Paint is sold by area, not by room
A gallon of paint does not cover a room — it covers an area. So every estimate reduces to three numbers: the paintable area of the walls, the number of coats, and the spread rate of the paint on the shelf. This calculator takes your room dimensions and returns the gallons to put on the counter.
The reason to run it instead of eyeballing it is that the error is expensive in both directions. Buy short and you make a second trip and risk a can that does not match the first exactly. Buy long and you store pails you never open. The maths is not hard; it just is not guesswork.
350 square feet per gallon is the planning number
One US gallon covers about 350 square feet in a single coat on smooth, properly primed interior drywall. That is the figure paint makers print on the can and the rate this calculator uses. It is a rate for a clean, sound, sealed surface — which is why it is a starting point and not a promise.
Four things pull the real number down:
- Texture. Knockdown, orange-peel and heavy stipple add surface area that has to be filled. A textured wall can fall to 250–300 square feet per gallon.
- Unprimed or porous surfaces. Bare drywall and raw wood soak the first coat, which then covers far less than the label rate.
- Large colour changes. Going dark to light, or covering a deep red, often wants a third coat or a tinted primer underneath.
- Application. Rolling lays paint down thinly and predictably; spraying loses product to overspray.
None of that changes the formula — it changes the rate you feel safe using. On textured or thirsty walls, treat 350 as optimistic and round up.
The formula behind the calculator
Three steps, in order:
wall area = 2 × (length + width) × ceiling height
paintable area = wall area − (openings × 21 sq ft)
gallons = paintable area × coats ÷ 350
The first line is the perimeter times the height: a room has two length walls and two width walls, so the perimeter is 2 × (length + width). The second line removes the space you are not painting — the calculator debits 21 square feet for every door or window you count, which is close to a standard 3 × 7 ft door. The third line multiplies by the coats and divides by the spread rate. The result is rounded up to whole gallons, because paint below a gallon is sold by the quart only.
Three worked examples
A 12 × 12 ft bedroom, 8 ft ceilings, one door and one window
Perimeter is 2 × (12 + 12) = 48 ft, so the wall area is 48 × 8 = 384 sq ft. Two openings remove 42, leaving a paintable 342 sq ft. Two coats is 684 sq ft of film, and 684 ÷ 350 = 1.95 gallons, so buy 2.
A 14 × 16 ft living room, 9 ft ceilings, three openings
Perimeter 2 × (14 + 16) = 60 ft; wall area 60 × 9 = 540 sq ft; three openings take 63, leaving 477 sq ft. Two coats is 954 sq ft of film, and 954 ÷ 350 = 2.73 gallons, so buy 3.
A 20 × 30 ft open-plan room, 10 ft ceilings, four openings
Perimeter 2 × (20 + 30) = 100 ft; wall area 100 × 10 = 1,000 sq ft; four openings take 84, leaving 916 sq ft. Two coats is 1,832 sq ft of film, and 1,832 ÷ 350 = 5.23 gallons, so buy 6. Price that one as a 5-gallon pail plus a single quart — 5.25 gallons — rather than six loose gallons.
Three rooms, three different buy counts. That is the point of running the numbers: the gap between 2 and 3 gallons on the middle room is a real trip to the shop.
Coverage at a glance
| Paint bought | Covers, one coat | Covers, two coats |
|---|---|---|
| 1 quart (0.25 gal) | 88 sq ft | 44 sq ft |
| 1 gallon | 350 sq ft | 175 sq ft |
| 2 gallons | 700 sq ft | 350 sq ft |
| 5 gallons | 1,750 sq ft | 875 sq ft |
The middle column is the one to remember: a gallon covers 350 square feet per coat, so it finishes 175 square feet of wall once two coats go on. A 12 × 12 room's 342 square feet of wall is almost exactly one gallon of finished coverage — which is why that room rounds up to two gallons of paint.
Why two coats is the real answer
Almost every repaint wants two topcoats, and the reason is not only opacity. A second coat evens out the sheen — uneven suction on the wall pulls the first coat in differently and shows up as dull and glossy patches under a raking light. It also hides the lap marks where one roller pass overlapped the last.
The instinct to buy "just enough for one thick coat" is the wrong instinct. A heavy single coat sags, dries slowly, and still hides the wall less well than two thin ones. Two thin coats beat one thick coat every time, which is why the calculator counts coats: set the number and it prices the job the way a painter would do it.
Ceilings, trim and doors are their own jobs
The calculator measures walls only. Treat a ceiling as a separate rectangle — floor area times coats — and give trim, doors and skirting their own line. Ceiling paint is usually flat and covers a little further than wall paint, so one gallon is often enough for a bedroom ceiling: a 12 × 12 ft ceiling is 144 sq ft, which is 0.41 gallons in a single coat. Doors and trim want a harder, scrubbable enamel, and a quart usually handles the trim in one room.
Buying smart: quarts, gallons and pails
Paint comes in quarts (0.25 gal), single gallons, and 1- or 5-gallon pails, and the price per gallon falls as the container grows. If the estimate lands at 5.2 gallons, a 5-gallon pail plus one quart is cheaper than six single gallons and saves a sixth can you will never finish.
Buy the whole job from one batch where you can. Colour is matched to a formula, but two cans mixed on different days can drift by a shade, and the drift shows on a large flat wall. Painting the room from a single 5-gallon pail sidesteps that entirely. Keep a labelled quart of each colour for touch-ups — colours are never quite the same on a re-tint, so the tin you save is the only one that will match in five years.
Primer is a separate purchase
The calculator counts topcoats, not primer. On bare drywall, bare wood, patched walls or a radical colour change you also need a primer coat, and primer covers less than paint — around 250–300 square feet per gallon. Two useful moves: use a PVA drywall primer on new board, and tint the primer toward the topcoat when you are going lighter, so two finish coats do the job instead of three.
Skip the primer when repainting a sound, previously painted wall in a similar colour. The existing film is already the sealed base a primer would provide.
Where the estimate usually goes wrong
- Measuring floor, painting walls. Floor area and wall area are different measurements. A 12 × 12 room has 144 sq ft of floor and 342 sq ft of wall.
- Forgetting the second coat. One coat is half the paint. The most common under-order is a single-coat figure used on a two-coat job.
- Trusting the label rate everywhere. 350 sq ft/gal assumes smooth primed drywall. On texture or bare board it is optimistic.
- Missing or double-counting openings. A door and a window do not need paint. Count each one once and let the 21 sq ft debit do the rest.
- Buying to the decimal. The last half-gallon always seems to come out of a fresh can. Round up.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below, and every worked example was produced by running the calculator itself rather than by hand. Where a value depends on a regional code, the calculator lets you override it rather than assuming one jurisdiction.
- US Environmental Protection Agency — Volatile Organic Compounds (VOCs) — why interior paints and coatings are formulated to VOC limits, and how those limits affect choosing and specifying a product.
- American Coatings Association — the US paint and coatings industry body — product categories, coatings technology and the coverage and application guidance the trade works from.
Figures, formulas and reference tables last checked: 12 October 2026.