Buying too little paint means a second trip to the store mid-project, and buying too much means shelves of half-used cans. Getting the estimate right comes down to understanding how paint coverage actually works, plus a few variables most people overlook. This guide walks through everything you need to know before you pick up a brush.
Most interior paints cover between 350 and 400 square feet per gallon on a smooth, previously painted surface. That number drops quickly in the real world. Flat and matte paints tend to cover better than glossy ones because they don't require as thick a film to look uniform. Ceiling paints are formulated to be thick and hide well in a single coat, while trim paints are thinner and usually need two.
Latex (water-based) paint is what most homeowners reach for on walls. It dries fast, cleans up with water, and has improved dramatically in durability over the past decade. Oil-based paint is still preferred by some for trim and cabinets because it levels out beautifully and hardens to a tough finish, but it takes much longer to dry and requires mineral spirits for cleanup. Coverage rates are similar for both types.
Higher-sheen paints (satin, semi-gloss, and gloss) are more transparent per coat, meaning you often need an extra coat to get solid, even coverage. Flat and eggshell finishes hide imperfections better and typically cover in two coats. If you're switching from a dark color to a light one, or vice versa, plan for three coats regardless of sheen.
Primer is not just a base coat. It seals the surface, improves adhesion, and can dramatically cut down on the number of paint coats required. You should always prime over bare drywall (the paper surface soaks up paint like a sponge), over patched areas, and when making a dramatic color change. Tinted primer matched to your topcoat color is worth the extra cost if you're going from a deep color to a pale one. Self-priming paints work well for routine touch-ups and repaints over the same color, but they are not a substitute for a dedicated primer coat on new or repaired surfaces.
Standard practice is to subtract half the area of each door and window from your total wall area. A standard interior door is about 20 square feet, so subtract 10. A typical double-hung window is around 15 square feet, subtract 7 or 8. This approach gives you a small buffer of extra paint, which you will want for touch-ups down the road. If your room has an unusual number of large windows or a sliding glass door, measure those openings more precisely and subtract accordingly.
Calculate the ceiling as its own separate area, length times width. Trim is best measured by running total linear feet, then multiplying by the width of the trim piece in feet to get square footage. A standard 3-inch baseboard on a 40-foot perimeter is only about 10 square feet, so a quart of trim paint goes a long way in most rooms.
Once you have your square footage, divide by the coverage rate on the paint can (not the theoretical maximum; use 350 sq ft per gallon as a conservative number). Multiply by the number of coats you plan to apply. Round up to the nearest gallon or quart. Most paint stores will mix custom colors in quarts and gallons, and some will tint five-gallon buckets for large projects at a cost savings. Buy all your paint from the same batch if possible. Even the same color can vary slightly between batches, and the difference shows on the wall.
Touch-up paint is one of the most overlooked parts of a painting project. Store leftover paint in a tightly sealed container, label it with the room name and color code, and keep it somewhere that won't freeze. When you touch up a scuff or nail hole six months later, use a small brush and feather the edges. Rolling a small patch almost always looks worse than the original mark. If you run out of paint and need to rematch a color, bring the original paint can to the store rather than relying on memory or a chip; the color code on the lid gives the mixer the exact formula.
The paint calculator lets you enter your room dimensions, number of coats, and surface type. It accounts for standard door and window deductions and tells you exactly how many gallons to buy.
