Article — Gallons Per Square Foot Calculator
Gallons per square foot calculator: paint, primer, stain and sealant
A gallon of standard interior latex paint covers 350-400 square feet on one coat, according to Sherwin-Williams and Benjamin Moore product specifications. So a 1,000 sq ft project at 2 coats needs about 5.3 gallons — round up to 6 at the store. The gallons per square foot calculator above handles the arithmetic for paint, primer, stain, sealant, epoxy and polyurethane. Real-world coverage runs 15-25% below the spec rate on textured walls, so the rounding-up rule doubles as a safety buffer.
Coverage depends on the product. Primer covers less per gallon than topcoat. Concrete sealant covers less than wood stain. The product picker in the calculator preloads typical rates from EPA and ASTM published data so the starting numbers are accurate.
How gallons per square foot works
Coverage rate is the area one gallon will cover at the recommended wet film thickness. ASTM D3276 specifies the laboratory wet thickness used for the rating — typically 4-5 mils for latex paint, 6-8 mils for primer. Manufacturers publish coverage on the can label, on data sheets, and on calculator pages such as the Sherwin-Williams paint calculator and the Benjamin Moore coverage guide.
per coat = area / coveragetotal = per coat × coatsbuy = ceil(total)The calculator multiplies area by number of coats, divides by coverage rate, and rounds up. For 1,000 sq ft of interior wall at 2 coats and 375 sq ft/gal: 1,000 × 2 / 375 = 5.33 gallons, rounded up to 6.
The 350-400 sq ft per gallon figure has been the industry rule of thumb since the 1950s. It comes from ASTM-standardised application at a 4-mil wet film thickness on smooth, primed drywall. Real-world coverage is usually lower because most walls are not laboratory-smooth: rolled application leaves slightly uneven thickness, textured drywall absorbs more paint, and porous masonry can cut coverage in half. The Green Seal GS-11 standard for paint coverage requires manufacturers to publish honest spec-sheet rates verified by independent testing.
Paint coverage by product type
Different products cover different areas per gallon because they apply at different wet film thicknesses and contain different solids percentages.
- interior latex paint = 350-400 sq ft/gal
- exterior acrylic paint = 300-350 sq ft/gal
- primer / sealer = 200-300 sq ft/gal
- wood stain = 200-400 sq ft/gal
- concrete sealant = 100-200 sq ft/gal
- epoxy floor coating = 250-350 sq ft/gal
- polyurethane varnish = 250-400 sq ft/gal
- high-build primer = 100-150 sq ft/gal
Higher coverage means thinner film and lower hide power. Premium paints often cover slightly less because they contain more pigment and binder per volume — the trade-off is better hide and durability. Builder-grade paint at 400+ sq ft/gal saves money on the gallon count but often needs a third coat to hide a dark previous colour.
Gallons per square foot formula
Three numbers go in, one comes out.
Step 1: measure area. For rectangular walls, area = (2L + 2W) × H. A 12 × 14 ft room with 8 ft ceilings has (24 + 28) × 8 = 416 sq ft of wall. Subtract about 20 sq ft per standard window and 21 sq ft per standard door.
Step 2: divide by coverage. 416 sq ft / 375 sq ft per gallon = 1.11 gallons per coat.
Step 3: multiply by coats and round up. 1.11 × 2 = 2.22 gallons, rounded up to 3. Always round up — fabric stores cut to the quarter-yard, but paint stores sell whole gallons (plus quarts for touch-ups).
How many coats of paint
Standard guidance from Sherwin-Williams and EPA renovation manuals:
One coat suffices for touch-ups, refreshing a similar colour, or premium one-coat formulations from major brands. Two coats is the standard for any meaningful colour change. Three coats are common for high-contrast shifts (red over white, black over beige) and for high-traffic areas where durability matters. New drywall almost always needs a primer coat first to seal the surface and equalise absorption.
Paint coverage surface factors
Manufacturer coverage rates assume smooth, sealed substrates. Real walls deviate.
Textured drywall (knockdown, orange-peel) typically reduces coverage by 15%. Popcorn ceilings cut it by 25%. Rough masonry, stucco and rough sawn wood cut coverage by 30-40%. A 1,000 sq ft popcorn ceiling that should need 2.67 gallons per coat actually needs about 3.5 gallons. Build the buffer into the order or expect a second store trip.
Other coverage thieves: porous unsealed wood, dramatic colour changes (especially red, deep blue, vivid yellow), large temperature swings during application, low-quality rollers that absorb paint instead of releasing it, and wind on exterior jobs that flash-dries the surface before the paint can level.
Calculating paint cost per square foot
Material cost runs roughly 8-15 cents per square foot for one coat of mid-range interior latex ($30-45 per gallon at 375 sq ft/gal). Two coats double that. Premium paints push material cost to 15-25 cents per square foot. Labour for a professional paint job typically runs $1.50-3.50/sq ft, dwarfing the paint cost.
For a 1,000 sq ft room (walls only, 2 coats), DIY paint cost is about $180-270 plus rollers and tape. The same job hired out runs $1,500-3,500. The labour-to-materials ratio is roughly 8-12 to 1 in most US markets. Painting yourself is the single largest cost saving in a home refresh.
Common paint calculation mistakes
Five errors account for most "I ran out of paint" trips back to the store.
Forgetting the second coat. The most common error. One coat almost never hides a colour change. Always plan on 2 coats unless touching up the same paint.
Ignoring texture. Spec coverage assumes smooth walls. Textured drywall and rough surfaces drink 15-40% more paint.
Skipping primer. Painting over bare drywall or major colour shifts without primer turns a 2-coat job into a 3-coat job — and the extra coat costs more than the primer would have.
Mixing dye lots. Buying the full quantity in one trip ensures all gallons come from the same batch. Mid-job purchases can show as subtle colour shifts between walls.
Confusing US and Imperial gallons. One US gallon = 3.785 L. One UK Imperial gallon = 4.546 L. Always check which gallon the supplier means, especially on imported specialty coatings.