Painting & Wall Calculators
Paint, primer, stain, wallpaper and drywall.
One US gallon covers 350 to 400 ft² per coat, so a 12 by 12 ft room with 8 ft walls takes about 2 gallons for two coats after deducting a door and a window. These six calculators cover paint, primer, stain, wallpaper, drywall and insulation.
All 6 painting & wall calculators
Walls are measured by perimeter, not by floor
The mistake that starts most wrong paint orders is using the floor area. A room's wall area is perimeter × height: a 12 by 12 ft room has a 48 ft perimeter, so at 8 ft it has 384 ft² of wall — well over twice its 144 ft² floor. Ceilings use the floor area; walls never do.
From that, deduct openings. A standard door is about 21 ft² and an average window about 15 ft². On a small room with two windows and a door that is over 13% of the wall area, which is the difference between two gallons and three.
What a gallon actually covers
Coverage is printed on the can, and it varies more between products than most people expect. These are the figures to plan with when you do not have the can in front of you.
| Product | Unit | Covers per coat |
|---|---|---|
| Wall paint | 1 US gallon | 350 to 400 ft² |
| Wall paint | 1 litre | ~10 m² |
| Primer, bare surface | 1 US gallon | 200 to 300 ft² |
| Deck stain, smooth wood | 1 US gallon | 250 to 350 ft² |
| Deck stain, rough or old wood | 1 US gallon | 150 to 250 ft² |
Two things pull real coverage toward the bottom of every range: surface porosity and application method. Bare drywall, new plaster and weathered timber drink far more than a previously painted wall, and spraying puts noticeably more material on the surface, and in the air, than a roller does.
Two coats is the assumption, not the upgrade
One coat is only realistic when you are repainting a similar colour on a sound, sealed surface. Any colour change, any bare substrate and any strong colour needs two, and deep reds and yellows routinely need three because the pigments have poor hiding power. Plan on two and treat one as the lucky outcome.
Primer is not an extra coat of paint; it is a different product doing a different job. It seals porous surfaces so the finish coat sits on top instead of soaking in, and it blocks stains. Priming bare drywall genuinely saves paint, because unprimed board can absorb a gallon over barely 200 ft².
Wallpaper, drywall and insulation count differently
These three are not liquids and do not follow coverage-per-gallon logic. Wallpaper comes in rolls of a fixed length, and usable yield depends on the pattern repeat: a large repeat wastes a slice off every drop, so two rolls of identical size can hang very different amounts of wall. Drywall is a count of fixed sheets, normally 4 × 8 ft, and the calculation is about how they lay out rather than about area alone. Insulation is sold by the batt or by the bag to a target R-value, which depends on where you live rather than on the size of the room.
Painting & Wall Calculators: common questions
How much paint do I need for a 12 by 12 room?
About 2 gallons for two coats. The walls are 384 ft² at 8 ft high, less roughly 36 ft² for a door and a window, so around 348 ft² per coat and 696 ft² in total, against 350 to 400 ft² per gallon.
How many square feet does a gallon of paint cover?
350 to 400 ft² per coat on a prepared, previously painted surface. Bare drywall, new plaster and weathered wood absorb considerably more, and primer on a bare surface covers only 200 to 300 ft² per gallon.
Do I measure the floor or the walls for paint?
The walls, which is perimeter × height. A 12 by 12 ft room has 384 ft² of wall at 8 ft high against only 144 ft² of floor, so using the floor area under-orders by more than half. Use the floor area only for ceilings.
Should I subtract doors and windows from the wall area?
Yes, for anything but a very large room. A door is about 21 ft² and a window about 15 ft². In a small room those openings are easily 13% of the wall area, which is often a whole gallon.