Paint Calculator
Square footage is the easy part. The hard part is that a gallon covers 350 square feet on a smooth primed wall and about 250 on bare or textured drywall — so the number on the can is a ceiling, not a promise.
Paint quantity calculator
What one gallon covers, by surface
| Surface | Coverage per gallon | Why |
|---|
Rooms of this size, priced out
| Room | Paintable area | Gallons (2 coats) | Cost |
|---|
Same 8 ft walls, two doors, two windows, ceiling included, 350 sq ft per gallon.
Why the can's coverage number is optimistic
Manufacturers rate coverage on a smooth, sealed, white surface under laboratory conditions. A real room is none of those things. Bare drywall absorbs the first coat like a sponge and can take 25 percent more paint than the label implies. Heavy texture such as knockdown or orange peel doubles the surface area at the microscopic level. Going from a deep saturated color to a light one usually takes three coats, not two, because the underlying hue keeps bleeding through.
The practical fix is to treat the label figure as a ceiling and pick the surface condition that matches what you are actually painting over. The default here is 350 square feet per gallon, which is what most painters assume for primed drywall. On a previously painted wall in good condition, 400 is realistic. On bare drywall or anything textured, 300. On masonry or stucco, 250 or less.
How the area is calculated
Wall area is the room's perimeter multiplied by the wall height. A 12 by 12 room has 48 feet of wall, which is 384 square feet at 8 feet tall. Doors and windows come out of that: a standard interior door is about 21 square feet, a standard window about 15. Two of each removes 72 square feet and leaves 312.
The ceiling is simple length times width, 144 square feet in this room. Adding it brings the total to 456. At two coats and 350 square feet per gallon that is 2.6 gallons, so you buy 3 — $114 at $38 a gallon. Cutting the ceiling drops it to 1.8 gallons, which rounds to 2.
Two coats, and when one is enough
Two coats is the default for a reason: the first coat seals and the second coat provides the uniform film that makes a wall look finished. One coat shows lap marks where each roller width overlaps, and it wears through faster at corners and door frames. The exceptions are a color-matched repaint over the same product in good condition, or a ceiling that is being refreshed in the same flat white.
Primer is a separate product and usually a separate line item. Bare drywall needs a drywall primer, not a paint-and-primer combination, because the paper face pulls moisture out of the paint film unevenly. Stains, water marks and knots need a stain-blocking primer or they will telegraph through two coats of finish.
The part the square footage misses
Trim, doors, window casings and baseboard are measured in linear feet, not square feet, so no wall-area formula captures them. A rough working figure is one quart for every 100 linear feet of baseboard at 3 to 4 inches tall, and paint for trim is a different product with a harder, more washable film than wall paint. On a typical bedroom, trim consumes a quart to a half gallon on its own.
Also missing: the ceiling you decide to paint after the walls are done, the closet interior, the patch you cut into the wall to move an outlet, and the second gallon you open at 9pm because the first ran short. Paint does not go bad quickly, so buying one extra gallon on a whole-room job costs nothing in the long run — but only if you are using a color you will want again.
Buying paint without overbuying
Paint is sold in quarts, gallons and five-gallon buckets. Quarts cost about 60 percent of a gallon, so they only make sense for touch-ups and trim. Five-gallon buckets cut the per-gallon price by a third but commit you to a single color and a single sheen, which is why contractors buy that way and homeowners usually should not.
Sheen matters more than most people expect. Flat hides wall imperfections and touches up cleanly but scuffs. Eggshell is the usual compromise for living areas. Semi-gloss belongs in kitchens, bathrooms and on trim, where scrubbing is routine. Buying the wrong sheen is the most common reason a paint job looks amateur, and it is not something the calculator can catch.
Frequently asked questions
How much paint do I need for a 12x12 room?
About 3 gallons for two coats on the walls and ceiling, at 8 foot walls and 350 square feet per gallon. Walls only, without the ceiling, comes to 2 gallons. Add a quart if you are painting the trim.
Is one gallon enough for a bedroom?
For a small bedroom of roughly 10 by 10 feet, one gallon covers the walls in a single coat at 350 square feet per gallon, but not two coats. Two coats of a 384 square foot surface needs about 2.2 gallons, so buy 3.
Does primer count as a coat?
No. Primer seals and provides adhesion; it is not a finish coat, and most primers are chalky and not scrubbable. Count primer separately at roughly the same coverage per gallon, then apply the finish coats on top.
How much does a gallon of paint cover on textured walls?
Roughly 250 to 300 square feet. Orange peel and knockdown texture increase the surface area the paint has to wet, and the deeper the texture the larger the penalty. Choose the textured option rather than trusting the label.
Why do I need more paint when changing colors?
A light color over a dark one needs a third coat because the old hue keeps showing through a partially opaque film. Red, orange and deep blue pigments are also inherently less opaque, so they need more coats even over white.
Should I buy extra paint for touch-ups?
Yes, about a quart. Paint matched later is never an exact match because sheen and tint batches drift, and a wall touched up with a different batch shows under side light. Label the can with the room and date.
Does this calculator include trim, doors and baseboard?
No. Trim is measured in linear feet and uses a different, harder paint. As a rule of thumb, allow one quart for every 100 linear feet of baseboard, and add a gallon if you are painting doors and casings in the same room.
Disclaimer: This calculator estimates quantities from room dimensions and typical coverage rates. Actual coverage depends on the product, the substrate, the application method and how much you load the roller. Confirm the figure on the label of the paint you are buying.
Citing this page? FigureMill, “Paint Calculator” (last reviewed October 31, 2026). Paintable wall and ceiling area from room dimensions, with door and window deductions, converted to gallons at a chosen coverage rate and costed per gallon. https://figuremill.com/materials/paint-calculator
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