Enter your wall area and coats, and get the number of gallons to buy — rounded up so you don't run out halfway up the wall.
Paint coverage comes down to three things: the area you're painting, how many coats, and the paint's coverage rate (about 350 square feet per gallon for most interior wall paint). Multiply wall area by coats, divide by coverage, and round up to whole gallons. Don't know your wall area? For a room, add up the wall lengths (the perimeter) and multiply by ceiling height.
We round up to whole gallons because running short mid-project means a trip back to the store and the risk of a slightly mismatched batch. Two coats is the honest default for most jobs — one coat rarely covers evenly, especially over a color change or fresh drywall. For a small job under a gallon, buying quarts can be cheaper, and the calculator tells you when that applies.
Large openings are worth subtracting so you don't overbuy. A standard interior door is roughly 20 square feet and an average window about 15 — enter the total in the openings field. For small trim and outlets, don't bother; the round-up covers it.
About 350 square feet per coat for typical interior wall paint. Rough or porous surfaces cover less, so check the label on your specific paint and adjust if it differs.
Two is the standard for most jobs — one coat rarely covers evenly, especially over a color change or new drywall. Use one coat only for a light refresh of the same color.
Gallons are cheaper per square foot for most jobs. For small projects under a gallon — an accent wall or trim — quarts can cost less. The calculator flags when a quart makes sense.