Paint Calculator
Wall area, surface and coats in. Gallons out, using the coverage rate your actual walls will give you rather than the one on the can.
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Openings deducted at 20 sq ft each, which covers a standard door or an average window. Rounded up to whole gallons.
Get the painting quotedWhy the number on the can is optimistic
A gallon of decent interior paint is rated at 350 to 400 square feet. Published coverage guidance is clear that the figure assumes the best case: a smooth wall, already primed, already painted, rolled in good conditions. It is a laboratory number printed on a product that gets used in houses.
Texture is what breaks it. A flat tape measure reading of 400 square feet of knockdown wall is more than 400 square feet of actual surface, because the roller has to cover every peak and trough. Bare drywall and fresh joint compound break it a different way: unsealed gypsum absorbs the first coat instead of holding it on the face. Published figures for those surfaces run well under the label.
This calculator uses the rate for the surface you pick, then rounds up to whole gallons. Buying short is the expensive mistake. A second trip for one more gallon risks a batch that does not quite match, and on a wall you can see the join.
The rates this uses
| Surface | Sq ft per gallon | Why |
|---|---|---|
| Smooth, already painted | 375 | The condition the label number is written for. Rolls out at close to the stated rate. |
| Light texture, orange peel or knockdown | 325 | Texture adds surface area the flat measurement does not see. Published figures for medium texture sit around 300 to 350. |
| Heavy texture or popcorn | 275 | Deep profile, and the roller has to work paint into it rather than across it. |
| Bare drywall or fresh joint compound | 250 | Unsealed gypsum and mud drink the first coat. Prime it and the second coat behaves normally. |
| Brick, block or stucco | 200 | Open pores and a surface that is not flat at any scale. The low end of every published range. |
First coat. A second coat on a surface the first coat has already sealed goes further, and the calculator counts it at 400 square feet a gallon.
Measuring the walls
Perimeter times ceiling height is the whole room. Add up the four wall lengths, multiply by the height, and that is your starting area before anything is taken out.
Deduct openings at roughly 20 square feet each. A standard interior door is about 21, an average window somewhere near 15, and a patio slider a good deal more. If the room has a wall of glass, measure it properly rather than counting it as one opening.
Ceilings are a separate number. So is trim, which is bought by the quart more often than the gallon. Our guide to exterior trim paint covers sheen and product choice for the outside of the house.
Primer is a separate question
Primer is not a coat of paint and it is not counted here. It is what you put on bare drywall, patched areas, bare wood and anything glossy so the topcoat behaves. Coverage varies so widely between products that the only honest figure is the one on the tin you are holding.
The practical rule: if the calculator result looks high, it is probably telling you to prime. Two coats of expensive finish paint on bare drywall costs more than one coat of primer and one of finish, and looks worse.
If you would rather not do it
We paint inside and out across the Triangle. House painting in Raleigh covers the work, and the two cost guides have real local numbers: interior painting cost by room and by square foot, and exterior painting cost by siding type. Staining a deck instead? The deck stain calculator handles railings and weathered boards, which paint maths gets wrong.
Common questions
How much paint do I need?
Work out wall area as perimeter times ceiling height, take off about 20 square feet for each door and window, then divide by the coverage rate for your surface. Smooth painted wall is about 375 square feet a gallon, textured nearer 275 to 325, bare drywall around 250.
How many square feet does a gallon of paint cover?
The label usually says 350 to 400, and that is for a smooth, primed, previously painted wall. Texture, bare gypsum, masonry and stucco all come in under it, some of them well under.
Do I need one coat or two?
Two, in almost every case where the color is changing or the surface is new. One coat is realistic only when you are repainting the same color on a sound wall. The second coat goes further than the first because the first one sealed the surface.
How much paint for a 12x12 room?
A 12 by 12 room with 8 foot ceilings has 48 feet of perimeter, so 384 square feet of wall. Take off a door and a window and you are near 350. Two coats on a smooth painted wall is about 2 gallons.
Should I count the ceiling?
Not in this calculator. Ceilings are usually a different product and often a different color, so they are their own calculation: length times width, at the same coverage rates.
Why did my last job need more paint than I bought?
Nearly always one of three things: the walls were textured and got counted as flat, the surface was bare or patched and drank the first coat, or the color underneath was dark enough to need a third pass. Deep reds and strong blues are the usual culprits for the third.
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