Find the paintable wall area of a room. Enter the room size, ceiling height, and how many doors and windows to subtract their openings.
Usage Tip
For wallpaper, keep the gross area and let the pattern repeat cover the openings as waste. The deductions here assume you skip painting behind the doors and windows.
Openings = doors × 21 + windows × 15 sq ft
Net wall = Gross − openings
Standard deductions assume a 3 × 7 ft door (21 sq ft) and a 3 × 5 ft window (15 sq ft); adjust the counts if yours are larger or smaller.
For paint, take the net wall area to the paint-coverage calculator for gallons — one gallon covers roughly 350 to 400 sq ft per coat.
Closets, soffits, and half-walls change the number; measure odd walls separately and add them.
How to Calculate Wall Area
For one wall, multiply its width by its height. For a whole room, add the lengths of all walls (the perimeter) and multiply by the ceiling height. Then subtract the doors and windows to get the net area you will actually paint or cover.
Example: a 12 by 14 ft room has a 52 ft perimeter; at 8 ft ceilings that is 416 sq ft gross. Subtract one door (21) and two windows (30) and the net is about 365 sq ft.
Door & Window Deduction Guide
Subtract the openings you will not cover. Typical sizes:
| Opening | Typical area |
|---|---|
| Standard door | 21 sq ft (3 x 7 ft) |
| Small window | 12 sq ft |
| Large window | 24 sq ft |
| Sliding glass door | 40 sq ft |
| Double / French doors | 40 sq ft |
For painting, many pros skip small deductions and just paint over the math as built-in waste; for wallpaper and drywall, always deduct, because the openings genuinely remove material.
Paint Estimation Guide
One gallon covers roughly 350 sq ft per coat. Take the net wall area, multiply by the number of coats, and divide by 350. Example: 365 sq ft at two coats is 730 sq ft of coverage, about 3 gallons (one gallon plus a quart is rarely worth the trip – round up).
| Net wall area | 1 coat | 2 coats |
|---|---|---|
| 200 sq ft | 1 gal | 2 gal |
| 400 sq ft | 2 gal | 3 gal |
| 800 sq ft | 3 gal | 5 gal |
Drywall Estimation Guide
Drywall comes in 4 ft wide sheets of varying length. Divide the area by the sheet coverage and round up:
| Sheet size | Coverage |
|---|---|
| 4 x 8 ft | 32 sq ft |
| 4 x 9 ft | 36 sq ft |
| 4 x 10 ft | 40 sq ft |
| 4 x 12 ft | 48 sq ft |
Example: 416 sq ft of wall divided by 32 is 13 sheets; add 10% for cuts and waste, so order about 15. Remember mud, tape, and screws separately.
Wallpaper Estimation Guide
Wallpaper rolls list a total area but only cover part of it after pattern matching and trimming. A single roll usually yields about 25 sq ft of usable coverage; a double roll about 56 sq ft. Divide net area by usable coverage and round up, adding extra for large pattern repeats.
| Roll type | Usable coverage |
|---|---|
| Single roll | about 25 sq ft |
| Double roll | about 56 sq ft |
| European roll | about 28 sq ft |
Accent Wall Planning
For a single accent wall, measure just that wall (width times height) and subtract any opening on it. Buy the accent color or paper for that area only, but keep enough of the main color to cut in around it. The Single Wall mode is built for exactly this.
Wall Size Examples
| Wall size | Area |
|---|---|
| 8 x 10 | 80 sq ft |
| 8 x 12 | 96 sq ft |
| 9 x 15 | 135 sq ft |
| 10 x 20 | 200 sq ft |
Waste Factor Recommendations
- Paint: round up to the next gallon; keep the leftover for touch-ups.
- Drywall: add about 10% for cuts and breakage.
- Wallpaper: add 10-15%, more for large pattern repeats.
- Textured or heavily patched walls absorb more paint – add a little.
Common Measuring Mistakes
- Using floor area instead of wall area for paint.
- Forgetting to multiply the perimeter by the ceiling height.
- Over-deducting tiny openings on a paint job, then coming up short.
- Ignoring pattern repeat on wallpaper.
- Measuring one wall and assuming the others match.
Frequently Asked Questions
How do I calculate wall area?
Multiply each wall width by its height, or take the room perimeter times the ceiling height, then subtract doors and windows.
Should I subtract doors and windows?
For wallpaper and drywall, yes – they remove material. For paint, small deductions are often left in as built-in waste; large openings should still be subtracted.
How much paint do I need for a room?
Net wall area times coats, divided by about 350 sq ft per gallon, rounded up. The Paint mode does this automatically.
How many drywall sheets do I need?
Divide wall area by the sheet coverage (32 sq ft for a 4×8) and add about 10% for waste.
How many rolls of wallpaper do I need?
Divide net area by the usable coverage per roll (about 25 sq ft single, 56 double) and round up, adding extra for pattern repeats.
Do textured walls need more paint?
Yes – texture, knockdown, and fresh patches increase surface area and absorption, so add a little extra.
Related Area Calculators
Note: deductions and material estimates are approximations. Opening sizes vary, irregular openings and textured walls change coverage, and paint, drywall, and wallpaper coverage differ by product – always confirm against the product label and add a waste allowance. General DIY guidance, not a professional estimate.