Paint Calculator – How Much Paint You Need and Which Cans to Buy
Most paint calculators stop at a number like "2.19 gallons". You are still left standing in the aisle choosing between three gallons, two gallons and a quart, or a 5-gallon bucket. This paint calculator goes one step further. It works out the paint for each product you are using (wall, ceiling and trim paint, plus primer when you need it). Then it picks the mix of cans that leaves the least paint over, and, if you enter prices, the cheapest mix.
Measuring a Room for Paint
For a whole room, measure the length and width along the floor from wall to wall, and the wall height from floor to ceiling. The wall area is the perimeter times the height: 2 × (L + W) × H. A 12 × 12 ft room with 8 ft walls has 2 × 24 × 8 = 384 ft² of wall. Add single walls, the triangular gable ends of a pitched roof, ceilings and trim as separate surfaces. Use the quantity field for identical rooms. For curved walls, sloped attic ceilings or stairwells, measure the area yourself and enter it as a known area.
Subtracting Doors and Windows
Openings are not painted, so they come off the total. A typical interior door is about 36 × 80 in (20 ft²) and a window about 36 × 60 in (15 ft²). Sizes vary a lot between countries and houses, so measure your own. The live diagram draws each opening as a cut-out, which shows you exactly what is being counted.
What "Coverage per Coat" Means
The coverage printed on a can is the area one litre or one gallon covers in a single coat on a smooth, sealed surface. US interior paints commonly list 350–400 ft²/gal. European and Australian cans often say "up to" 10–16 m²/L. The calculator starts at a cautious 350 ft²/gal (8.59 m²/L) in both systems. Enter the figure from your own can for a closer estimate. The paint needed is area × coats ÷ coverage, plus a small allowance for what stays in the tray and roller.
How Many Coats?
Plan on two coats for a new colour. One coat can be enough when you refresh the same colour over paint that is still in good condition. Going from dark to light, or using strong colours such as reds and yellows, often takes three coats. A tinted primer can save you that third coat.
When to Prime
Prime new drywall, bare plaster and bare wood, patched areas, water or smoke stains, and glossy surfaces. These either soak up paint unevenly or stop it from sticking. Turn on Prime first for any product and its surfaces are added to a separate primer estimate, with its own coverage and cans.
Textured and Porous Surfaces
Orange-peel and knockdown walls have more real surface than flat drywall. Popcorn ceilings, stucco, brick and block also soak paint in. The calculator adds 10% for light texture and 30% for heavy or porous surfaces. These are rules of thumb, so check the can for a masonry coverage figure.
Choosing Can Sizes
Buying the largest can first and filling in with smaller ones sounds sensible but often gets it wrong. For 7.58 L with 1, 2.5, 5 and 10 L cans, that approach buys 5 + 2.5 + 1 = 8.5 L, while 5 + 1 + 1 + 1 = 8.0 L is enough. The tool tries every combination of the sizes your store stocks. You can switch sizes off, or enter your own for brands that sell 3.7 L "gallons" or 0.75 L tins. With prices entered, it also shows when one 5-gallon pail costs less than four gallons and two quarts, even though you end up with more paint left over.