How much paint do I need?

Estimating paint without the guesswork

Painting a room comes down to one number: the wall area you actually have to cover. For a rectangular room that's twice the length plus twice the width, multiplied by the ceiling height. A 12 × 12 room with 8-foot ceilings has 384 square feet of wall before you take anything out.

Then you subtract the parts you won't paint. A standard interior door is about 21 square feet and an average window about 15, so a room with one of each drops to roughly 348 square feet. Multiply by the number of coats, divide by how far a gallon goes, and you have your answer.

How far does a gallon really go?

Manufacturers quote around 350–400 square feet per gallon, and that holds on smooth, primed drywall. New drywall, textured surfaces and deep or dramatic color changes all drink more paint, which is why two coats is the sensible default and why we always round up to whole gallons — running out halfway through a wall is how you get visible lap marks.

A worked example: a 14 × 12 bedroom

Say the room is 14 by 12 feet with 8-foot ceilings, one door and two windows, and you want two coats. The walls come to 2 × (14 + 12) × 8 = 416 ft². Take off 21 ft² for the door and 30 ft² for the two windows and you are left with 365 ft². Two coats at 350 ft² per gallon is 2.09 gallons.

The calculator rounds that up to 3 gallons, and it is the right call if you are buying single gallons. But look at the decimal before you pay: at 2.09 gallons, two gallons plus a quart covers the job. A quart costs more per ounce than a gallon, but less than a third of one, so it is the cheaper way to cover a small remainder. The rule of thumb: under 0.25 over a whole number, add a quart; over that, add a gallon.

The ceiling is a separate job. That same room has a 168 ft² ceiling, which takes about half a gallon per coat of flat ceiling paint. Buy it separately; wall paint in an eggshell or satin finish shows every roller mark on a ceiling.

How many gallons for common rooms

Two coats, 8-foot ceilings, one door and one window, 350 ft² per gallon. Walls only.

Coverage depends on the surface

If your walls fall in the lower rows, change the coverage box in the calculator instead of guessing. Going from 350 to 250 ft² per gallon adds 40% to the paint you need.

Mistakes that cost a second trip

• Skipping primer on new drywall or big patches. Bare joint compound soaks up the first coat and shows through as dull patches, a problem called flashing. A coat of drywall primer is cheaper than a third coat of paint.
• Not mixing cans together. Even cans tinted in the same order can differ slightly. Pour all the gallons for one room into a 5-gallon bucket and stir before you start, and a color shift halfway along a wall never happens.
• Going dark over light (or light over dark) with two coats. A big color change usually needs a third coat or a tinted primer. Plan for it rather than discovering it on day two.
• Forgetting closets and the back of the door. A walk-in closet can add 150 ft² of wall. Run it through the calculator as its own room.

What it costs

Mid-range interior wall paint usually sells for about $35 to $55 a gallon in the US, and premium lines run $60 to $80. For the bedroom above, that puts paint at roughly $90 to $165, plus $30 to $50 for a roller, tray, brush, tape and a drop cloth if you are starting from nothing. Prices vary by brand, sheen and region, so treat these as a starting point for your budget, not a quote.