Enter your room, and get the paint amount and a can combination to buy. Wall area × coats ÷ coverage — the calculator below does the rest.
Check the can: most interior wall paints state 350–400 sq ft per gallon per coat. We default to 350, the lower end — running short mid-wall costs more than a little leftover paint.
Add a room size above to see a can combination.
The calculator above does this automatically, but it helps to see it once: wall area, minus doors and windows, times the number of coats, divided by how far your paint spreads.
Add up the length of every wall (the perimeter) and multiply by the ceiling height. For a rectangular room, that is 2 × (length + width) × height.
Doors and windows don’t get paint. Subtract their combined area from the wall area — the calculator uses typical sizes, but measure your own if they run bigger or smaller.
Two coats means twice the area to cover, not twice the wall. A fresh color almost always needs two; a same-color touch-up might get away with one.
The can tells you how far a litre or gallon spreads for one coat. Divide the total area you’re covering by that number, and round up.
Prime first on dark colors you’re painting over, or on fresh plaster and new drywall — both drink paint and throw the coverage number off.
Changing color, not just refreshing it? Plan on two coats. One coat over a different color almost always leaves patches or shows the old color through.
Buy all your cans from the same batch number, printed on the label. Paint tint can vary slightly batch to batch, and it shows on a wall more than on a swatch.
Round up rather than down. A little leftover paint is useful for touch-ups later; running out mid-wall means a store trip and a visible seam where the batches meet.
Want the fuller walkthrough — trim paint, ceiling paint, and what eats more paint than you’d expect? Read the full paint calculation guide →
Measure the wall area (perimeter × height), subtract doors and windows, multiply by the number of coats, then divide by how many square feet or metres your paint covers per coat. The calculator above does this for you — just enter your room size.
Most interior wall paints cover 350–400 square feet per gallon, per coat — check your specific can, since it varies by brand and finish. Textured walls and dark colors typically cover less.
For a color change, almost always yes — one coat over a different color usually leaves the old color showing through in patches. A same-color touch-up can sometimes get away with one coat.
Multiply the wall’s width by its height to get the area, subtract any door or window on it, then divide by your paint’s coverage per coat (times two if you’re doing two coats). Use "I know the wall area" mode above to skip the room-wide math.
A little extra is worth it — it covers rounding, a heavier second coat, and future touch-ups. Buying by the can already rounds up, so you’ll usually end up with some left over without adding more on purpose.