Paint Coverage Calculator
Calculate exactly how many gallons of paint you need to buy for your walls based on room size, coats, and window/door openings.
Results are estimates. Check physical dimensions before purchasing.
Subtracts 21 sq ft per door
Subtracts 15 sq ft per window
Standard interior flat paint averages 350 sq ft per gallon.
How to calculate paint quantities
Calculating paint needs for a room involves finding the perimeter of the room, multiplying it by the wall height to find total wall square footage, subtracting any areas that won't be painted (such as doors and windows), and then accounting for the number of coats you plan to apply.
The Paint Calculation Formula
Wall Area = 2 × (Length + Width) × Height Net Paintable Area = Wall Area - (Doors × 21) - (Windows × 15) Total Area = Net Paintable Area × Coats Gallons Needed = Ceiling(Total Area / Coverage Per Gallon)
Why you should always round up
Paint is sold by the whole gallon or quart. If your calculations show you need 1.1 gallons of paint, you must buy 2 gallons. Buying exactly 1 gallon will leave you short, and attempting to stretch the paint by applying it too thin results in uneven coverage, streaks, and a finish that lacks durability. Keep any leftover paint in a sealed container for future touch-ups.
Gotchas & Special Circumstances
- Textured Walls: Textured surfaces (like stucco or popcorn texture) have up to 20-30% more surface area than flat drywall. Increase your paint budget accordingly.
- Drywall Primer: Bare drywall or fresh plaster absorbs paint very quickly. Apply a separate drywall primer first to prevent having to apply 3 or 4 expensive coats of color paint.
- Dark Colors: Transitioning from a very dark color to a light color (or vice versa) often requires a coat of high-hiding primer plus two coats of premium paint.
Frequently Asked Questions
For a standard 12x12 room with 8-foot high walls, you have about 384 sq ft of wall area. Subtracting standard space for one door and window leaves roughly 350 sq ft of paintable surface. A single gallon of paint covers 350 sq ft, so you will need 1 gallon for one coat, or 2 gallons for two coats.
A standard gallon of wall paint covers approximately 350 to 400 square feet. Textured walls, porous drywall, or dramatic color transitions might reduce coverage to 250-300 square feet per gallon.
Yes, standard professional practice is to apply two coats of paint for durability, color depth, and consistency. When changing colors dramatically, or painting raw drywall, you might need a separate primer coat plus two color coats.
A standard door is about 21 square feet, and a standard window is about 15 square feet. Our calculator allows subtracting these areas from the total room perimeter automatically so you don't overbuy.