Paint Coverage Calculator
Add every room, subtract the doors and windows, and get the primer and topcoat you need — in gallons and whole cans.
Walls sq ft, ceiling sq ft
Advanced options
Enter at least one room with a length, width, and height above.
Topcoat paint needed
gal
Buy × 1-gal cans of topcoat, plus of primer
sq ft gross − doors − windows = sq ft × coats ÷ = gal ✓
Common rooms, already worked out
Each link opens the calculator pre-filled with one room — 1 primer coat, 2 topcoats, 350 sq ft/gal.
How much paint will the job need?
How much paint do I need for a room?
Add the wall areas — twice the room length plus twice the width, multiplied by the ceiling height — then subtract the doors and windows. Divide by the paint’s coverage rate (about 350 square feet per gallon) and multiply by the number of coats. A typical 12-by-12-foot bedroom needs roughly 2 gallons for two coats.
How many square feet does a gallon of paint cover?
Most interior wall paints cover 350 to 400 square feet per gallon, per coat, on a smooth primed surface. Rough or porous walls, and big colour changes, lower that figure. This calculator defaults to 350 so the estimate stays on the safe side; adjust it to match your can.
Do I need primer, and how much?
Prime when you are covering bare drywall, patched repairs, stains, or a drastic colour change. Primer uses the same coverage math as paint. Set primer to one coat for most repaints, or zero if you are refreshing a similar colour over a sound painted surface.
Should I count the ceiling?
Only if you are painting it. Tick the “include ceilings” option and the calculator adds each room’s length times width to the paintable area. Ceilings are often painted with a dedicated flat ceiling paint, so you may want to run the ceiling total as a separate calculation.
How are doors and windows handled?
Each door is subtracted as 20 square feet and each window as 15 square feet — typical sizes. These are deducted from the total paintable area across all your rooms. If your openings are unusually large or small, adjust the room dimensions slightly to compensate.
Why does the calculator round up to whole cans?
Paint is sold in whole containers, and running out mid-wall risks a visible lap mark and a colour that does not quite match from a new batch. The calculator shows the exact gallons and the whole 1-gallon cans to buy. A little left over is also useful for future touch-ups.