How to Calculate Square Footage of a Wall

A single wall

Wall area = Width × Height

A wall 12 ft wide with an 8 ft ceiling is 12 × 8 = 96 square feet. That is the entire calculation for a plain wall.

Standard ceiling heights in US homes are 8 ft in older construction, 9 ft in most newer builds, and 10 ft or more in higher-end or period properties. Measure rather than assume — the difference between 8 and 9 ft across a room is a full extra gallon of paint.

A whole room

1. Add all wall lengths to get the perimeter. A 14 × 12 ft room is 14 + 12 + 14 + 12 = 52 ft.
2. Multiply by the ceiling height. 52 × 8 = 416 square feet of gross wall area.
3. Subtract the openings. One door at 21 sq ft and two windows at 15 sq ft each is 51 sq ft, leaving 365 square feet.

The wall and paint calculator takes the perimeter, height and opening counts and does all three steps, including the paint quantity.

Standard opening sizes

Use these when you have not measured the actual openings:

Opening	Typical size	Square feet
Interior door	3 × 7 ft	21 sq ft
Exterior door	3 × 6 ft 8 in	20 sq ft
Double door	5 × 7 ft	35 sq ft
Patio / sliding door	6 × 6 ft 8 in	40 sq ft
Small window	2 × 3 ft	6 sq ft
Average window	3 × 5 ft	15 sq ft
Large window	4 × 6 ft	24 sq ft
Picture window	6 × 5 ft	30 sq ft

When subtracting is worth it

Skip it when you have one or two average windows in a room you are painting. The overestimate becomes a useful waste allowance, and paint left over is more useful than paint you ran out of.

Always subtract when:

• The openings are large — a patio door alone is 40 sq ft, a meaningful fraction of a gallon
• There are several openings, so the total is 15% or more of the wall area
• You are calculating drywall, where each sheet costs real money
• You are calculating tile, where the material is expensive per square foot
• You are getting a quote and want to check the contractor’s numbers

The rule of thumb: paint is cheap enough to over-order, tile is not.

Gable walls and sloping ceilings

A gable end — the triangular section at the end of a pitched roof — splits into two shapes:

1. The rectangle from floor to the ceiling line: width × wall height
2. The triangle above it: (width × peak height) ÷ 2

For a 24 ft wide gable with 8 ft walls and a 6 ft rise to the peak: Rectangle = 24 × 8 = 192 sq ft. Triangle = (24 × 6) ÷ 2 = 72 sq ft. Total = 264 square feet.

A room with a sloping ceiling works the same way — measure the vertical wall portion as a rectangle and the sloping portion as a triangle or trapezoid. The triangle calculator handles the angled part.

Paint quantities

One US gallon covers about 350 square feet per coat on a smooth, previously painted surface. Plan on two coats for almost any job.

For 365 sq ft of wall over two coats: 365 × 2 = 730 sq ft of coverage, divided by 350 = 2.09 gallons. Buy 3 gallons, or 2 gallons and a quart.

Coverage drops when:

• Painting bare or newly patched drywall, which drinks the first coat
• The wall is textured — knockdown or orange peel adds surface area
• You are making a dramatic colour change, especially over a dark base
• Using deep reds, bright yellows or oranges, which frequently need three coats

Add the ceiling separately if you are painting it: ceiling area is the room floor area, length × width.

Drywall and tile

Drywall comes in 4 × 8 ft sheets of 32 sq ft each. Divide your net wall area by 32 and round up, then add about 10% for cuts. For 365 sq ft: 365 ÷ 32 = 11.4, plus waste, so order 13 sheets.

Wall tile is measured the same way but always subtract openings, and add 10% waste for straight layouts or 15% for diagonal. Tile is also sized in inches, so see the inches conversion guide — a 12 × 12 in tile is 1 sq ft, an 18 × 18 in tile is 2.25 sq ft.