How much paint do I need? (gallons & coats)
Paint is bought by the gallon, but you paint by the square foot, so the question is really: how many square feet of wall do you have, at how many coats, at what coverage? Here is the arithmetic, worked through.
Running out of paint mid-wall — or over-buying three gallons you will never use — both come from skipping one small calculation. The interior painting cost tool does it for you, but it is worth understanding so you can trust the number.
Step 1: wall area
Start with the perimeter of the room — add up the length of every wall — and multiply by the ceiling height. For a 12 × 15 foot room, the perimeter is 2 × (12 + 15) = 54 feet; at an 8-foot ceiling that is 54 × 8 = 432 sq ft of wall. That is the gross wall area before openings.
Step 2: subtract the openings
Doors and windows do not get painted the wall color, so subtract their area. A standard door is about 21 sq ft and an average window about 15 sq ft; one of each is roughly 36 sq ft. So the net wall area is 432 − 36 = 396 sq ft. The formula is wall area = perimeter × height − openings. Enter your real opening total — a room with a big picture window or double doors subtracts more.
Step 3: coats and coverage
A gallon of interior paint typically covers about 350–400 sq ft per coat; the calculator defaults to 375, which you can override for your product. Most jobs need two coats for even color, especially over a color change. So gallons = ceil(wall area × coats ÷ coverage). For our room: 396 × 2 ÷ 375 = 2.112, which rounds up to 3 gallons. You always round up — you cannot buy 2.112 gallons, and a little left over is your touch-up supply.
Why coverage is a band, not a fixed number
Coverage of 350–400 sq ft per gallon is a labeled planning band, not a guarantee. A smooth, previously painted wall in a similar color lands at the high end; a porous new drywall surface, a heavy texture, or a dramatic dark-to-light change drinks paint and lands at the low end (and may need a primer coat or an extra finish coat). Check your product’s label for its stated coverage and enter that number. See the paint coverage table for gallons at common room sizes.
Ceilings, trim and a sanity check
The wall calculation above is for walls only. If you are painting the ceiling, add its area (length × width — 12 × 15 = 180 sq ft here) as its own surface, usually in a different paint. Trim and doors are figured by length and count, not wall area, and use far less paint. As a sanity check: an average bedroom needs 2–3 gallons for two coats on the walls; if your calculation says 6, re-check your perimeter and height. And if you are also skim-coating or hanging new drywall first, size that with the drywall calculator.
The takeaway
Three numbers — perimeter, height and openings — give you wall area; two more — coats and coverage — give you gallons. Round up. That is the whole method, and it is why the answer for a standard 12 × 15 room at two coats is 3 gallons. Enter your own room and product numbers in the painting cost tool and add your $/gallon to get the material cost.
Primer and surface prep change the count
The two-coat assumption holds for repainting a wall in a similar color, but several situations add a coat — and therefore paint. Bare new drywall drinks the first coat, so it usually needs a primer (or a self-priming paint applied as an extra coat) before two finish coats. A dramatic color change — dark to light, or a bold color to a neutral — often needs a primer or a third coat to fully hide. Patched or skim-coated areas prime differently than the surrounding wall and can “flash” without a full priming coat. Each of these is really just another pass over the same wall area, so the arithmetic does not change — you increase the coats input in the painting cost tool from two to three, and the gallons rise accordingly. Budgeting the primer as its own coat is more honest than hoping two coats will cover, and it is far cheaper than running short a wall from the finish.
Sheen, second-coat coverage and the material line
Two details refine the material cost once you have the gallon count. First, sheen: flat and matte paints hide surface imperfections but mark more easily, while satin, semi-gloss and gloss are more washable — relevant in kitchens, baths and trim — and sometimes cover slightly differently, though the coverage band is broadly the same. Second, the second coat typically covers a touch better than the first because it goes over paint rather than a porous or contrasting surface, but you still budget both coats at full coverage to stay safe; the rounding-up rule absorbs the small difference. The paint you actually buy is gallons × your $/gallon, and quality matters here: a better paint with higher hide can genuinely need fewer coats, which can make the pricier can the cheaper job. Enter your product’s label coverage and your real price, and let the tool tell you the gallons — then add trim and ceiling paint as their own small quantities rather than lumping everything into one number.
Planning estimate. Coverage is a labeled band that varies by product and surface — check your paint’s label and buy a touch of extra for touch-ups.