Paint calculator
Enter your room size, openings, coats, and paint coverage to estimate wall area, gallons, and quarts for interior painting.
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How to use this paint calculator
Start by measuring the room length and width along the floor, then measure the wall height from the floor to the ceiling. The calculator treats the room as four straight walls, so the total wall area is the room perimeter multiplied by the wall height. If your room has alcoves, angled walls, a vaulted ceiling, or a large opening into another room, split the space into simple sections and add the results together.
Doors and windows are subtracted because those surfaces usually do not need wall paint. This calculator uses 20 square feet for each door and 15 square feet for each window, which is a practical planning shortcut for homeowners. After the openings are removed, the paintable area is divided by the coverage rate you choose: standard paint at 350 square feet per gallon or premium paint at 400 square feet per gallon.
Actual coverage can change based on the wall condition, roller nap, paint color, sheen, and surface texture. New drywall, patched walls, rough plaster, brick, paneling, and dramatic color changes often absorb more paint. The estimate includes 10% waste to help cover brush work, roller loading, small mistakes, and touch-ups after furniture goes back in place. If you are unsure about coverage, choose the standard 350 square foot option or buy one extra quart for small trim-ins and later repairs.
Use one coat when you are refreshing a similar color over a clean, well-painted wall. Choose two coats for most rooms, especially when changing color, covering marks, or painting a high-traffic space. Three coats may be useful for very deep colors, bright accent walls, or when a previous color still shows through after the second coat. Stir paint well, keep a wet edge as you roll, and box multiple cans together if color consistency matters across a large room.
Frequently asked questions
- How much wall area does one gallon of paint cover?
- Most interior paint covers about 350 to 400 square feet per gallon on a smooth, previously painted wall. Rough surfaces, new drywall, dark color changes, and heavy texture can lower that coverage.
- Should I subtract doors and windows before buying paint?
- Yes. Subtracting about 20 square feet per door and 15 square feet per window gives a more realistic paintable wall area. It is still smart to keep a small waste allowance for touch-ups and roller loss.
- Do I need primer before painting a room?
- Primer helps when painting new drywall, covering stains, switching from a dark color to a light color, or painting glossy surfaces. If the wall is clean and the color change is minor, paint-and-primer products may be enough.
- Is one coat of paint ever enough?
- One coat can work for freshening a similar color in good condition. Two coats usually look better and last longer, especially for color changes, patched walls, high-traffic rooms, and darker or brighter colors.
- How much extra paint should I buy?
- This calculator includes 10% waste for normal rolling, brushing, and touch-ups. If your walls are textured, damaged, or very porous, consider buying a little more or choosing the standard coverage setting.