Paint Coverage Calculator
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Find out how many gallons of paint you need from room dimensions or square footage — with door/window deductions and a wastage buffer built in.
Quick answer: A 12×12-ft room with 8-ft ceilings, 1 door, and 1 window needs about 2 gallons of paint for 2 coats of standard interior paint — plan to buy 3 one-gallon cans after the standard 10% wastage buffer. Interior paint typically covers 350 sq ft per gallon per coat on primed surfaces, per published US manufacturer spec sheets.
Paint Needed
Order quantity, includes 10% wastage buffer
Suggested purchase:
📐 Formula
Net paintable area = 2 × (Length + Width) × Height − (Doors × 21 sq ft) − (Windows × 15 sq ft). Gallons needed = (Net area ÷ Coverage rate) × Number of coats. Order quantity = Gallons needed × 1.10 (10% wastage buffer).
How to Use the Paint Coverage Calculator
Choose area or room-dimensions mode
Enter a direct square footage if you already know your paintable area, or switch to room-dimensions mode and enter length, width, height, and the number of doors and windows so the calculator can work out net wall area for you.
Select paint type and surface condition
Choose interior (previously painted), interior (new drywall/bare), exterior, primer, or ceiling. Each has a different published coverage rate per gallon, since new or textured surfaces absorb more paint than a smooth, previously painted wall.
Choose number of coats
Pick 1, 2, or 3 coats. Two coats is standard for most interior and exterior jobs. Three coats is recommended for new drywall, unprimed surfaces, or a major color change, such as covering a dark wall with a light color.
Review gallons and wastage-adjusted total
The calculator shows the net gallons needed before waste, plus an order quantity with a 10% wastage buffer and a practical can-size suggestion so you know exactly what to ask for at the paint counter.
Why Surface Condition Changes How Much Paint You Need
The single biggest variable in paint coverage isn't the color or the brand — it's what's underneath. A wall that's already painted and in good condition is relatively non-porous: the existing paint film seals the surface, so a fresh coat sits on top and spreads close to its full published rate, typically around 350 sq ft per gallon per coat. New drywall is the opposite. Bare gypsum board and drywall joint compound are porous and soak up the first coat of paint like a sponge, which is why new-drywall coverage drops to roughly 275 sq ft per gallon — about 20% less than a previously painted wall gets from the same can.
This is also why guessing your square footage instead of measuring it can throw off an order by a full can or more. If you're unsure of your room's exact dimensions, our square footage calculator can get you an accurate area first, before you plug numbers into this tool.
Why Primer Matters on New Drywall or a Major Color Change
Primer exists to even out exactly the absorption problem described above. On new drywall, primer seals the porous paper facing and joint-compound patches so the topcoat you apply afterward behaves like it's going onto a normal, previously painted wall — spreading evenly and hitting closer to its full coverage rate instead of disappearing into bare board. Skipping primer on new drywall usually means needing an extra topcoat just to get uniform color and sheen, which costs more in paint than the primer would have.
Primer also matters for major color changes, especially going from a dark existing color to a light new one — without it, the dark base can show through two or even three coats of light paint. The same applies to water stains, smoke damage, or glossy trim: a stain-blocking or bonding primer keeps the old surface from bleeding through or rejecting the new paint. If you're simply repainting a wall a similar shade over sound, previously painted drywall, primer is usually optional and you can go straight to the topcoat.
Worked Example: Painting a 12×12 Bedroom
Take a bedroom that's 12 ft long, 12 ft wide, with 8-ft ceilings, 1 standard door, and 1 standard window, being repainted with 2 coats of standard interior paint over previously painted drywall:
Gross wall area = 2 × (12 + 12) × 8 = 384 sq ft.
Deductions = 1 door (21 sq ft) + 1 window (15 sq ft) = 36 sq ft.
Net paintable area = 384 − 36 = 348 sq ft.
Gallons needed = (348 ÷ 350) × 2 coats ≈ 1.99 gallons — essentially the 2 gallons in the Quick Answer above.
Order quantity with a 10% wastage buffer = 1.99 × 1.10 ≈ 2.19 gallons, which rounds up in practice to 3 one-gallon cans (or, if the room were larger, a single 5-gallon bucket once the total climbs past around 4 gallons).
That 10% buffer isn't padding for its own sake — it covers the paint that ends up on the roller tray, brush, and drop cloth rather than the wall, plus it leaves enough left over in the can for future touch-ups without having to color-match a whole new gallon months later.
Application Method: Why Roller Nap and Texture Change Real-World Coverage
Published spreading rates from manufacturers like Sherwin-Williams and Behr assume a reasonably smooth wall and a standard application method — they're a starting point, not a guarantee. Roller nap thickness has a real effect on how much paint actually ends up on the wall versus how far it spreads: a short 3/8-inch nap, typical for smooth drywall, holds less paint and spreads it thinner, getting you closer to the full published coverage rate. A thicker 3/4-inch or 1-inch nap, used for stucco, popcorn ceilings, or heavily textured walls, holds more paint per pass and covers noticeably less area per gallon because paint fills into the texture's low points instead of just coating the surface.
Heavily textured surfaces — orange-peel drywall texture, brick, stucco, or rough-sawn wood siding — can cut real-world coverage by 15–25% below the smooth-wall figure this calculator uses. If your walls or exterior siding are heavily textured, it's worth budgeting an extra half-gallon to a full gallon beyond what the calculator suggests, especially for a first coat.
Choosing Interior vs. Exterior vs. Ceiling Paint
Interior, exterior, and ceiling paints aren't interchangeable, and the differences affect both coverage and durability. Interior paint is formulated for washability and low odor, and generally isn't built to handle UV exposure or temperature swings — using it outside leads to premature fading and cracking. Exterior paint is formulated to flex with temperature changes and resist UV, mildew, and moisture, which is also why its published coverage rate is a bit lower (around 300 sq ft per gallon) than smooth interior walls — exterior siding is rarely as smooth as interior drywall. Ceiling paint is typically a flat sheen formulated to hide imperfections and resist spatter and drips during application, and shares roughly the same 350 sq ft per gallon coverage as previously painted interior walls.
| Paint Type | Typical Coverage | Notes |
|---|---|---|
| Interior — previously painted | 350 sq ft/gal | Standard repaint over sound drywall |
| Interior — new drywall/bare | 275 sq ft/gal | More absorption; primer recommended first |
| Exterior | 300 sq ft/gal | Siding texture reduces coverage |
| Primer | 350 sq ft/gal | Seals porous or stained surfaces |
| Ceiling | 350 sq ft/gal | Flat sheen, similar to interior walls |
Once you've settled on paint quantities, it's worth stepping back and budgeting the rest of the project too — materials are only part of a renovation's total cost. Our budget calculator can help you plan labor, supplies, and paint together against your overall renovation budget.
Frequently Asked Questions
Calculate net paintable area by finding the gross wall area (2 × (length + width) × height) and subtracting standard deductions for doors (21 sq ft each) and windows (15 sq ft each). Divide that net area by your paint's coverage rate — typically 350 sq ft per gallon for previously painted interior walls — then multiply by the number of coats. Add a 10% buffer for touch-ups and future repairs before you buy.
Two coats is the standard for most interior and exterior jobs — it delivers even color and fully hides the wall underneath, even with quality paint. One coat can work for a same-color touch-up over sound existing paint. Three coats are recommended for new drywall, unprimed surfaces, or a major color change such as painting a dark wall a light color, since the first coat mostly gets absorbed or doesn't fully hide the old shade.
Yes, in most cases. Primer is essential on new drywall or bare wood, since raw surfaces absorb paint unevenly and would otherwise need extra topcoats. It's also recommended for major color changes (especially dark to light), water or smoke stains, and glossy surfaces paint won't stick to well. If you're repainting a similar color over sound, previously painted drywall in good condition, primer is usually optional.
One gallon typically covers about 350 sq ft per coat on a previously painted or primed interior wall, per published spreading rates from major US manufacturers like Sherwin-Williams and Behr. Bare drywall or new construction surfaces absorb more and typically cover closer to 275 sq ft per gallon. Exterior paint usually covers around 300 sq ft per gallon due to siding texture. Always check your product's label — actual coverage varies by brand and formula.
Use the same net-area method — gross wall area minus door and window deductions — but apply the exterior coverage rate, typically around 300 sq ft per gallon rather than the 350 sq ft used for smooth interior walls. Textured siding like stucco, brick, or heavy wood grain absorbs more paint and lowers real-world coverage further, so budget extra for textured exteriors. Multiply by your number of coats and add the standard 10% wastage buffer before ordering.
Sources & Methodology
Coverage rates used in this calculator are based on typical published spreading rates from major US paint manufacturers, not a government or standards body — home-improvement figures like these come from the manufacturers themselves rather than a regulator, unlike tax brackets or medical guidance elsewhere on this site. The 350 sq ft per gallon (previously painted interior), 275 sq ft per gallon (new drywall/bare), and 300 sq ft per gallon (exterior) figures reflect commonly cited spreading rates published by brands including Sherwin-Williams and Behr on their own product data sheets. Always check the specific can's label, since exact coverage varies by product line, sheen, and formula.