Paint Calculator
Work out how many gallons of paint you need for a room, including coats, doors and windows.
What you need
3
Gallons of paint — Rounded up. A gallon plus a quart is often cheaper than two gallons.
- Paintable area (openings removed)
- 365.0 sq ft
- Total area to cover across all coats
- 766.5 sq ft
- Exact gallons needed
- 2.2
- Trim and baseboard length
- 52.0 ft
Before rounding — useful for deciding between a gallon and a quart.
Room perimeter, for working out trim paint and masking tape.
Estimates for planning. Confirm quantities with your supplier before ordering — see the assumptions below for the figures used.
How to use the paint calculator
Paint coverage is quoted per gallon on the can, but the number that actually matters is your wall area after you subtract the doors and windows — and then multiply by the number of coats. Enter your room dimensions and this calculator handles all three.
What this calculator assumes
Every estimate depends on figures that vary by supplier and conditions. These are the ones used here, so you can sanity-check the result against your own quote.
- Each door is subtracted at 21 sq ft (3 x 7 ft) and each window at 15 sq ft (3 x 5 ft). Adjust the counts if your openings are much bigger or smaller.
- Default coverage is 350 sq ft per gallon, typical for interior latex on primed drywall. Bare drywall, textured walls and deep colours all cover less.
- Ceiling area is calculated from the room's floor area, and is only available when you enter room length and width.
- Primer is calculated as a single coat over the paintable area.
Frequently asked questions
How much does a gallon of paint cover?
About 350 to 400 square feet on smooth, primed drywall — roughly the walls of a 12 by 12 foot room in one coat. Textured walls, bare drywall and raw wood can drop that to 200 square feet, so check the can and lean toward the lower figure on a first coat.
How many gallons do I need to paint a 12x12 room?
A 12 by 12 foot room with 8 foot ceilings has 384 square feet of wall. Take off one door and two windows and you have about 333 square feet. Two coats is 666 square feet, so two gallons covers it with a little spare.
Do I need two coats of paint?
Almost always. One coat only works when you are repainting the same colour in the same sheen. Two coats gives you even sheen and full hide — most of the streaky, patchy results people blame on cheap paint are really a single coat.
Should I subtract doors and windows?
Yes for anything door-sized or larger. For one or two small windows the difference is within your waste allowance, but a room with four large windows and two doors can be 15% less wall than the raw dimensions suggest.
How much paint do I need for the ceiling?
Ceilings take the room's floor area, so a 12 by 12 room needs 144 square feet of coverage per coat — under half a gallon. Ceiling paint is usually flat white and does hide in one coat if the surface is sound.
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