Standing in the paint aisle trying to guess how many cans to buy? This paint calculator works out exactly how much paint you need from your wall dimensions, coat count, and surface condition, so you don’t end up short mid-job or stuck with three unopened cans in the garage.

Pick a surface type to load sensible starting dimensions, set your doors and windows, and the numbers update as you type.

Why people use this tool

• No more guessing at the store: get an exact number of cans, not just a rough liter or gallon figure, based on the can size you actually buy.
• Accounts for doors and windows: openings get subtracted automatically so you’re not buying paint for space that won’t get painted.
• Adjusts for surface condition: fresh drywall drinks up more paint than a smooth repaint, and this calculator factors that in instead of assuming every wall behaves the same.
• Works for any room or surface: interior wall, exterior wall, ceiling, full room, or a custom area if you already know the square footage.
• Optional cost breakdown: add paint, primer, and labor prices to see a full estimate before you commit to a quote.

How much paint you actually need

Paint volume comes down to area, coats, and coverage rate. This calculator starts with your wall area (or room perimeter times height, for a full room), subtracts any door and window area, then multiplies by your number of coats since a second or third coat needs just as much paint as the first.

That total gets divided by your coverage rate, the area one liter or one gallon actually covers, to get the raw amount of paint needed. A wastage allowance on top, typically 5-10%, covers roller loading, cutting in around trim, and the paint that never quite makes it out of the tray.

For example, a 4m x 2.4m interior wall with one door and one window works out to roughly 8.2m² of paintable area. At two coats and a coverage rate of 10m² per liter, that’s about 1.9L of paint before wastage. In imperial, a 13ft x 8ft wall with the same openings covers about 88ft², which at 350ft² per gallon and two coats needs roughly 0.55gal.

Doors, windows, and surface condition

Every door subtracts about 1.89m² (roughly 20ft²) and every window about 1.5m² (roughly 16ft²) from your wall area, standard sizes that cover most residential doors and windows without needing you to measure each opening individually. Set the count to zero if a wall genuinely has no openings, or if you’re painting a ceiling or a custom area where deductions don’t apply.

Surface condition matters more than most people expect. A smooth wall that’s been painted before soaks up paint at roughly the coverage rate printed on the can. Fresh drywall or plaster, on the other hand, is porous and can absorb 15% more paint on the first coat alone, while a heavily textured or rough surface, fresh or repainted, needs even more to get full coverage. This calculator applies a coverage adjustment for each condition so the numbers reflect what actually happens on the wall, not just the can label.

Coverage rate and cans

The default coverage rate, 10m² per liter or 350ft² per gallon, is a reasonable average, but it varies by paint brand, sheen, and color, especially with darker colors that often need an extra coat regardless of surface condition. If your paint can lists a different figure, enter it directly and the whole calculation adjusts.

Once the total volume is known, set your can size, commonly 4L or 1 gallon, and the calculator divides your total by that figure and rounds up, since a half-used can still means buying a full one. Switching between a 1L sample size and a 10L bulk can lets you compare which is more economical for your project without doing the math by hand.

Estimating cost

Toggle Estimate Cost and enter what you pay per liter or gallon of paint, plus a labor price per square meter or square foot if you’re pricing out a job rather than DIY-ing it. Turn on Include Primer Coat if the surface needs one, fresh drywall almost always does, and the calculator adds a separate primer line using the same wall area at a single coat.

The breakdown shows paint, primer (if included), and labor as separate line items next to the total, so you can see exactly where the budget goes before buying materials or accepting a quote.

For an oddly shaped wall or room, the Area Calculator can help work out the square footage first. To convert a supplier’s quote between liters and gallons, try the Volume Converter, and the Percentage Calculator is handy if you want to work out a custom wastage margin by hand.

These figures are estimates for planning purposes. Actual coverage varies by paint brand, application method, and wall texture, so buy slightly more than the calculator suggests if you’re painting a dark color over a light one or vice versa.