Multi-Room Paint Calculator & Shopping List
| Room | Walls (sq ft) | Ceiling (sq ft) | Paint (gal) |
|---|
A paint calculator built for whole projects, not single walls
Most paint calculators answer one room at a time and forget it the moment you leave. That's not how painting actually goes — you're doing the hallway, two bedrooms and maybe the ceiling while the furniture is already shoved into the middle of the house. This calculator keeps a running list of every room, does the gallon math per room, and rolls it all into one shopping list you can print and take to the store. Your project stays saved in your browser, so when you finally get to the second bedroom next weekend, the plan is still here.
The method, step by step
- Wall area: perimeter × ceiling height. A 12×12 room with 8-ft ceilings has 2 × (12 + 12) × 8 = 384 sq ft of gross wall.
- Subtract openings: 21 sq ft per door, 15 sq ft per window — the same standard deductions paint stores use. That 384 becomes 348 with one of each.
- Multiply by coats: two coats means painting the area twice. This is the step most people skip, and it's why "one gallon per room" folklore runs dry halfway along the second wall.
- Divide by coverage: 350 sq ft per gallon is a fair default for smooth, previously painted drywall. 348 × 2 ÷ 350 ≈ 1.99 gallons — so the shopping list says 2 gallons, not "about 2".
- Trim and primer ride along separately, because you buy them separately: trim paint by the quart from linear feet of baseboard, primer at one coat and ~300 sq ft per gallon.
What the shopping list rounds — and why
Exact math says things like "2.37 gallons", but stores sell gallons and quarts. The list rounds each total up to the nearest quart and converts four quarts into a gallon, so what you read is what you buy. If the wall total lands just over a whole gallon — say 2.05 — you'll see 2 gallons and 1 quart. Whether you actually buy the quart or stretch the gallons is your call; for the spare-paint side of that decision, our guide to how much extra paint to keep for touch-ups has real numbers.
Getting the inputs right
- Coverage is the honest lever. Fresh drywall, plaster patches and textured surfaces absorb more paint. Drop coverage to 250–300 for those, and read what a gallon really covers by surface if you're unsure.
- Count coats realistically. Like-on-like color refreshes sometimes pass with one coat; light over dark almost never does — here's the coat math for dark walls.
- Trim linear feet: walk the baseboard with a tape measure, or use the room perimeter as a decent stand-in and add door/window casings if you're painting those too.
- Primer is a separate product. The checkbox adds one primer coat over the same area. When you actually need it (bare drywall, stains, drastic color changes) is covered in our primer decision guide.
Frequently asked questions
How much paint do I need for a 12x12 room?
With 8-foot ceilings, one door and one window, a 12x12 room has about 348 sq ft of paintable wall. At 350 sq ft per gallon, two coats need almost exactly 2 gallons. Add the room above and the calculator shows the math line by line.
What coverage rate should I use?
Cans typically claim 350–400 sq ft per gallon, but that assumes smooth, sealed, previously painted walls. Textured, porous or bare surfaces drink more — plan on 250–300. The coverage field is editable, so match it to your walls.
How are doors and windows handled?
Each door deducts 21 sq ft and each window 15 sq ft — the industry-standard allowances. If your openings are unusually large (a sliding glass door, a picture window), count them as two.
Does the calculator include primer and trim?
Yes, separately — because you buy them separately. Primer is figured at one coat and ~300 sq ft per gallon. Trim is estimated from linear feet of baseboard and converted to quarts, since trim paint is usually a different sheen and product.
Should I round up or buy exactly what it says?
The shopping list already rounds up to whole gallons and quarts. If you are between sizes, a quart of extra wall color is cheap insurance for touch-ups — see our leftover-paint guide for how much spare paint is actually worth keeping.
Is my project saved anywhere?
Only on your own device. Rooms are stored in your browser so the plan is still here when you come back; nothing you type is uploaded or seen by us. The Clear button wipes it.
Related guides
Ceiling Paint vs Wall Paint: Why the Math Is Different
Ceiling paint vs wall paint math explained: why length-times-width with no deductions, one coat, and a flat sheen change how many gallons you buy.
Do You Need Primer? How to Count It Separately From Paint
Do you need primer? Five wall conditions that demand it, three that skip it, plus the math for counting primer gallons separately from paint.
How Much Paint for a 12x12 Room? The Real Two-Coat Math
How much paint for a 12x12 room? Two gallons covers two coats on 8-ft walls — here is the perimeter, opening, and ceiling-height math behind that number.
Leftover Paint: How Much Extra to Buy for Touch-Ups
Touch-up paint math from a year of field notes: how much extra to buy, quart vs gallon prices, and storage that keeps leftovers usable.
How Many Square Feet Does a Gallon of Paint Cover?
Paint coverage per gallon runs 250–400 sq ft depending on surface. Realistic numbers for smooth walls, texture, bare drywall, brick, and ceilings.
Painting Over Dark Walls: How Many Coats It Really Takes
Painting over dark walls takes more paint than the can promises. Real coat counts and gallon math for covering navy, black, and red walls.