Estimate interior paint in gallons and quarts for walls, ceiling, trim, and primer — adjusts for textured surfaces, sheen, and number of coats.
Advanced — coverage, opening sizes, prices, primer coverage
Reverse — I have N gallons, what room can I paint?
Enter how many gallons you already have, sheen, and coats — see the wall+ceiling area you can cover.
How the math works
Paint sizing is a two-step calculation: surface area first, paint volume second. Both steps have failure modes that quietly add a quart of waste — or a panicked second trip to the store.
- Wall area:
walls = 2 × (length + width) × ceiling_height - Subtract openings: each standard door is 21 sqft (3’×7′), each standard window is 15 sqft (3’×5′)
- Optional ceiling:
ceiling = length × width - Texture multiplier — smooth walls absorb 100%, lightly textured (knockdown, orange peel) absorb 15% more, popcorn or fresh drywall absorb 20–25% more
- Multiply by coats (2 is the safe default), then divide by coverage (350 sqft/gallon for eggshell on smooth walls)
The optimizer above turns that gallon number into a real shopping list — gallons round up to the next full can, with a single quart top-up only when you’re within a quart of the next gallon. Everywhere else, one extra gallon costs less than two or three quarts and leaves you a touch-up reserve.
Coverage by sheen and surface
Coverage data below comes from Sherwin-Williams and Benjamin Moore data sheets — the calculator defaults to the conservative end of the published range, since real-world rollers, primer condition, and tinted bases all push coverage down a few percent.
| Sheen | Smooth wall (sqft/gal) | Light texture (×1.15) | Heavy texture / popcorn (×1.25) |
|---|---|---|---|
| Flat / matte | 400 | ~348 | ~320 |
| Eggshell | 350 | ~304 | ~280 |
| Satin | 350 | ~304 | ~280 |
| Semi-gloss | 350 | ~304 | ~280 |
| High-gloss | 300 | ~261 | ~240 |
| Drywall primer (PVA) | 250 | ~217 | ~200 |
The numbers shift on premium lines (Benjamin Moore Regal Select hits 400–450 sqft/gal on smooth surfaces in lab conditions) but the multipliers stay the same. If your roller cover is brand-new and high-nap on a popcorn ceiling, expect coverage closer to the heavy-texture column.
When primer is non-negotiable
Primer toggle stays off by default because most repaints over existing latex don’t need it. Switch it on when any of these apply:
- Bare drywall
- The gypsum face paper drinks pigment unevenly. A coat of PVA drywall sealer (250–300 sqft/gal) flashes off in 30 minutes and gives you a uniform absorption surface — without it, joint compound seams telegraph through your finish coats forever.
- Bare wood (trim, doors, new construction)
- Tannin bleed-through ruins white finishes. Use an oil or shellac-based stain-blocking primer on knotty pine, oak, or anything previously stained.
- Glossy old paint
- Latex doesn’t bond to oil-based or high-gloss surfaces without a bonding primer (Zinsser 1-2-3, Sherwin-Williams ProBlock) or a thorough scuff-sanding to 150 grit.
- Dramatic color change or dark → light
- Going from burgundy to off-white in two finish coats almost never works; expect three or four coats without a tinted primer underneath. A grey-tinted primer matched to your finish color cuts this to two.
- Stain blocking (water marks, smoke, marker)
- Latex won’t seal these — they bleed through every layer. Use Kilz Original or BIN shellac for spot priming.
The calculator flags primer when you pick “bare drywall,” tick the dramatic color change box, or select “high-gloss” as your old surface. Tick the primer toggle and the result block adds the gallon count separately.
Worked example — bathroom 5×8, semi-gloss, walls only
This is the canonical case where the optimizer earns its keep. Buying two full gallons would waste nearly a gallon of paint on a small room.
- Wall area:
2 × (5 + 8) × 8 = 208 sqft - Subtract one door (21 sqft):
208 − 21 = 187 sqftof bare wall - Two coats:
187 × 2 = 374 sqfttotal paint area - Semi-gloss coverage at 350 sqft/gal:
374 / 350 = 1.07 gallons needed - Optimizer: remainder is 0.07 gal (≤ 0.25 gal threshold) → 1 gallon + 1 quart covers 1.25 gal of paint volume
1 gallon costs roughly $40–55 in semi-gloss; 1 quart adds $11–14. Total: $51–69. Buying two gallons would cost $80–110 and leave 0.93 gal sitting in your basement. Same logic holds for trim — measure your baseboard length and the calculator carves out a separate quart estimate at a 6″ stripe height. For shopping-list math on adjacent projects, the flooring calculator handles boxes-per-SKU and install-pattern waste in the same room footprint.
Real questions, real answers
Do I need 1 coat or 2?
Two is the safe default for almost every project. One coat works only when you’re repainting the exact same color over a fully-cured prior layer in the same sheen — and even then, manufacturers like Benjamin Moore explicitly recommend two coats “to achieve full color development.” Three coats become reality on red, deep yellow, or dark navy, where pigment density fights you for two passes before settling.
Is “paint and primer in one” a real primer?
For one job: repainting cured latex with a similar color, in a normal sheen, on a clean wall — yes. For everything else (bare drywall, bare wood, dark-to-light, stain blocking, glossy old surfaces) it is paint with slightly more solids, not a primer. The chemistry of a real PVA drywall sealer or a stain-blocking shellac is different from a tinted finish coat. Don’t treat the marketing label as a substitute for the right product on a problem surface.
How do I pick a sheen for the bathroom?
Semi-gloss or satin in any room with steam exposure. Flat/matte holds moisture and grows mildew along the ceiling line within 18 months in a poorly-vented bathroom. Semi-gloss wipes clean, resists splashes, and you’ll spot streaks fast enough to fix them. Eggshell is the bedroom/living-room default. If your project includes patching or replacing wallboard before priming, the drywall calculator covers sheet count, joint compound, and screw spacing.
Estimator only. Coverage drifts ±10% based on roller nap, paint quality, surface condition, and tinted-base pigment load. For deep custom colors, ask the retailer to print their pigment-load coverage chart — it shifts the per-gallon number on accent and ultra-deep bases. For accent walls done in tile or brick instead of paint, the tile calculator and brick calculator handle quantity and waste in the same single-room form.
