How to Calculate the Square Footage of a Wall

The square footage of a wall is its height times its width — the same multiplication as a floor, stood on its side. Where wall estimates go wrong is the subtracting: doors and windows are holes you don’t paint or paper, and whether you take them out depends on what you’re buying.

This guide covers measuring a wall, deducting openings the right way, and turning the result into paint, wallpaper or drywall. If you’re pricing paint specifically, the paint calculator does the coverage math once you have the area.

Height times width, one wall at a time

Measure each wall’s height from floor to ceiling and its width corner to corner, multiply the two, and add the walls together. A room 12 feet by 14 feet with 8-foot ceilings has two walls of 12 × 8 (96 sq ft each) and two of 14 × 8 (112 sq ft each) — 416 square feet of wall before any openings come out.

If the floor or ceiling isn’t level, measure the height in a couple of spots and use the taller reading for paint. You’d rather finish with a little left over than run short on the last wall.

Taking out doors and windows

How much you subtract depends on the material, but the standard sizes are worth committing to memory:

Opening	Typical size	Area to deduct
Interior door	3 ft × 6 ft 8 in	~21 sq ft
Exterior door	3 ft × 6 ft 8 in	~21 sq ft
Standard window	3 ft × 4 ft	~12–15 sq ft
Patio / sliding door	6 ft × 6 ft 8 in	~40 sq ft

For paint, many decorators don’t bother deducting a single door or window — the offcuts and the second coat absorb it, and it leaves a useful margin. Once a wall has a large opening, such as a patio door or a picture window, or several windows, take them out or you’ll badly over-buy. For wallpaper and tile, deduct the big openings but keep part of that area as waste, because you hang or lay full pieces and trim them to fit.

From wall area to paint

Paint coverage runs about 350 to 400 square feet per gallon per coat on a smooth, previously painted wall. It’s less on bare drywall, rough plaster, or a strong color change, all of which drink the first coat. Plan on two coats for anything more than a touch-up.

So 416 square feet of wall over two coats is roughly 832 square feet of coverage — a little over two gallons at 400 sq ft per gallon. The paint calculator works this out and rounds to whole cans, and the drywall calculator does the same for board if you’re building the wall rather than finishing it.

Walls that aren’t rectangles

Not every wall is a clean rectangle. Split the awkward ones into a rectangle plus a triangle:

• Gable ends — the triangular wall under a pitched roof is a rectangle up to the eaves plus a triangle on top. The triangle is ½ × base × height.
• Stairwell walls — usually a rectangle plus a right triangle that follows the stair line, or a trapezoid if the two ends are different heights.
• Sloped or vaulted ceilings — the top of the wall becomes a triangle or trapezoid; measure the tall side and the short side and average them for a quick estimate.

Work out the rectangle and the triangle separately with the triangle calculator, then add the two.

The mistakes that throw wall estimates off

1. Measuring one wall and multiplying by four. Rooms are rarely square, so one long wall skews the whole figure.
2. Deducting every small opening for paint, then coming up a half-can short because the margin is gone.
3. Forgetting the ceiling when you’re painting that too — it’s its own length × width on top of the walls.
4. Buying paint for one coat. Bare board and repainted-dark walls almost always need two.

Frequently asked questions

How do you calculate the square footage of a wall?

Multiply the wall’s height by its width, both in feet. An 8-foot-tall wall that’s 14 feet wide is 112 square feet. For a whole room, do each wall and add them up, then decide whether to subtract the doors and windows based on what you’re buying.

Do you subtract doors and windows when measuring a wall for paint?

For a single door or window, most painters leave it in — the extra covers spills, second coats and a bit of spare. For big openings like patio doors or picture windows, or a wall with several windows, take them out, or you’ll buy far more paint than the job needs.

How many square feet does a gallon of paint cover?

Roughly 350 to 400 square feet per coat on a smooth, already-painted surface. Bare drywall, rough textures and dramatic color changes cover less because they soak up more. Since most jobs need two coats, a gallon realistically finishes about 175 to 200 square feet of wall.

How do I measure a wall with a sloped or vaulted ceiling?

Split it into a rectangle and a triangle. Measure up to the lowest ceiling point for the rectangle, then treat the sloped part on top as a triangle — half its base times its height. Add the two. For a rough number, you can instead average the tall and short heights and multiply by the width.

How do I find the total wall area of a room?

Measure each wall’s height and width, multiply each pair, and add all four (or however many) walls together. A rectangular room only needs two measurements repeated — two walls at one width, two at the other — but check the ceiling height in more than one place if the house isn’t new.

The short version

Wall square footage is the easy part; the judgment is in what you subtract and how many coats you’re covering.

• Multiply height by width for each wall, then add the walls together.
• Deduct big openings; leave small ones in for paint as a built-in margin.
• Figure paint at 350–400 sq ft per gallon per coat, and plan for two coats.
• Break gables and sloped-ceiling walls into a rectangle plus a triangle.

Once you have the wall area, the paint calculator turns it into cans and coats, and the main square footage calculator handles the measuring math if the walls aren’t simple rectangles.