Paint Quantity Calculator

About This Tool

The Paint Quantity Calculator takes the guesswork out of buying paint for your next room makeover. Nothing is more frustrating than running out of paint mid-wall or ending up with three extra gallons collecting dust in the garage. This calculator uses industry-standard coverage rates for different paint finishes, accounts for doors and windows you will not be painting, factors in multiple coats, and includes primer estimates — giving you a precise gallon count that saves money and eliminates waste. Whether you are a first-time DIY painter or a seasoned home improver, accurate paint estimation is the foundation of a successful project.

How Paint Coverage Works

Paint coverage is measured in square feet per gallon and varies by finish type. Flat and matte paints offer the highest coverage at approximately 400 square feet per gallon because their thin, non-reflective formula spreads easily and absorbs into the surface. As you move up in sheen — eggshell, satin, semi-gloss, and gloss — the coverage per gallon decreases because shinier paints contain more binders and resins that create a thicker film. High-gloss paint covers approximately 325 square feet per gallon, about 20% less than flat paint. This calculator adjusts the gallon estimate based on your selected finish, ensuring accuracy regardless of sheen level.

The Importance of Primer

Primer is a preparatory coating that serves multiple critical functions. First, it seals porous surfaces like bare drywall, new plaster, or unfinished wood, preventing paint from soaking in unevenly and creating blotchy results. Second, it blocks stains, smoke residue, and dark colors from bleeding through the topcoat. Third, it creates a uniform base that helps paint adhere properly and display its true color. Primer typically covers about 300 square feet per gallon, slightly less than paint. While modern paint-and-primer-in-one products work for simple repaints on previously painted surfaces, they cannot replace dedicated primer for bare surfaces, stain blocking, or dramatic color changes.

Why Multiple Coats Matter

Two coats of paint is the industry standard for a reason. The first coat provides base coverage but often looks uneven, with visible brush strokes, roller marks, and areas where the underlying color shows through. The second coat fills in gaps, evens out color density, and creates the smooth, uniform appearance you expect. For dramatic color changes — painting dark over light or light over dark — a third coat may be necessary. Using primer before paint is more cost-effective than adding extra topcoats because primer is cheaper per gallon and provides a better base. This calculator multiplies the paintable area by your selected number of coats to give you the total coverage needed.

Accounting for Doors and Windows

Standard doors and windows represent significant unpainted area. A standard interior door at 3 feet by 7 feet is 21 square feet, and a typical window at 3 feet by 5 feet is 15 square feet. In a room with two doors and two windows, that is 72 square feet you will not be painting — nearly a fifth of the wall area in a small room. Failing to exclude these areas leads to overbuying. However, for rooms with many small or unusually shaped windows, the standard deduction may underestimate excluded area. In those cases, measure the actual openings. This calculator uses industry-standard door and window sizes for its deductions, with the count adjustable to match your room.

Choosing the Right Finish for Each Room

Selecting the appropriate paint finish is as important as choosing the right color. Flat finishes hide wall imperfections beautifully and are ideal for ceilings and formal rooms, but they scuff and stain easily. Eggshell is the most versatile — it provides a subtle warmth without obvious shine and is easy to clean with a damp cloth. Satin offers a gentle luster that works well in high-traffic areas like hallways and family rooms. Semi-gloss is the standard for kitchens, bathrooms, and all trim because it resists moisture, grease, and scrubbing. High gloss is reserved for accent pieces, doors, and cabinets where maximum durability and a dramatic sheen are desired. Many professional painters use eggshell on walls and semi-gloss on all trim in the same room for an elegant contrast.