Paint Calculator: How Many Gallons Do You Need?

Enter your room dimensions and we subtract doors and windows, apply your coats and coverage rate, and tell you exactly what to buy — rounded up to real can sizes.

What else do you need?

Worked example: the 12 by 12 bedroom, line by line

Start with the room this page uses as its reference: 12 feet by 12 feet with 8 foot ceilings, one door, and two windows. Wall area comes first: two times the length plus the width, multiplied by the height, which is 2 times 24 times 8, or 384 square feet of wall before openings. Openings come out next, using the standard sizes in the dataset behind this calculator: 21 square feet for the door and 15 square feet for each window, so 21 plus 15 plus 15 removes 51 square feet and leaves 333 square feet of paintable wall. Coats and coverage turn area into gallons: 333 square feet times two coats is 666 square feet of coverage needed, and at the standard planning rate of 350 square feet per gallon that is about 1.9 gallons before waste. With the recommended 10 percent waste allowance for cutting in, roller absorption, and touch ups, the raw figure rises to just over 2 gallons, and the buy plan rounds up to real can sizes, so the honest answer is buy 2 gallons and expect a small amount left for touch ups. Run the same room at the low end of the coverage range and the answer changes: at 250 square feet per gallon, which rough or porous surfaces can produce, two coats with waste need close to 3 gallons, which is why the coverage field is editable instead of fixed.

That same two gallon answer prices out quickly with the tiers in the paint dataset. At the budget tier of $25 per gallon, paint alone is $50, and with about $40 of supplies the room totals about $90. At the mid grade tier of $45 per gallon, paint is $90 and the room totals about $130. At the premium tier of $65 per gallon, paint is $130 and the room totals about $170. All three sit inside the typical do it yourself range for a 12 by 12 bedroom of roughly $130 to $240 quoted in the answers below once a third gallon, a quart for trim, or a primer coat enters the job.

How coverage actually works

The coverage rate is the one number on this page that deserves a second look, because it is a claim about a smooth, primed surface rather than a promise about your wall. Manufacturers claim 250 to 400 square feet per gallon for one coat, and 350 is the standard planning figure this calculator uses by default. Fresh drywall, rough texture, and porous or unprimed surfaces drink more paint and land near the low end of that range, while a smooth wall that was already painted in a similar color can land near the high end. Color change matters as much as texture: a strong color shift usually needs the full two coats to read evenly, even when the first coat looks close. This is also why the calculator subtracts doors and windows instead of ignoring them. On a small room, openings are a meaningful share of the wall, and skipping the subtraction overstates the job by roughly a quart, which is real money at premium prices and a wasted partial can in the garage. When in doubt, plan at 350, keep the waste allowance on, and buy from a store that will take back an unopened gallon.

Common mistakes that leave people short, or stuck with extra paint

The most common mistake is planning for one coat because the first coat looks good while it is wet. Paint dries darker and thinner than it looks on the wall, and thin spots that hide under work light show up clearly in daylight, so two coats is the planning standard on this page for a reason. The second mistake is turning off the waste allowance to make the number smaller. Cutting in, roller absorption, and the paint that stays in the tray and the can routinely consume that 10 percent, and running out near the end risks a second trip and a mismatched batch. The third mistake is using the coverage claim from the can as a guarantee on a rough or unprimed surface, where real coverage can fall toward 250 square feet per gallon. The fourth mistake is forgetting that doors and windows still need a decision even though they are subtracted from the wall area: if you are painting trim and doors too, that paint is a separate, smaller purchase, not part of the wall gallons. The last mistake is buying quarts to top up a job that needs gallons. Quarts are the most expensive way to buy paint per ounce, so when the raw figure lands just over a whole gallon, rounding up to the next gallon is usually the cheaper and safer buy.

Frequently asked questions