Paint Gallons
Figure gallons needed for walls and ceiling, with doors/windows deducted.
Estimates only — verify with local code and the product label.
How this works
Wall area starts as perimeter times wall height, then subtracts standard allowances for doors and windows — 21 SF per door and 15 SF per window are common painter's-estimate averages for typical residential openings. If your doors or windows run unusually large (patio sliders, picture windows), measure them directly instead of relying on the average.
Gallons needed is that net area, multiplied by however many coats you're applying, divided by the paint's coverage rate. 350 SF per gallon is used as the default here because it sits in the middle of most interior paint's published spread-rate range (typically 250-400 SF/gallon on primed drywall) — the actual number is always printed on your paint can, and it's worth using that instead once you've picked a product.
Ceiling area, if you include it, is simply room length times width, run through the same coats-and-coverage math separately from the walls (ceiling paint is often a different product/sheen than wall paint, so keeping the two gallon counts apart is usually more useful than a single combined number).
The result always rounds up to a whole gallon, since paint is sold that way — a little extra left over is normal and useful for future touch-ups.
Frequently asked
Why 21 SF for a door and 15 SF for a window?
These are standard averages used across the painting trade for a typical residential interior door (about 3' x 7') and a typical window opening. Oversized openings should be measured directly and subtracted by hand.
Do I need a separate primer coat in this count?
No — this only totals your topcoats. If you're priming bare drywall or a dramatic color change, add that as its own coat with its own coverage rate, since primer often covers differently than finish paint.
Questions about a project? Contact us.