all.

Both rooms have the same four walls. The leaning one holds less floor, and nothing in the wall lengths says which room you are standing in — the diagonal d does.

So measure one more line: corner to corner, across the room. That diagonal splits it into two triangles whose three sides you now know, and a triangle’s area follows from its sides alone. For walls of 16, 12, 16 and 12 ft with a diagonal of 20 ft:

triangle 1: 16, 12, 20 =  96
triangle 2: 16, 12, 20 =  96
total                  = 192 square feet

That room is square: 16, 12 and 20 is the 3-4-5 triangle scaled up, so its corners are right angles. Leave the same four walls and stretch the diagonal to 24 ft and the room leans over — the area drops to 171 sq ft — about 11% less floor to cover. The calculator’s Four walls + diagonal shape does this arithmetic and shows you each corner’s angle.

Check your corners while you are there. Measure the other diagonal as well. In a room that is truly square, opposite walls are equal and both diagonals are equal; if the two diagonals differ, the corners are not 90° and the difference tells you by how much. It is the same check a carpenter makes before fixing a floor, and it is the reason the calculator asks for the second diagonal and tells you what it should read before you measure it.

Step 4: take off what is not floor, add what is

A kitchen island, a fixed cupboard or a staircase comes off the total. A bay window or an alcove goes on. Measure each one as its own little rectangle.

For flooring, take off anything permanent that sits on the floor and will not be covered:

a kitchen island or fitted units
a fireplace hearth
a staircase opening
a built-in wardrobe, if it will not be floored underneath

Leave everything movable in: a sofa is not a hole in the floor.

Feet, inches and metres

You measured	To get square feet	To get square metres
feet × feet	multiply them	× 0.09290304
inches × inches	÷ 144	÷ 1550.0031
metres × metres	× 10.763910417	multiply them
yards × yards	× 9	× 0.83612736

The conversion factors are exact definitions rather than rounded approximations, and every one of them is listed with its source on the methodology page.

How much extra to buy

Measured area is not order quantity. Flooring gets cut, tiles break, and a pattern has to line up, so trades add a waste allowance on top:

straight-laid flooring or plain tile: about 10%
a diagonal or herringbone layout, or a room full of angles: 15% or more
carpet from a roll: depends on the roll width, and often more than 10%

The calculator’s Waste and cost panel applies the percentage to the area and multiplies by your price, so you get the quantity to order and the money it costs in one place.

Mistakes that cost money

Rounding too early. Round 12.54 m² down to 12 m² in three rooms and you have lost 1.62 m², about 17 sq ft — a box of flooring. Keep the decimals until the last step.
Mixing units. A length in feet times a width in metres is not an area. Convert first.
Measuring the ceiling. Sloping walls, bay windows and radiator recesses mean the ceiling and the floor are different shapes. You are covering the floor.
Forgetting the doorway. Flooring usually runs into the threshold; the opening is part of the room for materials, even when it is not part of the room for a listing.
Assuming the listed size is right. A property listing may follow a measuring standard that includes or excludes different spaces. Measure the floor yourself.

Typical room sizes

A number is easier to trust when you know what it usually comes to. Each room below has its own page with the building-code minimums, the survey data and a drawing of what fits — a bedroom, a kitchen, a living room and a garage — and room sizes sets them side by side.

Room	Worked example	Sizes on its page
Bedroom	12 × 12 ft (144 ft²)	70 ft² – 422 ft²
Kitchen	16 × 12 ft (192 ft²)	195 ft² – 424 ft²
Living room	20 × 14 ft (280 ft²)	70 ft² – 421 ft²
Garage	22 × 20 ft (440 ft²)	194 ft² – 494 ft²

Does square footage include the walls?

It depends on what the number is for.

To buy materials — flooring, tile, carpet, underlay — measure the floor inside the walls. That is what this calculator does, and it is what a supplier needs.
To describe a property, national standards decide what counts: whether a basement, a garage or a room with a low ceiling is included, and whether the measurement is taken inside or outside the walls. Those numbers are not interchangeable with a floor measurement, so do not use one where the other is expected. For a US house measured the way an appraisal does it, see how to measure the square footage of a house.

Open the calculator and put your measurements in — it handles the splitting, the cutouts and the conversions, and it shows the working for every room.