Paint calculator
Paint calculator:
how much do you need?
Calculate wall and ceiling paint in gallons or litres. Add each wall, subtract doors and windows, choose coats and primer, and get paint cans, rollers, brushes and tape with waste included.
Enter measurementsRoom measurements
Based on your result
What you'll need
Quantities update after every calculation. Product brands and prices can be chosen at your preferred store.
- Interior paint
- Wall and ceiling finish
- Calculate above
- Primer
- For bare or patched surfaces
- Calculate above
- Roller covers
- One for each finish stage
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- Angled brushes
- Cutting in and trim
- 2 brushes
- Painter's tape
- Edges, trim and sockets
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The maths
How we work out your paint
Each wall you select is calculated on its own, so its doors and windows come off before coverage and waste are applied. Painting the ceiling or adding a primer coat simply adds another surface with the same maths — nothing is averaged, and quantities are only rounded at the very end.
Coverage assumes smooth, previously painted interior walls. Fresh plaster, bare wood and textured or very dark surfaces drink more paint, which is exactly what the primer coat and the waste margin are there to absorb.
paintable = wall area − doors − windows
ceiling = length × width (if included)
paint = Σ paintable × coats ÷ 350 × 1.05
primer = Σ paintable ÷ 350 × 1.05
- 350 sq ft
- per gallon, per coat
- 21 sq ft
- removed per door
- 15 sq ft
- removed per window
- +5%
- waste & touch-ups
- 2 coats
- typical for a repaint
- ft ↔ m
- converted automatically
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01
Measure every wall
Enter width and height separately for each wall you plan to paint, in feet or metres — the calculator keeps them apart so odd-shaped rooms still add up.
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02
Remove its openings
Doors and windows are deducted from the specific wall where they appear, so you are not buying paint for glass and panels.
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03
Add ceiling and primer
Painting the ceiling adds its floor-sized area. A primer coat is counted as one full pass over every painted surface — worth it on bare or patched walls.
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04
Apply coverage and coats
One gallon covers about 350 square feet per coat. We multiply by your coats, add 5% for waste and round up to a practical quantity to buy.
Good to know
Paint questions, answered
Most repaints need two coats for an even finish. Big colour changes or bare drywall usually need primer plus two top coats.
Prime bare drywall, patched areas, or when moving from a dark colour to a light one. Sound paint in a similar colour usually does not need it.
About 350–400 square feet per coat on smooth walls. Rough or porous surfaces use more, so the calculator uses 350 square feet.
The extra 5% covers roller loading, touch-ups and small measurement differences so the job is not left unfinished.
Ceilings are usually a flat white or a shade lighter than the walls, which keeps the room feeling taller. Tick “Include ceiling” to add its area, and set a separate ceiling colour in the form.
Matt hides wall imperfections and suits low-traffic rooms; eggshell and satin are slightly shinier and wipe clean, which is handy in hallways, kitchens and kids’ rooms. Both cover at roughly the same rate.
On a small room, not by much. On a large room with patio doors or several windows the openings can remove a whole gallon, which is why each wall subtracts its own doors and windows.
Enter just that wall and set “Walls to paint” to 1. For a single feature wall you will normally need well under a gallon, but two coats are still worth it for an even, streak-free finish.
Yes — the calculator gives you the quantity in gallons, so you can price it up against any brand or finish. Premium paints sometimes cover a little more per gallon, but 350 square feet keeps the estimate safe.