Paint Calculator

How much paint do I need?

Paint coverage is an area calculation with two adjustments: you subtract the doors and windows you won't paint, and you multiply by the number of coats. This tool does both, then divides by how far a gallon (or liter) actually spreads to tell you how many cans to buy.

The formula

• Wall area = 2 × (length + width) × height. The two-times-perimeter form covers all four walls of a rectangular room in one step.
• Subtract openings — about 20 sq ft per door and 15 sq ft per window (1.9 m² and 1.4 m²). These are typical sizes; large picture windows or double doors count for more.
• Multiply by coats, then divide by coverage. One US gallon covers roughly 350 sq ft per coat; one liter covers about 11 m².

Why two coats is the realistic default

A single coat almost never looks finished — it leaves roller streaks and lets the old color show through, especially over a color change or fresh drywall. Two coats is the standard for a quality result, so the tool defaults to it. Going from a dark wall to a light one may even need a primer plus two coats.

What changes coverage

• Surface texture: smooth drywall is close to the 350 sq ft figure; rough stucco, brick, or popcorn texture can drop it by a third because the paint fills every pit.
• Porosity: bare drywall and patched areas drink the first coat — prime first or expect to use more.
• Application: spraying wastes more than rolling; thick coats cover fewer square feet but aren't actually better.

FAQ

Does this include the ceiling?

No — it estimates walls only. To add the ceiling, calculate it separately as length × width and add that area, since ceiling paint is often a different product.

Should I buy exactly the amount shown?

Round up to whole cans and keep a little extra for touch-ups. Buying all your paint in one batch also guarantees a consistent color — cans mixed at different times can vary slightly.

How many coats should I plan for?

Two coats is the realistic default and what the tool assumes, because a single coat usually leaves roller streaks and lets the old color show through. A big color change, or painting over bare drywall, may need a primer plus two coats.

Why might I need more paint than the estimate?

Coverage drops on rough or porous surfaces — stucco, brick, popcorn texture, and bare drywall can use up to a third more than the 350 sq ft per gallon baseline. Spraying also wastes more than rolling, so add a margin for those conditions.

Does my data stay private?

Yes. The room dimensions and openings you enter are processed entirely in your browser and are never uploaded or stored anywhere.

Is the paint calculator free and mobile-friendly?

Yes. It's free with no sign-up and runs in any modern phone or desktop browser, so you can use it while standing in the room you're about to paint.