Paint Coverage Calculator
Calculate how much paint you need for your walls and rooms. Includes multiple coats and gallons needed.
Reviewed by the Calcura editorial team · Last reviewed
- Question
- How much paint do I need for a room?
- Answer
- gallons = wall_area × coats / 350, where 350 ft² per gallon is the industry-standard coverage for latex wall paint (Sherwin-Williams and Benjamin Moore spec). Wall area = perimeter × ceiling height, minus windows and doors (~21 ft² per standard door, ~15 ft² per window). A 12' × 14' room with 8' ceilings (416 ft² walls, minus 50 ft² of openings) at 2 coats needs ≈ (366 × 2) / 350 ≈ 2.1 gallons.
▸Show formula & caveatsHide formula & caveats
- Formula
gallons = (wall_area − openings) × coats / 350 ft²/gal- Caveats
- Textured walls, primer coats, or color-change repaints often need more — add 20–30% buffer.
- Dark colors over white typically need 3 coats; round up the gallon count to buy 1-gallon cans.
- Paint containers ≈ 3.785 L per US gallon. Buy cans, not part-cans — opened paint is hard to reuse.
What This Calculator Does
Estimates paint volume for a rectangular room's walls given length, width, ceiling height, and coat count. Uses the Sherwin-Williams/Benjamin Moore industry-standard coverage of 350 ft² per gallon for latex wall paint applied by roller on a smooth or lightly textured surface. Returns gallons or liters and the total wall surface area.
When To Use This Calculator
Repainting a standard bedroom
12 × 12 ft with 8 ft ceilings: 2 × (12+12) × 8 = 384 ft² × 2 coats ÷ 350 = 2.2 gal → buy 3 gallons.
Estimating an open-plan living and dining area
30 × 20 ft with 9 ft ceilings: 900 ft² × 2 coats = 1,800 ft² ÷ 350 = 5.14 gal → buy a 5 gal bucket and a quart.
Budgeting for a hallway requiring 3 coats over deep red
25 × 4 ft hallway, 8 ft ceiling = 464 ft². 3 coats = 1,392 ft² ÷ 350 = 4 gal.
Confirming a contractor's paint material line item
Contractor quotes 4 gallons for a 14 × 16 × 9 ft bedroom × 2 coats. Check: 540 ft² × 2 / 350 = 3.1 gal → 4 gal with waste is reasonable.
How It Works
Wall surface area is computed as perimeter × height — specifically 2(L + W) × H — which assumes rectangular rooms with no adjustments for openings. The total is multiplied by coat count, then divided by the 350 ft²/gal coverage figure. Coverage is a manufacturer spec that assumes smooth drywall, 4-mil wet film thickness, and a high-density roller. Real-world variability comes from texture (sand/popcorn can drop coverage 20–30%), color opacity (one-coat whites vs three-coat deep reds), and application method (brush eats ~10% more than roller). The tool deliberately doesn't subtract doors or windows — the 20–30 ft² of subtraction typically offsets trim cut-in and touch-up waste anyway.
Formula
wall_area_ft² = 2 × (L + W) × H | gallons = (wall_area_ft² × coats) / 350 | liters = gallons × 3.78541Real-World Example
Scenario
Homeowner painting a 14 × 18 ft master bedroom with 9 ft ceilings, changing from mid-tone navy to a pale gray. Two coats of a premium paint-and-primer-in-one at ~400 ft²/gal claimed coverage, but they're using the conservative 350 ft² figure because navy-to-gray is a challenging transition.
Calculation
Wall area: 2 × (14 + 18) × 9 = 576 ft². Two coats = 1,152 ft². 1,152 / 350 = 3.29 gallons. Round up to 4 gallons (or 3 gallons + 1 quart) to have cushion for touch-ups. In liters: 3.29 × 3.78541 = 12.46 L.
Result
Buy 4 gallons (or 12.5 L). The extra gallon covers overlaps, lap-mark rework, and closet interiors — and a sealed leftover gallon covers touch-ups for years.
Common Mistakes to Avoid
Mistake: Forgetting ceilings and trim when budgeting paint
Solution: This calculator only covers walls. Ceilings take another pass at ~350 ft²/gal, and trim/doors are typically painted in semi-gloss — budget a separate quart or gallon.
Mistake: Using 400 ft² coverage for textured or porous surfaces
Solution: Heavily textured walls, new drywall, and raw wood can drop real coverage to 250–300 ft²/gal. Use 250 for rough texture, 300 for new drywall, 350 for smooth repaint.
Mistake: Trusting 'one-coat' paint claims for dark-over-light changes
Solution: Paint-and-primer claims one-coat coverage only for modest color jumps. Deep reds, vivid blues, and black always need two coats plus usually a tinted primer.
Mistake: Buying just enough and running short at the end
Solution: Always round up and add ~10% for cutting in, rolled-over trim mistakes, and touch-ups. Unopened quarts and gallons are returnable at most retailers.
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When This Isn't the Right Tool
Spray-applied paint on cabinets, doors, or trim
Use instead: Spraying has ~30% higher overspray loss than rolling. Use the manufacturer's spray-specific coverage rate, not the wall-paint 350 ft²/gal number.
Exterior siding, stucco, or masonry
Use instead: Exterior paints use different coverage ranges (250–400 ft²/gal depending on texture). Use the exterior product's specific data sheet.
Angled ceilings, vaulted walls, or irregular shapes
Use instead: Perimeter × height undercounts sloped surfaces. Measure each wall individually and sum the areas, or take off the drawings.
How to Verify This Calculation
Sanity check: a 10 × 10 ft room with 8 ft ceilings has 320 ft² of wall. Two coats = 640 ft² / 350 = 1.83 gal. If the tool returns wildly different numbers, unit (ft vs m) was wrong. 1 sq m = 10.764 sq ft.
Sources
- Sherwin-Williams — paint coverage guide — Industry standard for 350–400 ft²/gal coverage on smooth drywall, cited across the US paint industry.
- Benjamin Moore — product data sheets — Manufacturer coverage specs at the per-product level for topcoats and primers.
- Master Painters Institute — painting specifications — Industry guide for wet-film thickness, surface prep, and coverage assumptions used in commercial specs.
- BIPM — SI Brochure — Basis for 1 US gallon = 3.785411784 L and 1 ft² = 0.09290304 m² exact conversions.
Reviewed by the Calcura editorial team · Last reviewed