Paint + Primer Coverage Calculator — Gallons & Leftover
Painting a room? This free paint and primer coverage calculator estimates exactly how many gallons you need for your walls — and unlike other calculators, we show you exactly how much paint you'll have left over for future touch-ups. Enter your room dimensions or wall area directly, choose your surface type, number of coats, and paint tier, and get an instant breakdown of paint, primer, and total cost. Built for DIY homeowners and small contractors, it accounts for doors, windows, and ceiling area automatically, so you buy exactly what you need.
How to use this calculator
1. Measure your room or enter wall area directly
Grab a tape measure and get the length and width of the room, plus the ceiling height — the default is 8 ft, standard for most US homes, but change it if yours runs taller. Prefer to skip the geometry? Switch to the "By wall area" tab and type in your total square footage directly; that's handy if you already have blueprints or a contractor's measurement. Either way, toggle "Paint ceiling too?" on if you're covering the ceiling in the same color, since that adds real square footage to your total.
2. Choose your surface type and number of coats
Pick the surface that matches your walls: bare drywall, previously painted, textured, or masonry. This matters more than most people expect — bare drywall soaks up close to 30% more paint per gallon than a smooth, previously painted wall, because raw drywall paper and joint compound are porous. Then set your number of coats. Two coats is the default and the right call for most repaints. Drop to one coat only if you're matching a very similar color on a wall already in good shape; bump to three if you're covering a dark color with something much lighter.
3. Pick a paint tier and primer option
Choose economy, mid, or premium paint — this changes both the price per gallon and the coverage rate, since better paint carries more solids and spreads further. Leave "Include primer?" on Auto and the calculator recommends primer automatically for bare drywall (it always needs it) and skips it for previously painted walls in decent shape. If you know your project needs primer regardless of surface — say, you're covering a water stain or going from a dark wall to white — force it to "Always include" instead.
4. Review your paint, primer, and leftover estimate
The results update instantly as you type — no button to click. You'll see your paintable area (after doors and windows are subtracted), the exact gallons needed for your chosen coats, and how many gallons to buy, rounded up to a whole can since paint doesn't sell by the quart at most stores. Below that sits our leftover box: how much paint you'll have left after the project, in gallons and in square feet of future touch-up coverage. That's the number most calculators skip, and it's the one that tells you whether to save the can or start planning your next room.
The formula we use
Here's the exact math, start to finish. First we work out your wall area: perimeter × height, using 2 × (length + width) × ceiling height. If you're painting the ceiling too, we add length × width on top of that. Next we subtract openings — 20 sqft per door and 15 sqft per window, the standard sizes for a residential interior door and window — to get your paintable area.
From there: paintable area ÷ coverage rate = gallons per coat, multiplied by your number of coats for total gallons needed. Coverage rate is the part most calculators fudge. We use a matrix based on surface and tier — previously painted drywall at mid-tier covers 400 sqft per gallon, while bare drywall at the same tier covers only 300, because raw drywall is more porous. These rates are deliberately conservative: manufacturers like Dulux advertise coverage 40-70% higher on their tins, measured in ideal lab conditions on smooth, primed surfaces. Real walls have texture, humidity, and roller technique working against them, so we'd rather you buy slightly more than run short mid-coat.
Finally, we round up to a whole gallon can — even when the math says 1.67 gallons, you're buying 2 — and that rounding gap is exactly where the leftover number comes from.
A worked example
Let's walk through a real bedroom: 12 ft × 12 ft, 8 ft ceilings, previously painted walls, one door, two windows, mid-tier paint, two coats.
Wall perimeter: 2 × (12 + 12) = 48 ft. Wall area: 48 × 8 = 384 sqft. Subtract openings: 1 door (20 sqft) + 2 windows (30 sqft) = 50 sqft. That leaves 334 sqft of paintable area.
Previously painted drywall at mid-tier covers 400 sqft per gallon, so one coat needs 334 ÷ 400 = 0.835 gallons. Two coats: 1.67 gallons. Paint sells by the whole gallon, so you round up to 2 gallons — a Behr Premium Plus can from Home Depot runs $44, so 2 gallons costs $88.
Here's the part other calculators skip: those 2 gallons cover 800 sqft between them, but you only needed 668 sqft for two coats. That's 132 sqft of leftover coverage — enough for touch-ups on scuffed corners, nail holes, or a scrape from moving furniture, for years to come. Previously painted walls in decent shape don't need primer, so your total project cost lands at $88 flat: no primer line, no surprises.
Regional cost context
US paint prices vary by tier more than most people expect — and primer often costs more per gallon than the cheapest paint tier, which catches first-time painters off guard.
As an Amazon Associate we earn from qualifying purchases. Estimates are for planning purposes only — verify with local suppliers and building authorities before buying materials or starting work.
| Product | Tier | Price | Container |
|---|---|---|---|
| Economy | Economy | $27 | 1 gallon |
| Mid | Mid | $44 | 1 gallon |
| Premium | Premium | $63 | 1 gallon |
| Primer | Kilz 2 | $25 | 1 gallon |
Prices from Home Depot, current as of our last check — see our Methodology page for sources and verification dates.
Economy tier — Glidden Premium — gets you a usable finish for rentals, ceilings, or rooms you'll repaint again soon. Mid-tier Behr Premium Plus is the sweet spot for most owner-occupied bedrooms and living rooms: better hide, better washability, and a smoother finish than economy. Premium — Behr Marquee — markets itself as a paint-and-primer-in-one on similar colors, and its thicker formula genuinely covers better, though we still budget for two coats on any real color change.
Kilz 2 primer, at $25 a gallon, actually undercuts economy paint slightly — a reminder that primer isn't the "cheap" line item people assume. All prices above reflect our last Home Depot check (see our Methodology page for how we verify prices); expect a few dollars of regional and seasonal swing either way.
Shop on Amazon: Behr Marquee interior paint · Zinsser Bulls Eye 1-2-3 primer
A quick safety note: lead paint
If your home was built before 1978, there's a real chance old layers under your current paint contain lead. The EPA's Renovation, Repair, and Painting (RRP) Rule requires certified contractors for work disturbing more than 6 sq ft of painted surface in pre-1978 housing — and DIYers should test first with an EPA-recognized lead test kit before sanding or scraping. This calculator doesn't budget for lead abatement or extra prep time, so plan separately if your home falls in that window. When in doubt, check current guidance at epa.gov before you start.
Common mistakes to avoid
- Skipping primer on bare drywall. New drywall is porous and drinks your topcoat unevenly, leaving flat spots and wasted paint. Primer first, always, on anything unpainted.
- Forgetting to subtract doors and windows. A room with three windows and two doors has 90 sqft less wall than its raw perimeter × height suggests — miss that, and you'll misjudge both your total and your leftover.
- Buying paint across two trips without checking the batch number. Even the same color and tier can shift slightly between production runs, and a visible seam on a wall is a common DIY regret. Buy all your gallons in one trip if you can.
- Rounding down "to save money." If the math says 1.67 gallons, that's 2 gallons in the real world — paint doesn't sell in fractions, and running short mid-wall means a hardware store trip while wet paint dries unevenly without you.
- Ignoring texture and surface condition. A textured "orange peel" wall or masonry surface eats more paint than a smooth one. Our coverage matrix accounts for this — but only if you pick the right surface type up top.
- Planning a bigger project? If you're also building a deck, see how many boards you'll need with our composite decking calculator. Adding a retaining wall to the yard too? Check our retaining wall block calculator for blocks and gravel.
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Frequently asked questions
How much paint do I need for a 12×12 room?
For a 12×12 room with standard 8 ft ceilings, budget about 2 gallons of paint for two coats. Here's the math: your wall area is roughly 384 sqft (perimeter × height), and after subtracting a typical door and two windows you're painting about 334 sqft. At a mid-tier coverage rate of 400 sqft per gallon on previously painted drywall, two coats need 1.67 gallons — which rounds up to 2 gallons, since paint sells by the whole can. Bare drywall needs more.
Do I need a separate primer coat?
Bare drywall always needs primer — the short answer is yes for anything unpainted. Primer seals the porous drywall paper and joint compound so your topcoat covers evenly instead of soaking in and looking patchy. Previously painted walls in good condition usually don't need it, unless you're covering a dark color with something much lighter, where primer saves you a third topcoat. Our calculator recommends primer automatically based on your surface type, and you can override it either way.
How many square feet does a gallon of paint cover?
It depends on your surface and paint tier — there's no single number. Previously painted drywall covers 350-450 sqft per gallon depending on tier, while bare drywall covers only 250-350 sqft per gallon, since raw drywall is more porous and soaks up more paint. Textured walls and masonry cover even less. These are deliberately conservative figures — most tins advertise coverage 40-70% higher, measured on smooth surfaces in ideal lab conditions that rarely match a real wall.
How much paint do I need for two coats?
Two coats roughly doubles your one-coat number, but not exactly, since you round up to a whole gallon can at the end rather than after each coat. For a 12×12 room, one coat needs about 0.84 gallons at mid-tier coverage on previously painted walls; two coats need 1.67 gallons, which rounds up to 2 gallons. One coat can work for a close color match on a wall already in good shape; three coats suit dramatic color changes.
How much extra paint should I buy for touch-ups?
You don't need to buy extra — the leftover from rounding up to a whole gallon can usually covers it. Because paint only sells in whole cans, buying 2 gallons for a job that needs 1.67 gallons leaves roughly 0.33 gallons, enough for touch-ups on scuffs, nail holes, and scrapes for years afterward. Our calculator shows this leftover figure — in gallons and square feet of future coverage — for every project, so you know what you'll have on hand.
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As an Amazon Associate we earn from qualifying purchases. Estimates are for planning purposes only — verify with local suppliers and building authorities before buying materials or starting work.
Last updated: August 2026. Estimates are for planning purposes only. Verify with local suppliers before purchase. Coverage rates are generic averages; actual coverage varies by product, application method, and surface condition — check your specific paint can's label before buying.