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Paint calculator

How much paint, and where that number came from.

Room by room, adjusted for your wall texture, with the doors and windows taken out. The total is not the point — being able to open the arithmetic and check every step by hand is the point. Nothing is sent to a server, and a half-finished estimate survives a refresh.

The whole job

Touch-up adds 5 percent, which is the allowance estimating.ts gives for it. The leftover can is the paint that will still match your wall in two years.

Suggested from the perimeter. Crown runs the perimeter at the ceiling the same way base runs it at the floor, so a room with both has twice the perimeter. Casings get added by the opening — measure them if they matter.

How this was calculated

Enter a length, a width and a ceiling height and the arithmetic appears here.

What to buy

Put a length, a width and a ceiling height into the room above and the numbers land here.

Why 350 square feet per gallon and not 400
We plan at 350 square feet per gallon, one coat, on smooth sealed drywall. The can usually says 350 to 400. We use the bottom of that range, and here is the arithmetic for why. Theoretical coverage = 1604 x volume solids / dry film thickness in mils. The 1604 constant is pure unit conversion - a US gallon is 231 cubic inches, spread out one mil thick. Run a real product through it: Sherwin-Williams SuperPaint Interior Latex Flat is 43 percent volume solids at 1.6 mils dry, so 1604 x 0.43 / 1.6 = 431 square feet per gallon theoretical. The label on that same can says 350 to 400. That gap of roughly 7 to 19 percent is the manufacturer's own built-in application allowance, before a single real-world loss. Then add what stays in the tray, on the roller cover, in the brush and in the can. Add cutting in, which eats paint out of proportion to the area it covers. Add any porosity at all, and add the fact that texture presents more actual surface than a tape measure reports. Field data runs 10 to 20 percent under manufacturer claims. So 400 is the best case on a technical data sheet and 350 is the realistic case on a job. A calculator that starts at 400 sends you back to the store mid-wall.
Why a door counts as 20 square feet
We count a standard door at 20 square feet and a standard window at 15. On the door: a full 3 foot by 7 foot door computes to exactly 21 square feet, but that is not the most common door in an American house. Most US interior doors are 30 to 32 inches wide by 80 inches tall, which is 16.7 to 17.8 square feet, and a 3 foot by 6 foot 8 inch door is 20. So 20 is the honest default; use 21 only if the doors really are a full 3 by 7. On the window: 15 square feet is a defensible middle. A 3 by 4 window is 12 square feet, a 3 by 5 is 15, a 4 by 5 is 20. Real windows vary too much for any single number to be right, so treat it as a planning figure and measure the big ones. Three rules go with these numbers. First, deduct nothing under about 3 square feet - it falls inside the normal waste allowance. Second, always fully deduct the large stuff: fireplaces, wall-to-wall built-ins, passthroughs, sliding glass doors, and anything over roughly 30 to 40 square feet. Third, and this is the one most calculators get wrong: deduct openings for material, not for labor. An opening removes 20 square feet of fast open rolling and adds perimeter cut-in, masking and detail work that is slower per square foot than open wall. Deducting it from a labor estimate makes the estimate worse, not better. A real estimator computes material with deductions and labor without them.
How the trim math works
One quart covers roughly 100 to 150 linear feet of standard trim per coat, and 125 is the working midpoint. Quarts = total linear feet x coats / 125. A gallon runs about 400 to 600 linear feet at two coats. A typical three-bedroom house has 500 to 800 linear feet of trim all in, which comes out to 1 to 1.5 gallons for two coats, or about a quart per average room. Crown moulding is not a special case: it runs the full perimeter at the ceiling exactly the way baseboard runs it at the floor, so a room with both simply has twice the perimeter. Add casings by the opening. One caveat that matters: 125 assumes standard profiles, roughly 3 to 4 inch baseboard and 2 to 3 inch casing. Tall 5 to 7 inch base, built-up multi-piece crown and deep ornate profiles can easily double consumption. Multipliers of 1.0 / 1.4 / 1.8 for standard / tall / ornate are a reasonable way to handle that, but those multipliers are our own inference rather than a published figure, and we are labelling them as such.

Deduct openings first, then multiply by the number of coats, then round up - in that order. And round up to what you can actually buy. Paint is sold in gallons and quarts, not in decimals, so 2.14 gallons means three gallons. Running short on a color-matched job mid-wall is a much worse outcome than a leftover can, and the leftover can is the touch-up paint you will want in two years anyway.

Order all of a color at once and box it - combine every gallon into one container and mix before any of it goes on a wall. Batch-to-batch color variation is small but it is visible on a large plane in raking light, and it is unfixable after the fact. Standard practice on any job over a couple of gallons.

A "how many coats" dropdown is the wrong question. Ask about the color change instead, because the two directions are not symmetric. Dark over light is a base-chemistry problem: deep and ultra-deep bases carry far less titanium dioxide to make room for colorant, so reds, deep yellows, oranges and some deep blues routinely need three or more coats. The fix is a grey-tinted primer stepped toward the topcoat. Light over dark is a hiding-power problem against a dark substrate, and it needs a white or tinted primer coat or three coats of topcoat. Like over like on a sound repaint is genuinely one to two coats, and sometimes one is the honest answer.

Primer coverage does not follow paint coverage, because a primer is a sealer rather than a film-builder. PVA runs about 19 percent volume solids against 43 percent for a premium wall paint, and at the same 4.0 mils wet it leaves half the dry film. It is doing a different job. This is also why primer gets its own coverage table on this page instead of a percentage applied to the paint number. Plan primer volume at roughly 0.8 to 1.2 times the topcoat volume, plus 5 to 10 percent.

Compute material with opening deductions and labor without them. A door removes 20 square feet of fast rolling and adds cut-in, masking and detail work that is slower per square foot than open wall. Many working estimators do not deduct openings at all on small residential jobs, because not deducting builds in roughly the same 10 to 15 percent overage you would add anyway and it removes the risk of running short on a custom color mid-wall. Vertigraph offers a clean simplification for homeowner-facing math: subtract a flat 10 percent for openings in typical residential.

A painting estimate is a labor estimate with some paint in it. Regional cost data puts labor at 75 to 95 percent of the total, and names surface condition as the most important single cost driver: walls that need patching, sanding and priming cost significantly more than walls that do not. Any square-foot price that ignores condition will mislead. Every number on this page is a material planning tool, not a quote - the quote comes from walking the actual house.

How to use it properly

  1. 1

    Measure the room

    Length and width in feet, plus the ceiling height. Measure the room, not the furniture. If the room is L-shaped, treat it as two rectangles and add them.

  2. 2

    Count doors and windows

    Only the ones in the walls you are painting. Do not deduct anything under about three square feet — that falls inside the normal waste allowance.

  3. 3

    Pick the texture

    This is the input that moves the answer most. Southern California tract housing is overwhelmingly orange peel and knockdown, not smooth drywall.

  4. 4

    Check the arithmetic

    Open "how this was calculated" on each room. Every step is printed with its numbers in it. If something looks wrong to you, change it.

The whole chain, in one line

Perimeter times ceiling height, minus the openings, divided by your texture’s square feet per gallon, times the number of coats, times one plus the waste factor. That is the entire wall-color calculation. The ceiling runs separately at its own texture, the primer runs at a different rate because a sealer is not a paint, and the trim is counted in linear feet rather than square feet.

The constants: 350 square feet per gallon on smooth drywall, 20 square feet per door, 15 per window, 125 linear feet of trim per quart per coat, and a 10 percent base waste factor — fifteen percent as soon as the surface has any texture in it.

The order matters

Deduct the openings first, then multiply by the number of coats, then round up — in that order. And round up to something you can actually buy. Paint is sold in gallons and quarts, not in decimals, so 2.14 gallons means three gallons.

This calculator rounds once, at the project total, rather than room by room. The reason is that you are going to order all of a color at once and box it together. If your rooms are different colors, round each color on its own.

The wrong question is "how many coats"

The right question is how big the color change is, because the two directions are not symmetric. Dark over light is a base-chemistry problem: deep bases carry very little titanium dioxide so that colorant has room, which is why reds, deep yellows and oranges routinely need three coats or more. Light over dark is a hiding-power problem against a dark substrate.

The fix in both directions is the same and it is cheap: a grey-tinted primer stepped toward the topcoat. One coat of tinted primer costs less than a third coat of deep-base premium paint. It is in the primer list above.

Rather have somebody measure it?

A material estimate is not a quote. Juan walks the house, checks the condition of the walls, and writes the scope down. No charge for that.

(760) 809-0025Get a free estimate
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