How to Measure a Room for Paint: Wall Area Minus Windows & Doors

To measure a room for painting: 1) Measure the room perimeter by adding the lengths of all walls together; 2) Multiply perimeter by wall height to get gross wall area; 3) Subtract openings (deduct 21 sq ft per standard door, 15 sq ft per standard window, and 40 sq ft for patio sliders); and 4) Divide net wall square footage by 350 sq ft/gal to determine paint gallons required per coat.

Homeowner using a 25 foot tape measure to record wall length and ceiling height in an empty room

Published 2026-03-01 · Updated 2026-03-01 · 8 min read

Standard Architectural Deduction Values

• Standard Interior Entry Door: 3 ft × 7 ft = 21 sq ft.

• Standard Double-Hung Window: 3 ft × 5 ft = 15 sq ft.

• Large Picture Window: 5 ft × 6 ft = 30 sq ft.

• Closet Bifold Doors: 4 ft × 7 ft = 28 sq ft.

• Patio Sliding Glass Door: 6 ft × 7 ft = 42 sq ft.

Worked Example: Master Bedroom (14×16 ft with 9 ft Ceilings)

Calculating wall paint for a 14×16 room with 2 entry/closet doors and 3 standard windows.

Order 3 gallons of interior eggshell wall paint.

Do professional painters always deduct windows and doors?

For quick estimates, pros sometimes do not deduct standard windows, treating the opening area as a buffer for waste and cutting-in overspray.

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