A wall square footage calculator finds the total paintable area of a room's walls, subtracts the doors and windows, and converts the result into the gallons of paint you need. Instead of guessing or over-buying, you get an accurate paintable square footage in seconds. This free tool from Anchor AI Tools works in feet or meters and estimates paint for one or two coats.
Below the calculator you will find the wall area formula, a worked example for a real bedroom, standard door and window sizes, a paint coverage reference, and clear answers to the questions painters and homeowners ask most.
Quick Answer: To calculate wall square footage, add up the length of every wall, multiply by the ceiling height to get total wall area, then subtract the area of each door and window. The formula is Paintable Wall Area = (Perimeter × Height) − Openings. For a 12 ft × 14 ft room with 8 ft ceilings, one door, and two windows: (52 × 8) − 45 = 371 sq ft of paintable wall. At about 350 sq ft of coverage per gallon, that is roughly 1 gallon for one coat.
Enter the room's perimeter (or its length and width) and the ceiling height, then subtract the doors and windows. The calculator returns total wall area, paintable wall square footage, and an estimate of how many gallons of paint you need.
Measure paintable wall area and the paint needed for one or two coats.
Room dimensions
Rectangular room: length + width are doubled to get the perimeter automatically.
Openings to subtract
Uses standard sizes: 21 sq ft per door, 12 sq ft per window (≈1.95 / 1.11 sq m).
Need the floor area instead of the walls? Use the room size calculator for a single room, the floor plan square footage calculator to total a whole layout, or the main square footage calculator for any area type.
Wall square footage is the surface area of a room's vertical walls, measured for painting, wallpapering, or paneling. Unlike floor square footage, which measures the ground a room covers, wall square footage measures the area you actually coat with paint — so it depends on the ceiling height and on how many doors and windows interrupt the walls.
The figure that matters for a paint job is the paintable wall area: total wall surface minus the openings. Painting over doors and windows wastes both paint and money, which is why every accurate estimate subtracts them.
This calculation is the foundation of paint estimating. As paint manufacturers such as Sherwin-Williams note in their coverage guidance, one gallon of interior paint typically covers about 350 to 400 square feet per coat on smooth, primed surfaces — so knowing your paintable square footage directly tells you how many gallons to buy.
Paintable Wall Area = (Perimeter × Ceiling Height) − Openings
Perimeter of a rectangular room = 2 × (Length + Width)The calculation runs in three short steps:
Paintable wall area is the total wall surface minus every door and window opening.
🛏️ Scenario
A 14 ft × 12 ft bedroom with 8 ft ceilings, one door, and two windows, painted with two coats.
Perimeter: 2 × (14 + 12) = 52 ft
Total wall area: 52 × 8 = 416 sq ft
Openings: (1 door × 21) + (2 windows × 12) = 21 + 24 = 45 sq ft
Paintable area: 416 − 45 = 371 sq ft
Paint is sold in whole gallons and quarts, so round up. For this room, buying three gallons covers two coats with a little to spare for touch-ups.
If you do not want to measure every opening, these U.S. standard sizes give a reliable estimate. The calculator above uses the bolded standard areas by default.
| Opening | Typical Size | Area to Subtract |
|---|---|---|
| Interior door | 3 × 7 ft | 21 sq ft |
| Standard window | 3 × 4 ft | 12 sq ft |
| Large picture window | 6 × 4 ft | 24 sq ft |
| Sliding patio door | 6 × 7 ft | 42 sq ft |
| Small/bath window | 2 × 3 ft | 6 sq ft |
Once you know the paintable wall square footage, paint quantity is simple division. One gallon of interior wall paint covers roughly 350 square feet per coat on a smooth, primed surface. Porous, textured, or unprimed walls absorb more, so coverage drops toward 250 to 300 sq ft.
Gallons = (Paintable Area ÷ 350) × Number of Coats
Round up to the next whole gallon — paint is sold whole.| Paintable Area | 1 Coat | 2 Coats |
|---|---|---|
| 200 sq ft | 1 gal | 2 gal |
| 350 sq ft | 1 gal | 2 gal |
| 500 sq ft | 2 gal | 3 gal |
| 700 sq ft | 2 gal | 4 gal |
| 1,050 sq ft | 3 gal | 6 gal |
Always buy a little extra for touch-ups and a second coat on darker colors, which usually need two coats for full coverage.
Forgetting to subtract openings. Skipping doors and windows overstates the area and leads to buying too much paint. Always remove them for a paint estimate.
Using floor area instead of wall area. Wall square footage depends on ceiling height, not the floor footprint. A small room with high ceilings has more wall to paint than its floor size suggests.
Ignoring the ceiling and trim. If you are painting the ceiling too, calculate it separately as length × width. Trim and doors often use a different paint and finish.
Assuming one coat. Most repaints and all color changes need two coats. Estimate for the coats you will actually apply.
Not rounding up. Running short mid-job means a second store trip and a possible color-batch mismatch. Round gallons up and keep the receipt.
This tool is built for homeowners planning a repaint, DIY decorators buying the right amount of paint, professional painters quoting jobs, landlords refreshing rental units, and interior designers budgeting a project. Anyone estimating paint, wallpaper, or wall panels needs paintable wall square footage first.
Pairing it with other estimates? The percentage calculator adds tax or a waste margin to your paint order, the price per square foot calculator works out labor cost per square foot, and the time to decimal calculator converts crew hours for an accurate quote.
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