Project Parameters
Length of the longest room wall.
Width of the room.
Standard ceilings are 8 or 9 ft; high ceilings are 10–12 ft.
Standard door opening is ~21 sq ft.
Average window opening is ~15 sq ft.
Instant Estimate
Ceiling Paint Needed
Flat white ceiling paint minimizes surface imperfections.
Net Wall Surface Area
Deductions for Openings
Coverage Per Gallon
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Guide
How to calculate paint for an interior room
One gallon of quality interior paint covers approximately 350 to 400 square feet with one coat. To find your requirement, calculate room wall perimeter multiplied by height, subtract doors and windows, and multiply by two coats.
Measure Wall Perimeter
Add all four wall lengths together: 2 × (Length + Width).
Multiply by Ceiling Height
Multiply perimeter by ceiling height to obtain gross wall surface area.
Deduct Openings
Subtract 21 sq ft for each interior door, and 15 sq ft for each standard window.
Double for 2 Coats & Divide by 350
Multiply net square feet by 2 coats and divide by 350 sq ft per gallon. Always round up to the next full gallon.
Mathematics
Formulas used
Net Wall Square Footage
(2 × (Length + Width) × Ceiling Height) − Doors & WindowsActual paintable wall area inside the room.
Gallons to Buy
Ceil((Net Wall Area × Number of Coats) / 350)Standard interior coverage rate per gallon.
Answers & FAQ
Frequently asked questions
How many gallons do I need for a 12x12 room?▼
A 12x12 room with 8 ft ceilings has roughly 384 sq ft of gross wall area (around 320 sq ft net). For two coats, you will need 2 gallons of wall paint.
Is 1 gallon of paint enough for a bedroom?▼
For a very small bedroom (10x10) receiving a single refresh coat, 1 gallon may barely suffice. However, 90% of bedrooms require 2 gallons for standard two-coat coverage.
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