RenoPeek

Paint calculator

Enter the room dimensions and this returns gallons, counting two coats and subtracting the doors and windows you are not painting. It works from 350 square feet per gallon, the conservative end of the 350 to 400 range Sherwin-Williams publishes.

Room dimensions

Paint needed

Enable JavaScript to calculate, or use the formula belowThe formula and the coverage rate are printed below, so this works on paper too.

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The formula

Wall area is the perimeter multiplied by ceiling height, minus the openings. Perimeter is twice the length plus twice the width, so a 14 by 12 room at 9 feet gives 52 feet of perimeter and 468 square feet of wall before subtractions.

Openings come off at 20 square feet per door and 15 per window — a standard 3-by-6-foot-8 door and an average double-hung window. Multiply what remains by the number of coats, divide by 350, and round up, because paint is sold in whole cans.

StepArithmeticExample, 14 × 12 × 9 ft room
Perimeter2 × (length + width)52 ft
Gross wall areaperimeter × ceiling height468 sq ft
Openings(doors × 20) + (windows × 15)50 sq ft
Net areagross − openings418 sq ft
Total to covernet × coats836 sq ft at two coats
Gallonstotal ÷ 350, rounded up3 gallons

Why 350 and not 400

Sherwin-Williams publishes 350 to 400 square feet per gallon, and both numbers are true — of smooth, primed, previously painted drywall. Texture, bare patches, skim coat and any porous surface pull the real figure toward the bottom of that band or below it. Estimating at 400 and discovering 350 means a second trip to the store mid-wall, which is worse than a spare quart.

When you also need primer

Primer is a separate calculation and covers less: HomeAdvisor prices it at $10 per gallon and it typically spreads over 200 to 300 square feet rather than 350. Three situations genuinely need it — bare drywall or patched repairs, a dramatic color change in either direction, and any surface with stains or smoke damage that would bleed through. Painting a beige wall a slightly different beige does not.

What it costs to have it done

HomeAdvisor puts a professional interior job at $2.75 per square foot of wall area, or $4.70 including trim and ceilings, with a national average of $2,022. The striking part is the split: labor is 75 to 95 percent of the total. Paint for the example room above runs about $40 to $50 at their quoted $13 to $17 per gallon, so almost everything you would pay a painter is time.

That ratio is the argument for doing it yourself, and it is the only remodeling task where the argument is this lopsided. For comparison, what interior painting costs covers the full breakdown, and kitchen remodel costs shows what happens when labor is only a quarter of the bill.

Questions people ask

How much does a gallon of paint cover?
Sherwin-Williams states that one gallon of interior paint covers about 350 to 400 square feet, which is roughly a 10-by-10-foot room. This calculator uses 350 as the working figure, because the lower end of that band is what textured, porous or previously unpainted walls actually deliver.
How many coats of paint do I need?
Two coats is the working default for any color change, and the calculator assumes it. One coat is realistic only when repainting the same color on a sound surface. Going from a dark color to a light one usually needs primer plus two coats, and pricing that as two coats will leave you a gallon short.
Should I subtract doors and windows?
Yes, once there are more than a couple. A standard door is about 20 square feet and an average window about 15, so a room with two doors and three windows has around 85 square feet you are not painting. On a small bedroom that is most of a gallon at current coverage rates.
How much does it cost to paint a room professionally?
HomeAdvisor puts interior painting at $2.75 per square foot of wall area, or $4.70 including trim and ceilings, with a national average job at $2,022. Labor is 75 to 95 percent of that total, which is why doing it yourself changes the number so dramatically compared with other remodeling work.
Does paint quality change how much I need?
It changes coats more than coverage. Cheaper paint spreads similarly but hides worse, so it often needs a third coat where a premium paint needs two. HomeAdvisor lists flat paint at $13 per gallon against $35 for high-gloss enamel, and a third coat of cheap paint costs more in labor than the saving on the can.
Sources