DIY & Materials calculator
Enter your room's dimensions, doors, windows, and coats, and get gallons of paint needed — coverage rate and door/window deductions cross-verified from Sherwin-Williams, Benjamin Moore, Behr, and Lowe's, not a single unsourced figure.
A planning estimate — check the coverage rate on the can you buy.
How much paint you need depends on wall area, openings, coats, and coverage rate — not a flat number per room size. This calculator turns your own room dimensions into a gallon figure you can take straight to the store.
Room dimensions
Standard ceiling height is 8-9 ft.
Each standard door ≈ 20 sq ft subtracted.
Each standard window ≈ 15 sq ft subtracted.
Most quality paints need 2 coats.
400 for smooth walls; lower it to 350 or less for textured or porous surfaces.
Gallons to buy
2.5 gallons
Rounded up to the nearest half gallon — exact need is 2.33 gal.
At a glance
Use the rounded “gallons to buy” figure for purchasing, and lean toward the larger can when you're between sizes — running out partway through a wall and getting a slightly different mix from a new batch is a visible problem. If you're painting walls and ceiling with different products, check the “how this was calculated” breakdown to see the wall and ceiling areas separately, and price them out individually rather than assuming one gallon figure covers both products.
Net paintable area
2 × (L + W) × height − doors×20 − windows×15Plus room length × width for the ceiling, if included.
Gallons
Gallons = (area × coats) ÷ coverage rate, rounded up to the nearest half gallonCoverage rate defaults to 400 sq ft/gallon, adjustable for texture.
A 12 ft × 10 ft room with 9 ft ceilings, 1 door, 2 windows, 2 coats, painting the ceiling too, at the default 400 sq ft/gallon. Wall perimeter is 2×(12+10) = 44 ft, so gross wall area is 44×9 = 396 sq ft. Subtracting 20 for the door and 30 for the windows leaves 346 sq ft of net wall, plus a 120 sq ft ceiling — 466 sq ft per coat. At 2 coats that's 932 sq ft, or 2.33 gallons, which rounds up to 2.5 gallons to buy.
Multiply the wall perimeter (2 × (length + width)) by the ceiling height to get gross wall area, subtract about 20 sq ft per door and 15 sq ft per window, multiply by your number of coats, then divide by the paint's coverage rate — typically 350-400 sq ft per gallon per coat. This calculator does that math and rounds up to the nearest half gallon for you.
Most quality interior paints cover 350-400 sq ft per coat per gallon on smooth, primed drywall, per Sherwin-Williams and other manufacturer sources. This calculator defaults to 400 sq ft/gallon and lets you lower it — textured walls, porous surfaces, and lower-quality paint all cover less.
Two coats is standard for most interior repaints — Benjamin Moore's own calculator assumes 2 coats by default. A dramatic color change (especially light-to-dark or dark-to-light), bare or highly porous drywall, or a deep accent color can need 3 coats or a tinted primer first.
For new or bare drywall, a dramatic color change, stain-blocking, or glossy and oil-based surfaces needing better adhesion, yes. Primer is a separate estimate from this calculator — it typically covers less per gallon (roughly 200-300 sq ft) than finish paint, since it's applied thicker to seal the substrate.
Usually not — ceiling paint is typically a flat or matte product formulated to hide roller marks and resist moisture, a different product from wall paint even though this calculator totals wall and ceiling area together at one coverage rate for a single convenient gallon estimate. Buy and apply them as separate products if their sheens differ.
This calculator subtracts 20 sq ft per standard door and 15 sq ft per standard window, the majority convention across manufacturer and retailer sources. Some sources use a smaller window deduction (around 10 sq ft) — if your windows are unusually large or small, adjust the room dimensions or treat it as a rough estimate either way.
For walls and ceilings, gallons are almost always cheaper per square foot than quarts. Use quarts mainly for trim, a small accent wall, or to top up a fraction of a gallon — if your result lands at, say, 2.3 gallons, two gallons plus a quart is usually more economical than three full gallons.
This calculator covers walls and ceilings, not trim. Trim is typically a different, semi-gloss product and is estimated by linear feet rather than area — budget a separate quart or two rather than folding it into the wall total.
Disclaimer
Paint quantities are estimates based on standard coverage rates. Actual usage varies by paint brand, color, sheen, surface texture, and application method (roller vs. spray). Treat the gallon figure as a planning estimate, and check the coverage rate printed on the can you actually buy.
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