Paint Calculator

Every paint can states a coverage rate on the back label, usually somewhere between 250 and 400 square feet per gallon depending on the sheen, the base, and how heavily the paint is applied. A wall-and-ceiling latex paint typically covers about 350 square feet per gallon per coat on a primed, smooth surface. That number is the anchor of every paint calculation.

The math: multiply the perimeter of the room in feet by the ceiling height in feet to get the total wall area, subtract the square footage of doors and windows that will not be painted, multiply by the number of coats, and divide by the coverage rate.

A worked example: a room with a forty foot perimeter and an eight foot ceiling has 320 square feet of wall. Subtract about forty square feet for a typical door and two windows and you get 280 square feet of paintable area. Two coats at 350 square feet per gallon works out to (280 x 2) / 350 = 1.6 gallons. Round up to two gallons at the store.

Interior wall paint on drywall covers close to the label rate. Exterior paint on wood siding covers less because the surface is porous and the first coat soaks in. Plan on 250 to 300 square feet per gallon for the first coat on unpainted or heavily weathered siding.

Primer is a special case. A tinted primer on drywall covers about 300 to 350 square feet per gallon. A stain-blocking primer over water damage or heavy stains covers about 200 to 250 square feet per gallon because it needs to be applied heavier to seal the substrate. Always price primer and finish coats separately.

Color changes drive coat counts. Painting light over light or matching an existing color takes two coats. Painting a dark color over a light color takes three coats or two coats over a tinted primer. Painting a light color over a dark color takes three coats, sometimes four, because the darker pigment bleeds through.

Trim paint is calculated by linear feet times height. A four inch tall baseboard around a forty foot perimeter is 40 x (4/12) = 13.3 square feet of trim. Crown molding, casing around doors and windows, and chair rails add up quickly on an older home. On a full trim job in a 1,500 square foot house, plan on two to three gallons of trim paint.

Ceilings are the room length times the width. A twelve by fourteen foot room has a 168 square foot ceiling, which is half a gallon at one coat and just under a gallon at two coats. Ceiling paint is typically flat, thicker than wall paint, and covers at closer to 300 square feet per gallon.

Cabinetry is where paint calculations often go wrong. A run of standard uppers and lowers in a kitchen has far more surface area than the linear footprint suggests, because you paint the face frames, the doors on both sides, the drawer fronts, and the exposed cabinet ends. Plan on one gallon of primer and one to two gallons of enamel finish for a small kitchen, more for a large one.

Sheen affects coverage indirectly by affecting how forgiving the paint is on imperfect surfaces. Flat and matte paints hide roller stipple and drywall texture the best but scrub the worst. Eggshell and satin hide texture reasonably well and clean up better. Semi-gloss and gloss show every roller mark and every trowel line on the drywall but wipe clean easily and are the standard for trim and doors.

Brand differences are real. Premium contractor-grade paints from major brands cover better and often hit the manufacturer's stated coverage rate in real use. Bargain paints usually need an extra coat, which erases any per-gallon savings and adds a full day of labor. On a paying job, premium paint is almost always the right economic choice; the material cost delta is tiny compared to labor.

For exterior work, quality matters even more. A premium 100 percent acrylic exterior paint on Texas siding holds its color and integrity ten to twelve years. A budget exterior paint fades and chalks in five. That is a real long-term cost to the homeowner and a real reputation cost to the painter.

Painting is a repeat, recurring, referral-heavy business, and the first customer in a neighborhood turns into three or four more once the truck is parked in the driveway. Getting that first customer is the whole game, and in 2026 that starts with a Google search.

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