Paint Coverage Calculator

Estimating paint quantity accurately is one of the most common challenges homeowners face before a painting project. Buy too little and you risk running out mid-job — leaving visible lap marks where fresh and dry paint meet. Buy too much and you waste money on cans that sit in your garage for years. This paint coverage calculator takes the guesswork out by using your room's actual measurements combined with standard industry deductions.

The calculator starts by computing the gross wall area — the total surface area of all four walls before any subtractions. It then deducts standard allowances for each door (21 sq ft) and each window (15 sq ft), giving you the net paintable wall area. If you choose to paint the ceiling, the room's floor area is added to the total. Finally, the combined surface area is multiplied by your chosen number of coats and divided by the paint's rated coverage per gallon to produce the exact gallons required.

The calculator rounds up to the nearest whole gallon for the recommended purchase quantity, because partial gallons are rarely sold at the same price-per-square-foot efficiency. A time estimate is also provided, using a rate of 1.5 hours per gallon — a reasonable average for a single experienced painter using a roller on smooth interior walls.

Understanding each variable helps you dial in a more accurate result. The coverage per gallon field defaults to 350 sq ft, which sits in the middle of the typical 300–400 sq ft range printed on most interior latex paint cans. High-sheen paints and primers often cover less; thick, textured surfaces can reduce coverage by 15–25%. For very dark colors requiring full opacity over a light base, plan for an additional coat beyond what the label suggests.

Every can of paint lists a theoretical coverage on its label — typically between 300 and 400 square feet per gallon for interior latex paints. This rating is measured under ideal laboratory conditions: smooth, primed surfaces at a uniform film thickness. Real-world coverage almost always falls short for several reasons.

Surface texture is the biggest variable. A smooth, freshly primed drywall surface may genuinely reach 400 sq ft per gallon, but a sand-textured wall, stucco, or brick can consume 25–40% more paint per coat because paint fills the crevices. When in doubt, enter a lower coverage figure — 280 to 300 sq ft per gallon — to build in a buffer for textured surfaces.

Paint color also affects how many coats you need. Switching from a bright white to a deep navy or forest green often requires three coats instead of two because dark pigments do not spread as uniformly over light backgrounds. The number-of-coats field in the calculator lets you account for this directly. Conversely, applying a fresh coat of the same existing color can sometimes be done in a single coat with a quality paint-and-primer formula.

The type of paint finish — flat, eggshell, satin, semi-gloss, or high-gloss — has less impact on coverage per gallon than surface texture but does affect application technique. Higher-sheen finishes are more self-leveling, reducing roller stippling, but they also highlight surface imperfections more readily.

A key accuracy feature of this calculator is automatically subtracting the area occupied by doors and windows from the gross wall area. Painting professionals use standard industry deductions rather than measuring every opening individually, because interior doors and windows cluster around average sizes in residential construction.

A standard interior door is assumed to be 3 feet wide by 7 feet tall, giving a surface area of 21 sq ft. This deduction removes the area you will not paint on the wall — typically just the door frame opening, since the door itself is a separate painting project if desired. A standard window is assumed to be 3 feet wide by 5 feet tall, equaling 15 sq ft. These are conservative estimates; many windows are smaller, meaning the deduction is a slight over-correction that works in your favor by leaving you with a small buffer.

If your room has an unusually large picture window, a sliding glass door, or a bay window grouping, consider adding extra windows in the count to approximate the larger glass area. For a standard 6-foot sliding patio door, entering two windows (2 × 15 = 30 sq ft) is a reasonable approximation. The goal is accuracy within 10%, which is sufficient for purchasing decisions since paint is sold in fixed can sizes anyway.

Do not deduct for built-in furniture, fireplace surrounds, or wainscoting unless you plan to cut in precisely around each one. Many painters simply roll over these edges and address them separately — the small amount of paint applied to trim that gets repainted in white or another color is negligible in terms of coverage.

The number of coats is arguably the most consequential variable in this paint coverage calculator. Applying a second coat can nearly double your total paint requirement and project time, so getting this right saves real money. Professional painters follow a consistent set of guidelines based on substrate condition and color transition.

One coat is appropriate when you are applying the same color and sheen over a surface in good condition, using a high-quality paint-and-primer product, or doing a refresh coat in a utility space where absolute perfection is not needed. One coat over a dramatically different color almost always shows bleed-through, especially with yellows, reds, or any dark hue over white.

Two coats is the standard recommendation for most residential projects — a color change on previously painted surfaces in good condition, full coverage on newly primed drywall, or any space where the finished look matters. Two coats are also standard practice when applying any mid-sheen to high-sheen finish, because the first coat seals the surface and the second levels out to a consistent sheen.

Three or more coats are needed for dramatic color changes (deep colors over white or vice versa), unprimed bare surfaces, or when painting over water-stained or smoke-damaged walls. Stains should be spot-primed with a stain-blocking primer first; otherwise they can bleed through indefinitely regardless of how many finish coats you apply.

When using the calculator, always enter the total number of finish coats you intend to apply. If you plan to apply a coat of primer separately, calculate primer and topcoat quantities independently — primer typically covers 200–300 sq ft per gallon and should be entered as a separate estimate with one coat.

Once the paint calculator gives you a gallon figure, translating that into an actual purchase requires a few practical considerations. Paint is sold in quarts (0.25 gallon), one-gallon cans, and five-gallon buckets. For projects requiring less than one gallon, buy quarts — they are cost-effective for small rooms or accent walls. For projects over four gallons of the same color, a five-gallon bucket is almost always cheaper per gallon than individual cans and ensures perfect batch-to-batch color consistency.

Color consistency is a critical reason to buy all your paint at once. Even with the same paint formula, cans mixed on different days at different stores can have subtle color variations visible once dried on a wall. Professional painters "box" their paint — pouring all cans into a single large bucket and stirring together — before starting, to guarantee uniformity across the entire room.

Keep one quart for touch-ups. Nail holes, dings, and scuffs happen months or years after the original project. Having the exact matching paint in storage makes future repairs invisible. Label each can with the room name, paint brand, color name, and formula code before storing. Paint stored properly at room temperature lasts three to five years.

The calculator's recommended purchase rounds up to the nearest whole gallon. If the exact amount is very close to a whole number (for example, 1.95 gallons rounding to 2), trust the rounding — the small buffer covers application losses from brush and roller nap absorption, paint left in the tray, and the inevitable waste of paint that dries in the can between coats. If you are well under a threshold (say, 1.2 gallons rounding to 2), consider whether a quart purchase would bridge the gap more economically.