Paint Coverage Calculator

What one gallon of paint covers

Paint cans carry a coverage number on the label, and for interior wall paint it is usually 350 square feet per gallon. That figure assumes one coat on a smooth, sealed surface, applied at the spread rate the maker lists. It is a rate, not a promise. A gallon poured onto bare drywall soaks in and covers less. A gallon rolled over a factory primed door covers more. The 350 figure sits in the middle, which is why it is the default in the calculator above.

The word that matters in that sentence is coat. Coverage is quoted per coat, so a room that needs 344 square feet of paint needs 688 square feet of product when you roll two coats. Most paint makers tell you to apply two coats for a reason: the first coat seals and the second coat delivers the color. One heavy coat looks fine while it is wet and shows thin, blotchy patches once it dries, and those patches cannot be fixed by touch up because the sheen will not match.

Coverage also drops when the wall is rough. Orange peel texture, knockdown finish, brick and concrete block all present more surface than their flat dimensions suggest. A cinder block wall can take twice the paint of a smooth wall of the same size. Add 15 to 25 percent for light texture and 30 to 50 percent for heavy texture or masonry, and treat that as a starting point rather than a rule.

How the calculator builds your wall area

The math is plain geometry. A room has four walls, and the two pairs are the same size, so the wall area is 2 x (length + width) x height. A 12 by 14 foot room with 8 foot ceilings gives 2 x 26 x 8, which is 416 square feet of wall. Add the ceiling at 12 x 14, or 168 square feet, if you are painting that too, and the total surface is 584 square feet before any deductions.

Then subtract the openings you are not painting. Two interior doors at 21 square feet each take 42 square feet off, and two windows at 15 square feet each take another 30, so the paintable wall drops to 344 square feet. Multiply by the number of coats and divide by the coverage per gallon to get the gallons you need. In this room that is 344 x 2 = 688 square feet, and 688 divided by 350 is 1.97 gallons.

Ceiling height is the input people get wrong most often. A tape measure from floor to ceiling includes the trim and the gap above the baseboard, and in newer homes the number is often 9 feet rather than 8. Measure in two or three spots, because ceilings sag and floors slope, and use the tallest reading. Underestimating height by six inches on a 12 by 14 room costs about 26 square feet of wall per coat, which is a quarter of a gallon you will have to go back for.

Doors and windows: how much to subtract

Subtracting openings is worth doing, but it is not worth agonizing over. A standard interior door is 3 feet by 7 feet, or 21 square feet, and a standard single window is about 3 feet by 5 feet, or 15 square feet. Trim, casing and the small strip of wall inside the frame usually get painted with the wall, so these numbers are close enough for a paint order.

A room with one door and one window gives back 36 square feet, about a tenth of a gallon per coat. Skip the deduction and you buy slightly more paint than you need, which is the safer error. Deduct every opening and you may land one gallon short after the second coat, which is the expensive error because a second trip to the store can mean a different batch of tint.

Two situations call for skipping the deduction. Small rooms with lots of trim, where the cutting in around each opening uses nearly as much paint as the opening would have, and rooms where you are spraying, because overspray and masking waste more than the openings save.

One coat or two: why the multiplier matters

Every coverage number on a can is a one coat number, and almost every job is a two coat job. That single fact doubles your order, and it is the reason people finish a room with one wall left. Paint a dark color over a light one, or a light color over a dark one, and the second coat is not optional: the first coat will look translucent no matter how thick you roll it.

Look at the 10 by 10 row. One coat needs 248 square feet, which is 0.71 gallons, so a single gallon would do. Two coats need 496 square feet, which is 1.42 gallons, so you buy two. The jump from one can to two is where most of the money goes, and it is also where most of the regret comes from when a homeowner tries to stretch a gallon across four walls.

Three coats happen more often than people expect. Going from a deep red or navy to white, painting bare drywall with a tinted topcoat, or covering a patch that was spackled and not primed all push you to a third coat. Add a coat in the calculator and watch the gallon count move before you leave for the store.

Coverage by paint type, sheen and surface

Sheen changes coverage because it changes how the film flows out. Flat paint has more pigment and less binder, spreads easily and hides wall imperfections, and it usually hits the top of the coverage range. Higher gloss means more binder, a tighter film and a surface that shows every ridge, so it covers slightly less per gallon and needs a smoother wall to look right.

Primer is the line item people forget. New drywall, fresh joint compound and bare wood all absorb paint at a much higher rate than a previously painted wall, so a primer coat at 250 square feet per gallon can eat a full gallon in a room that would take two thirds of one for the finish coats. Budget primer separately and do not fold it into the topcoat estimate.

Low odor and low VOC labels are worth a look for a bedroom or a nursery, since the paint smell lingers longest in a room with the door closed. The EPA guide to volatile organic compounds indoors explains what those labels measure and why ventilation still matters on a low VOC product.

Rounding up: gallons, quarts and the can you keep

Paint is sold in quarts, gallons and five gallon buckets. A quart covers roughly 87 square feet at the 350 rate, so it is the right buy when the calculator says you need 1.1 gallons and the store is out of your color in gallons. Two quarts cost more per ounce than a gallon, but they beat a second trip.

Always round up to the next whole container. If the math says 1.97 gallons, buy two gallons. If it says 2.1, buy three, because the leftover half gallon is what saves the wall you scuff in six months. Touch up paint that comes from the same can matches; touch up paint mixed from a new can six months later usually does not, even with the same color code, because tint bases drift between batches.

Store the leftover in the original can, label the lid with the room and the date, and keep it somewhere that does not freeze. Latex paint separates in a garage that drops below freezing and never mixes back smooth. A sealed gallon keeps for years, and a quart of the right color on a shelf is worth more than the eight dollars it cost when a chair backs into the hallway wall.

One more container trick: five gallon buckets are cheaper per gallon but only make sense on a whole house job. Paint that sits in a five gallon bucket for a month skins over and loses its ability to be stirred back, so buy the size you will finish in one season.

Where the 350 number goes wrong

Three things push real coverage below the label. The first is application method. A roller with a thick nap holds more paint and leaves more on the wall, while a sprayer loses 20 to 30 percent to overspray unless you are careful. The second is surface prep. Dust, grease and sanding residue cause the paint to bead and pull, and you end up rolling the same spot twice. The third is color change, which is a hiding problem rather than a coverage problem, and it is solved with primer rather than with more topcoat.

Homes built before 1978 deserve a pause before any scraping or sanding. Paint in that era can contain lead, and sanding turns it into dust that a vacuum does not fully remove. The OSHA lead page covers worker protection and safe work practices, and HUD's lead safe housing guidance explains the disclosure and remediation rules that apply to rental property. Test kits are cheap and take minutes. On a pre-1978 house, test before you sand, and use a wet sanding method or a chemical stripper instead of dry scraping.

Treat the number from this calculator as a purchasing estimate, not a specification. Buy one extra quart on a color matched job, keep the receipt, and return the unopened can if the second coat finishes early. Every calculation runs in your browser and nothing you type is uploaded or stored.