How the estimate works
Paint is sold by volume but spent by area, so the whole calculation is: how much area, divided by how far a unit of paint spreads. The area side is the room's perimeter times the wall height, minus openings you won't paint:
Paint = (2 × (length + width) × height − doors − windows) × coats ÷ coverage rate
A standard interior door subtracts about 20 sq ft (1.9 m²) and an average window about 15 sq ft (1.4 m²) — close enough for buying paint, since the waste margin lives in the round-up to whole cans anyway. If you tick "include ceiling", the tool adds length × width to the area.
The number that actually moves the answer: coverage rate
Brand calculators quietly assume their own product's spreading rate, which is why two of them can disagree by a full gallon on the same room. Rates genuinely differ: quality interior paint on smooth, primed drywall spreads 350–400 sq ft per US gallon (10–12 m² per litre), but bargain paint, textured walls, fresh plaster, or a first coat over bare drywall can pull that down to 250–300 sq ft (7–9 m²). That's why the coverage field here is editable — copy the spreading rate off your actual can and the answer adjusts instantly. When you don't know yet, the defaults (350 ft²/gal, 10 m²/L) are the industry's usual planning figures.
Buying cans, not litres
The store doesn't sell you 1.73 gallons. The tool rounds up to your chosen can size and also shows how much of the last can will be left over — useful for judging whether to size down (say, a gallon plus a quart instead of two gallons) and for keeping touch-up paint. A practical tip: buy all cans of a custom-tinted color at once and box them (intermix into one container) so slight tint variations between cans don't show as banding on a wall.
Frequently asked questions
Does this include primer?
No — primer spreads at a different rate and is calculated separately. If you're priming, run the tool once with your primer's coverage rate at 1 coat, then again for the paint.
What about trim, doors, and baseboards?
Trim uses different paint (usually semi-gloss) in small quantities; a quart or litre typically handles a room's trim. This calculator covers walls and optionally the ceiling.
Should I subtract the wall space behind furniture?
No — paint the whole wall. Furniture moves; unpainted rectangles behind the sofa are forever (or at least very awkward).
My room isn't rectangular. What do I enter?
Measure the total perimeter of all walls you'll paint, divide by 2, and enter that as "length" with width 0 — the formula 2 × (L + W) then reproduces your true perimeter. Everything else works the same.
Planning a bigger project? Pair this with the concrete calculator for the rest of your materials list.