Wall area formula for a rectangular room
Add length and width, multiply by two for the perimeter, then multiply by wall height. Deduct openings that will not receive paint or another finish.
Example: 12 × 14 ft bedroom
Perimeter is 2 × (12 + 14) = 52 ft. At 8 ft high, gross wall area is 416 sq ft. Subtract one 21 sq ft door and two 15 sq ft windows to get 365 sq ft of net wall area. The ceiling adds 168 sq ft when included.
Standard opening allowances
| Opening | Default deduction | Measure exactly when |
|---|---|---|
| Interior door | 21 sq ft | Double, oversized, or includes sidelights |
| Window | 15 sq ft | Picture, bay, or unusually sized |
| Sliding glass door | 40–50 sq ft | Always—sizes vary |
| Open passage | Width × height | Always—sizes vary |
How to measure irregular rooms
Measure each wall separately when the room is L-shaped, has bump-outs, or changes height. Multiply each section's width by height and add the results. For triangular gables, use ½ × base × height.
Frequently asked questions
Should every opening be deducted?
Deduct substantial areas that will not be coated. Leaving very small openings in a paint estimate can form part of a modest allowance, but exact deductions are better for costly finishes.
How do I measure wall height?
Measure floor to ceiling in at least two places and record changes rather than assuming every wall is identical.
What about closets and alcoves?
Treat walk-in closets as separate rooms. Measure alcoves and any closet interior included in the project rather than relying on the main-room perimeter.
Turn area into coating quantities
Wall area alone does not include coats, coverage, allowance, or container rounding. Use the paint calculator for wall finish, the ceiling paint calculator when the overhead surface uses another product, and the primer calculator for a required preparation coat.
Field measurements take priority over standard opening allowances.
