How Many Gallons of Paint Do You Need?
Paint coverage is straightforward on paper — a gallon covers approximately 350 to 400 square feet on a smooth, previously painted surface. In practice, it depends on the surface texture, the color change, and the paint quality. Going from dark to light (or white to a deep color) typically requires three coats instead of two. Textured walls absorb more paint. New drywall needs a primer coat before the finish coats.
The Paint Calculator accounts for all of this. Enter your room dimensions, the number of doors and windows (which are subtracted from the wall area), the number of coats, and the per-gallon coverage rate for your specific paint. It returns the exact number of gallons — rounded up, because paint stores don't sell partial gallons.
A reliable rule of thumb: one gallon covers a 10×10-foot room (walls only, no ceiling) with two coats. For ceiling paint, figure the ceiling area separately — ceiling paint is formulated differently (thicker, more drip-resistant) and often has different coverage rates than wall paint.