How Much Paint Do I Need for a Room?

Paint is sold by the gallon, and most retailers will not accept returns on custom-tinted products. A gallon of quality interior latex typically costs $35 to $60, so over-ordering by two or three gallons on a whole-house project adds up quickly. Under-ordering is equally frustrating: stopping mid-wall to buy another gallon often means the new batch looks slightly different because tint machines vary and fresh paint sheen differs from paint that has been open for days.

Beyond cost, the right quantity affects project timing. A second coat needs the first coat to dry fully, usually four to six hours for latex paint. Running out halfway through coat two means waiting until the next day to finish, which extends prep time, delays furniture moving, and pushes back other trades if you are renovating an entire room.

Professional estimators also account for surface condition. New drywall absorbs more paint on the first coat than previously painted walls. Textured surfaces like orange peel or knockdown can reduce effective coverage by 10 to 20 percent compared to smooth drywall. Getting the math right upfront means fewer surprises and a cleaner finish.

Start by measuring the length and width of the room in feet. For standard rectangular rooms, multiply the perimeter (twice the length plus twice the width) by the ceiling height to get gross wall area. A 12-by-14-foot room with 8-foot ceilings has a perimeter of 52 feet and gross wall area of 416 square feet.

For rooms with alcoves, bay windows, or angled walls, break the space into rectangles, calculate each section separately, and add the totals together. Do not forget closets that you plan to paint — measure their interior walls the same way. If you are painting the ceiling, add length times width as a separate surface.

Round measurements to the nearest half foot. Precision within a few inches does not change your gallon count, but grossly underestimating a bump-out or stairwell wall can leave you short.

Standard interior doors cover roughly 20 square feet of wall space. A typical double-hung window accounts for about 15 square feet. Multiply the count of each opening by these values and subtract from your gross wall area to get net paintable area.

If your doors or windows are non-standard — oversized patio sliders, floor-to-ceiling windows, or small basement hopper windows — measure them individually and subtract their actual area instead of using defaults. Built-in bookcases, fireplaces with surrounding trim, and large archways may also reduce paintable area if you are not painting those surfaces.

When in doubt, subtract the openings. It is better to slightly under-estimate wall area and buy an extra quart than to over-estimate and end up with a full unused gallon of a color you will never use again.

Most interior walls need two coats for even color and proper hide, especially when covering a darker existing color or painting new drywall with a bold hue. Multiply your net wall area by the number of coats to get total coverage needed. Two coats on 350 net square feet requires 700 square feet of coverage.

Coverage rates are printed on every paint can, usually between 350 and 400 square feet per gallon for smooth surfaces with one coat. Premium paints with higher solids content often cover closer to 400 square feet, while cheaper paints or deep colors may only achieve 300 to 350 square feet per gallon.

Divide total coverage needed by the per-gallon coverage rate to get raw gallons. For 700 square feet at 375 square feet per gallon, you need 1.87 gallons before overage. Always use the coverage number from your specific product rather than a generic estimate.

Add 10 to 15 percent overage for touch-ups, cutting-in waste, and uneven surfaces. On textured walls or when rolling near popcorn ceilings, lean toward 15 percent. This buffer also covers the paint left in the tray, on rollers, and in the can rim that you cannot recover.

Round up to practical purchase sizes. Paint is sold in quarts, gallons, and five-gallon buckets. If your calculation comes to 1.7 gallons, buy two gallons rather than one gallon plus two quarts — the price difference is small and having a full spare gallon for future touch-ups is worth it.

For ceilings painted a different color, calculate ceiling area separately with its own coat count and coverage rate. Flat ceiling white often covers in one coat over a previous white ceiling, while walls almost always need two.

New drywall and patched walls are thirsty. The first coat on bare joint compound and paper face can soak up 30 to 50 percent more paint than a sealed surface. Some painters use a dedicated primer coat first, which changes your gallon math entirely — primer and topcoat are calculated separately.

Dark-to-light color changes require a tinted primer or extra topcoats. Going from navy blue to off-white may need three coats of topcoat even with primer. Going from light to dark usually hides in two coats but uses more product per coat because deep bases have less white pigment.

Bathrooms and kitchens benefit from moisture-resistant formulas, which sometimes have slightly lower coverage than standard latex. Exterior paint calculations follow the same formula but use higher overage (15 to 20 percent) because siding texture, lap joints, and weather conditions increase waste.

• Forgetting to multiply by the number of coats, which cuts your estimate in half and guarantees a store run mid-project.
• Using gross wall area without subtracting doors and windows, leading to over-ordering by one or more gallons in smaller rooms.
• Assuming one coat will suffice when covering new drywall, dark colors, or patched walls that need two or three coats for full hide.
• Ignoring the coverage rate on the specific paint can and defaulting to 400 sq ft per gallon when your product only covers 325.
• Buying quarts to save money when the math calls for nearly two full gallons — two quarts cost almost as much as a gallon and leave you short.
• Not budgeting separate paint for ceilings when they are a different color or finish than the walls.