Understanding your result
The headline is the paintable wall area: all the walls you entered minus the openings. The sub-line gives the gross area and the openings, or the ceiling and the total if you ticked the ceiling. The three figures show the gross walls, the openings and either the walls plus ceiling or a quick paint figure at 350 sq ft per gallon and two coats.
The table goes wall by wall, so you can spot a mistyped dimension, then gives the gross area, the openings with what was counted, the paintable area, the ceiling if included, the perimeter of the walls entered, the area in the other unit system and the quick paint figure. The perimeter is worth a glance: it is the total width of the walls and should match the room’s outline.
This is the measurement step. The area goes straight into the wall paint calculator’s measured-area mode, the paint coverage calculator and the paint cost calculator. For tall walls with a sloping top, the stairwell calculator handles the average height, and for wallpaper the roll calculator needs the perimeter and height rather than the area.
How we calculate this
Each wall is a rectangle. Walls with a sloping top, under stairs or a cathedral ceiling, are a rectangle plus a triangle; enter the average height, which gives the exact area for a straight slope. The standard openings are the ones every calculator on the site uses, listed on the standard sizes page. The quick paint figure uses the site’s default coverage and allowance and is there only as a sense of scale; the wall paint calculator does it properly with your can’s figure.
The measuring guide walks through alcoves, bay windows, sloped ceilings and closets, and the master guide shows a whole room measured by hand and carried through to cans.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Door | 20 sq ft | A standard 36 × 80 in interior door |
| Window | 15 sq ft | A typical 3 × 5 ft double-hung window |
| Default room | 12 × 14 ft, 8 ft walls | A medium room, as in the whole-house estimator |
| Quick paint figure | 350 sq ft per gallon, 2 coats, 5% | The site’s wall paint defaults |
| Unit conversion | 1 m² = 10.7639 sq ft | Exact, from 0.3048 m per ft |
Assumptions last reviewed October 8, 2026.
The calculator does not model walls that are not rectangles beyond the average-height approach, curved walls (measure along the curve) or the edges of window reveals, which are trim. It measures; the products are in the other calculators.
Two worked examples
A 12 × 14 room, four walls
8 ft walls, one door and two windows, no ceiling.
- Walls: 12 × 8 + 14 × 8 + 12 × 8 + 14 × 8 = 96 + 112 + 96 + 112 = 416 sq ft
- Openings: 1 × 20 + 2 × 15 = 50 sq ft
- Paintable: 416 − 50 = 366 sq ft, which is 34 m²
- Quick paint: 366 ÷ 350 × 2 × 1.05 = 2.2 gallons
The perimeter of 52 ft matches the outline of a 12 × 14 room, which confirms the four widths.
An L-shaped living room and a two-wall alcove
Six walls of 16, 4, 8, 8, 8 and 12 ft, all 9 ft high, two doors, three windows.
- Walls: 144 + 36 + 72 + 72 + 72 + 108 = 504 sq ft
- Openings: 2 × 20 + 3 × 15 = 85 sq ft
- Paintable: 419 sq ft; quick paint 2.51 gallons
Separately, an alcove of two walls, 10 and 15 ft by 9 ft, with one window, a 25 sq ft fireplace surround and a 10 × 15 ft ceiling: 225 sq ft gross, 40 of openings, 185 sq ft of wall and 150 of ceiling, 335 sq ft in all.
Where to find your inputs
Widths. Along the floor at the baseboard, corner to corner. Round to the nearest inch or centimetre; rounding to the foot on six walls adds up.
Heights. Floor to ceiling, measured in a corner. For a sloping wall, the average of the low and high ends.
Doors and windows. Every one in the walls you measured, including closet doors.
Other openings. Width times height of the fireplace surround, built-ins or tiled areas.
Ceiling. Length times width of the floor beneath it.
Common mistakes
- Measuring one wall and multiplying by four. Rooms are rarely square.
- Using the floor area. A 12 × 14 room has 168 sq ft of floor and 416 sq ft of wall.
- Forgetting the closet. If you paint inside it, its walls are walls.
- Subtracting tiny openings. Switches, vents and small windows are not worth the arithmetic.
- Counting the same corner twice. Measure each wall face once, from corner to corner.
- Mixing units. Switch the toggle before typing metres.
Questions people ask
- How do I calculate the square footage of a wall?
- Multiply its width by its height. A 14 ft wall that is 8 ft high is 112 sq ft. For a room, add the walls and subtract the openings; a 12 × 14 room with 8 ft walls has 416 sq ft of wall, and with a door and two windows taken out 366 sq ft to paint.
- What sizes are doors and windows?
- The calculator subtracts 20 sq ft for a door (a standard 36 × 80 in slab) and 15 sq ft for a window (about 3 × 5 ft). Large or unusual openings go in the other openings field as their own area, measured width times height.
- How do I measure an L-shaped room?
- Walk round it and measure every wall face, which for an L shape is six walls. Enter six walls. The perimeter the calculator reports is the sum of the widths, which you can check against the room's outline.
- Should I include the ceiling?
- Only if it takes the same paint. Ceiling paint is usually a different product, so most people measure the ceiling separately. Tick the ceiling box when you want the total area for a primer, a sealer or a quote.
- What should I count as other openings?
- Anything on the wall you will not paint and that is big enough to matter, such as a fireplace surround, a built-in bookcase, a tiled splashback, a large mirror or an archway. Below about 10 sq ft it is not worth subtracting.
- Does the result convert to square metres?
- Yes. The table shows the paintable area in the other unit system, and switching the toggle to metric takes measurements in metres and reports square metres. One square metre is 10.764 sq ft.