Quick answer: A gallon of paint covers roughly 250 to 400 square feet depending on the surface. For average walls and two coats, plan one gallon per 175 square feet. Measure walls, subtract 20 square feet per door and 15 per window, divide by coverage rate, multiply by coats, then add 10 percent for waste.
Paint Coverage Calculator
Find out exactly how much paint to buy — no more half-empty cans.
Assumes standard latex paint. A door ≈ 20 sq ft, a window ≈ 15 sq ft (per Behr’s official measuring guide). Includes 10% extra for waste and touch-ups. For bare wood or drastic color changes, apply primer first, then your finish coats on top. Primer seals the surface; it does not count as a finish coat.
Paint coverage is the wall area one gallon of paint covers in a single coat, expressed as square feet per gallon.
How to Use the Calculator
- Enter your total wall area in square feet. Measure each wall’s width, multiply by the ceiling height, and add the walls together. Painting a house exterior? Enter the total siding area instead.
- Subtract doors and windows. Enter how many of each you are skipping. The calculator deducts 20 square feet per door and 15 per window, following Behr’s official measuring guide.
- Pick your coats. Two is the standard for most jobs. Choose one if you are refreshing the same color on walls in good shape, or three for drastic color changes and rough surfaces.
- Choose the surface type. Smooth covers 400 square feet per gallon, average covers 350, and rough or textured covers 250.
- Add a price per gallon (optional). If you know what your paint costs, enter it to get a project cost estimate next to the quantity.
- Hit calculate. You get the gallons to buy, the smartest can combination of gallon and quart cans, and your estimated material cost.
What Each Input Means
Total area
The paintable surface in square feet. For a room, that means the walls only, not the floor. A 12 by 15 foot room with 8 foot ceilings has 432 square feet of wall.
Doors and windows
The openings you will not paint. Each standard door removes about 20 square feet and each window about 15, per Behr’s measuring guide. Deducting them keeps you from overbuying.
Number of coats
How many full layers of finish paint you plan to apply. Two coats is the norm, because the first coat seals and the second builds full color and durability.
Surface type
How thirsty the surface is. Smooth primed drywall sips paint at 400 square feet per gallon. Average walls sit at 350. Textured walls, stucco, and brick drink it at 250.
Price per gallon (optional)
Enter your paint’s price and the calculator multiplies it by the gallons you need, so you see the material cost before you ever reach the store.
The Formula and Methodology
The calculator runs one formula:
Gallons to buy = ((wall area − door and window deductions) ÷ coverage per gallon) × coats × 1.10, rounded up to the nearest quart.
The 1.10 factor adds 10 percent for waste, roller absorption, and touch-ups. The coverage rates sit inside the ranges the major manufacturers publish: Behr rates its paints at 250 to 400 square feet per gallon, Sherwin-Williams at 350 to 400, and Benjamin Moore at 400 to 450. The door and window deductions, 20 and 15 square feet, come straight from Behr’s official measuring guide. Most online paint calculators show a number with no explanation. This one shows its math and names its sources, so you can sanity-check every result.
Quick Reference Tables
Coverage per gallon by surface type
| Surface type | Coverage per gallon | Typical surfaces |
|---|---|---|
| Smooth | 400 sq ft | Primed drywall, previously painted smooth walls |
| Average | 350 sq ft | Most interior walls in lived-in homes |
| Rough / textured | 250 sq ft | Knockdown, orange peel, stucco, brick |
These defaults sit inside the manufacturer ranges: Behr 250 to 400, Sherwin-Williams 350 to 400, Benjamin Moore 400 to 450 square feet per gallon.
Gallons needed for common room sizes (two coats, average walls, no deductions)
| Room size (8 ft ceilings) | Wall area | Gallons to buy |
|---|---|---|
| 10 × 10 ft | 320 sq ft | 2.25 |
| 12 × 12 ft | 384 sq ft | 2.5 |
| 12 × 15 ft | 432 sq ft | 2.75 |
| 14 × 18 ft | 512 sq ft | 3.25 |
| 20 × 20 ft | 640 sq ft | 4.25 |
Figures include the 10 percent waste allowance and rounding up to the nearest quart. Subtract doors and windows for the exact number, the calculator does this for you.
Smart can combinations
| Gallons to buy | Best can combination |
|---|---|
| 1.25 | 1 × 1-gallon can + 1 × 1-quart can |
| 2.25 | 2 × 1-gallon cans + 1 × 1-quart can |
| 2.5 | 2 × 1-gallon cans + 1 × 2-quart can |
| 3.75 | 3 × 1-gallon cans + 3 × 1-quart cans |
| 4.5 | 4 × 1-gallon cans + 1 × 2-quart can |
| 10 | 2 × 5-gallon buckets |
Buying the exact combination beats grabbing whole gallons, since quarts cost less per project than paint you will never open. For 5 gallons or more, buckets are usually cheaper per gallon than individual cans.
Worked Example: 12 × 15 Room, Two Coats
Say you are painting a 12 by 15 foot bedroom with 8 foot ceilings, two doors, and two windows, using average-wall paint at 350 square feet per gallon. Here is the full math:
- Wall area: 2 × (12 × 8) + 2 × (15 × 8) = 432 sq ft
- Deductions: 2 doors × 20 + 2 windows × 15 = 70 sq ft
- Paintable area: 432 − 70 = 362 sq ft
- Per coat: 362 ÷ 350 = 1.03 gallons
- Two coats: 1.03 × 2 = 2.07 gallons
- Add 10% waste: 2.07 × 1.10 = 2.28 gallons
- Round up to the nearest quart: buy 2.5 gallons, that is 2 × 1-gallon cans + 1 × 2-quart can
At a typical $45 per gallon, the paint costs about $112.50. The calculator runs this exact sequence for any numbers you enter.
7 Common Mistakes When Estimating Paint
- Measuring the floor instead of the walls. Paint covers walls, so floor square footage is the wrong starting point. A 12 by 12 room has 144 square feet of floor but 384 square feet of wall.
- Forgetting to subtract doors and windows. Two doors and two windows remove 70 square feet. Skip the deduction and you will buy roughly an extra quart you do not need.
- Counting primer as a finish coat. Primer seals, it does not finish. Plan your two finish coats on top of the primer, not instead of one of them.
- Ignoring surface texture. Textured walls can need 40 percent more paint than smooth ones. Using the smooth rate on stucco is the fastest way to run out mid-project.
- Buying the exact calculated amount. Spills, roller absorption, and touch-ups are real. The 10 percent buffer exists because almost every job needs it.
- Assuming one thick coat equals two thin ones. Thick coats sag, dry unevenly, and still cover poorly. Two normal coats always beat one heavy one.
- Forgetting the ceiling or trim decision. Decide before you shop whether the ceiling and trim get painted too, and whether they need a different paint. Surprise additions mean surprise store trips.
2026 Paint Cost Guide (Typical US Ranges)
Interior paint typically runs $30 to $70 per gallon in 2026, with premium lines priced higher. These are typical ranges, not quotes, and prices vary by region and retailer. Five-gallon buckets usually cost less per gallon than single gallons, which matters on bigger jobs.
| Project | Paint needed (approx.) | Typical material cost |
|---|---|---|
| 12 × 12 room, 2 coats | 2.5 gallons | $75–$175 |
| 12 × 15 room, 2 coats | 2.5 gallons | $75–$175 |
| 20 × 20 room, 2 coats | 4.25 gallons | $128–$298 |
Costs above cover paint only. Add primer, rollers, brushes, tape, and drop cloths on top. Exterior paints and primers sit in their own price bands, so price those separately if your project goes outside.
Most manufacturers rate a gallon at 250 to 400 square feet per coat. Behr lists 250 to 400, Sherwin-Williams lists 350 to 400, and Benjamin Moore lists 400 to 450. Smooth primed drywall lands near the top of the range, while rough or textured surfaces land near the bottom. Always check the label, since coverage varies by product line.
A 12×12 room with 8-foot ceilings has 384 square feet of wall space. After subtracting a door and a window, you get about 349 square feet. At 350 square feet per gallon with two coats plus 10 percent waste, you need about 2.2 gallons, so buy 2.25 gallons: two 1-gallon cans and one 1-quart can.
No. Primer seals the surface and gives topcoat something to grip, but it does not build color or finish the way paint does. Count primer separately, then plan your finish coats on top of it, usually two. One exception: on previously painted walls in good shape, you can often skip primer and just apply two coats of paint.
Textured walls drink more paint. Knockdown, orange peel, and popcorn textures typically yield 200 to 250 square feet per gallon, versus 350 to 400 on smooth drywall. Our calculator uses 250 square feet per gallon for rough surfaces, which sits at the conservative end of Behr’s 250 to 400 range. Spraying uses even more, so keep the waste buffer.
A 2,000 square foot house usually has 1,500 to 2,000 square feet of paintable wall area, since floor space includes garages and overhangs you do not paint. At 300 square feet per gallon with two coats plus waste, plan roughly 11 to 15 gallons. Rough siding, brick, or stucco pushes you toward the higher end.
Buy a little extra. Running out mid-wall forces a store trip, and a new batch can dry a slightly different shade. The calculator’s 10 percent waste allowance usually covers this, but for color-critical rooms, rounding up one extra quart is cheap insurance. Store leftovers sealed and labeled for touch-ups.
Measure each wall’s width and multiply by the ceiling height, then add the walls together. For a 12 by 15 room with 8-foot ceilings, that is 432 square feet. Subtract 20 square feet per door and 15 per window, following Behr’s measuring guide. Ignore baseboards and trim unless you plan to paint them too.
Two coats is the standard for a reason. The first coat seals and evens out the surface, the second builds full color and durability. One coat can work when repainting the same color on walls in good shape with quality paint. New drywall, repairs, or color changes almost always need two.
Often yes. Deep reds, navy, and black cover poorly because their pigments are less opaque, and going from light to dark (or dark to light) usually needs a tinted primer plus two finish coats. Some painters add a gray primer coat under dark colors to cut it to two total. Budget an extra coat when switching extremes.
Measure length times width for the ceiling area, then divide by the paint’s coverage rate, usually 350 square feet per gallon for flat ceiling paint. A 12×12 ceiling is 144 square feet, so one gallon covers two coats with room to spare. Textured or popcorn ceilings need about 25 percent more.
One gallon covers 350 to 400 square feet on smooth walls, around 300 on lightly textured surfaces, and 250 or less on rough surfaces like stucco or brick. Those are the manufacturer rates from Behr, Sherwin-Williams, and Benjamin Moore. Most jobs need two coats, so plan on roughly half the single-coat coverage in practice.
Paint itself is the small part. At 30 to 70 dollars per gallon (typical 2026 US range), a 2,000 square foot home needing 10 to 15 gallons for two coats costs about 300 to 1,000 dollars in paint. Hiring a professional usually brings the total to a few thousand dollars depending on prep work, stories, and region, since labor is most of the bill.