Painting Calculator: How Much Paint Do You Need?

One of the most common mistakes homeowners make before a painting project is buying too little paint and needing to make a second trip — sometimes finding the exact color is no longer available in the same dye lot. The opposite problem, buying far too much paint, leads to wasted money and leftover cans that need disposal. Accurate square footage calculation is the key to getting the right amount the first time. To calculate the wall square footage of a room, start by measuring the perimeter of the room — add up the length of all four walls. Multiply the total perimeter by the ceiling height. This gives you the gross wall area. From this number, subtract the area of doors and windows, since you will not be painting those surfaces. A standard interior door is approximately 21 square feet and a standard window is about 15 square feet. Subtract each door and window from your gross wall area to get the net wall area to be painted. Most interior paints cover between 350 and 400 square feet per gallon for a single coat on a smooth, primed surface. Divide your net wall area by 350 to get a conservative estimate of gallons needed for one coat. For two coats, which is standard for most color changes and new drywall, multiply by two. Color changes require extra planning. When painting over a dark color with a light one, or a light color with a dark one, you may need three coats for full coverage. Some highly saturated colors — deep reds, navy blues, rich greens — are notoriously difficult to cover and may require an additional coat even when painting over a neutral. In these cases, a tinted primer in a shade close to your final color can reduce the number of finish coats needed. Ceiling calculations are simpler. Measure the length and width of the room and multiply them to get the ceiling area in square feet. Ceilings typically need one coat of ceiling paint, which is usually a flat white formulation with slightly different properties than wall paint. For trim — baseboards, door casings, window frames, and crown molding — a quart of paint typically covers 100 to 125 linear feet. Measure the linear footage of all trim in the room to estimate how much trim paint you need. Primer adds another layer to the calculation. Fresh drywall, repaired surfaces, and drastic color changes all benefit from a primer coat before the finish paint. Primer typically covers 200 to 300 square feet per gallon. Account for primer separately from your finish paint calculation. A practical tip: once you know how many gallons you need, round up to the next whole gallon rather than buying by fractions. Leftover paint is useful for touch-ups, while running short mid-project is always an inconvenience.

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