Wall Square Footage Calculator
Enter your room dimensions and the number of doors and windows to get the net wall area in square feet. The calculator then tells you how many gallons of paint or primer you need and how many 4x8 drywall sheets to buy, with an optional cost estimate. Switch between feet, inches, and metres at any time.
How to measure walls for a room
Measure the length and width of the room floor in feet. Multiply the total perimeter (2 x length + 2 x width) by the ceiling height to get gross wall area. Then subtract the area of each door and window opening. The result is the net wall area, which is the surface you actually need to paint, tile, or panel. For a rectangular room 14 ft x 12 ft with 9 ft ceilings the perimeter is 52 ft and the gross wall area is 52 x 9 = 468 sq ft. Subtract one door (20 sq ft) and two windows (24 sq ft) and you get 424 sq ft of net wall area.
How much paint do I need?
A typical latex flat or eggshell paint covers 350 to 400 sq ft per gallon per coat. This calculator uses 350 sq ft per gallon as a conservative estimate so you do not run short. Most rooms need two coats for full, even coverage, especially when changing colors. Dark colors and vivid reds or yellows may need a third coat. Primer uses a slightly thicker formula and covers around 300 sq ft per gallon. Always round up to a whole gallon when purchasing, and buy from the same production lot to guarantee a color match.
Drywall sheet count and the 10% waste rule
Standard drywall panels are 4 ft x 8 ft (32 sq ft each). Professionals add at least 10% to their sheet count to cover the inevitable waste from cutting around outlets, corners, and openings. The calculator applies this waste factor to the gross wall area (before deducting openings) because the cut-away pieces around doors and windows are still consumed during the hang. For a room with 468 sq ft of gross wall area you need ceil(468 x 1.10 / 32) = 17 sheets. Larger panels (4x10 or 4x12) reduce the joint count and finishing labor but cost more per sheet.
Tips for an accurate wall area measurement
Use a laser distance meter for rooms with furniture since it is faster and more accurate than a tape. Measure wall height at two or three points because floors and ceilings are rarely perfectly level. For rooms with cathedral or vaulted ceilings, measure the wall height at the eaves and calculate the triangle area of the upper section separately. Bay windows and bump-outs add wall area on the inside face, so measure each facet individually and add them together. Closets, alcoves, and knee walls each have their own perimeter and must be calculated separately if you are painting or drywalling them.
Standard deduction sizes for common openings
| Opening type | Typical size | Area (sq ft) | Notes |
|---|---|---|---|
| Standard interior door | 2'8" x 6'8" | 17.8 | Frame opening, no casing |
| Standard exterior door | 3'0" x 6'8" | 20 | Includes screen/storm door clearance |
| Sliding glass door | 6'0" x 6'8" | 40 | Double panel unit |
| Small window | 2'0" x 3'0" | 6 | Bathroom or utility |
| Standard residential window | 3'0" x 4'0" | 12 | Most common single-hung |
| Large picture window | 4'0" x 5'0" | 20 | Living room feature window |
| Garage door (single) | 9'0" x 7'0" | 63 | If wall is an attached garage |
These are the industry-standard areas used to deduct doors and windows from gross wall area. Measure your own openings for the most accurate result.
Frequently asked questions
What is the formula for wall square footage?
Wall square footage = room perimeter x ceiling height, minus the area of any doors and windows. Room perimeter is 2 x (length + width). So a 14 ft x 12 ft room with 9 ft ceilings has a perimeter of 52 ft and a gross wall area of 52 x 9 = 468 sq ft. Subtract 20 sq ft per door and 12 sq ft per window for the net area.
How many gallons of paint do I need for a 12 x 12 room?
A 12 x 12 room with 9 ft ceilings has a perimeter of 48 ft and a gross wall area of 432 sq ft. Subtract one door (20 sq ft) and two windows (24 sq ft) for a net wall area of about 388 sq ft. At 350 sq ft per gallon you need roughly 388 / 350 = 1.1 gallons per coat, so 2 gallons for two coats. Round up and buy 2 gallons.
Should I measure gross or net wall area for drywall?
Use gross wall area (before deducting doors and windows) for drywall because the sheet must be cut to fit around openings, so the cut-off strips are consumed and cannot be reused for another piece. Then add 10% for additional waste from trimming corners and fixing mistakes.
How do I calculate wall square footage for an irregular room?
For L-shaped or other irregular rooms, break the floor plan into rectangles, calculate the perimeter of each rectangle, multiply by ceiling height, and add the results together. Shared interior walls that divide two sections of the room must not be counted twice. For rooms with angled walls, measure the actual wall length along the surface and multiply by ceiling height.
Do I include doors and windows in the square footage for painting?
That depends on whether you are painting them. If you are not painting the door or window trim and surfaces, subtract their area from the gross wall area. This calculator subtracts them by default. If you are painting all surfaces including woodwork, use the gross wall area instead.
How much extra paint should I buy for touch-ups?
Buy at least 10-15% more than your calculated amount. Touch-ups rarely match perfectly unless you use paint from the same production lot (same batch number). Store the leftover paint in an airtight container away from freezing temperatures, labeled with the room name and paint color code.