How do you calculate wall paint needed?
Multiply room perimeter by wall height, subtract doors and windows, multiply by the number of coats, add waste, then divide by the paint coverage per gallon and round up.
Free painting tool
Free Wall Paint Calculator
A wall paint calculator estimates how many gallons of paint a room needs from wall dimensions, coats, openings, and coverage per gallon. Use this tool to plan wall paint, primer, waste allowance, and material cost before buying supplies.
Room and paint inputs
Paint shopping estimate
3 gallons of wall paint cover 794.2 sq ft after 10% waste.
Paint cost
$126
Primer cost
$56
Paintable area
361 sq ft
Adjusted coverage
794.2 sq ft
Wall area
1 room before openings
448 sq ft
Net paintable wall area
2 doors and 3 windows removed
361 sq ft
Finish coat coverage
2 coats before waste
722 sq ft
Waste allowance
10% extra for touch-ups, roller loss, and wall texture
72.2 sq ft
Planning notes
- Buy 3 gallons of wall paint for about $126 before tax or supplies.
- Plan on 2 gallons of primer for about $56.
- Estimated paint and primer cost is $182 with the prices entered here.
How to use the wall paint calculator
- 1
Enter the room size
Add room length, width, wall height, and the number of matching rooms.
- 2
Set doors, windows, and coats
Subtract common openings and choose how many finish coats you plan to apply.
- 3
Adjust paint coverage
Enter the square feet covered by one gallon of paint and any waste allowance for touch-ups or porous walls.
- 4
Add primer if needed
Turn on primer for bare drywall, patched walls, dramatic color changes, or stain blocking.
- 5
Review gallons and cost
Use the rounded gallon estimate to compare paint quantities, primer needs, and rough material cost before shopping.
Wall paint calculator FAQ
How much area does one gallon of paint cover?
Most interior wall paints cover about 350 to 400 square feet per gallon on smooth walls. Porous, textured, or dark walls may cover less.
Should I subtract doors and windows when estimating paint?
Yes. This calculator subtracts 21 square feet per door and 15 square feet per window as practical planning defaults.
How many coats of paint should I estimate?
Two coats is a good default for most interior repainting projects. Use one coat for close color touch-ups and three coats for major color changes without primer.
Do I need primer before painting walls?
Use primer on bare drywall, repaired patches, stains, glossy surfaces, or when changing from a very dark color to a light color.
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