Paint Calculator
The paint aisle mistake runs in one direction: buying one gallon for a job that needs two, because wall area hides inside room length and width. A 12 × 10 ft room with 8 ft ceilings carries 352 square feet of wall before openings - minus two doors and two windows, times two coats, that's roughly 1.6 gallons, which means buy two. This calculator walks the same steps: room dimensions in, doors and windows subtracted, coats multiplied, coverage divided, purchase rounded up.
Coverage runs 350-400 sq ft per gallon on primed, smooth walls - the calculator uses the conservative 350 so the estimate errs toward enough. The note carries the trade's batch-matching rule: paint is tinted per batch, so finishing a wall from a second, unmatched can is the visible mistake the rounding-up prevents. Metric mode computes liters at 10 m² per liter. Room numbers are remembered for the next room in the same house.
How to use
- Enter room length, width and wall height.
- Set doors and windows - standard 21 and 12 sq ft openings subtract automatically.
- Pick coats (2 is default) and read gallons or liters, rounded up for touch-ups.
Frequently asked questions
How much paint do I need per room?
Wall area = (length + width) × 2 × height, minus openings (21 sq ft per door, 12 per window), times coats, divided by 350 per gallon. A 12x10x8 room with two doors and two windows needs about 1.6 gallons for two coats - buy two, because partial gallons cannot be matched later.
Does 2 coats double the paint?
Nearly - the second coat uses slightly less because the primed first coat seals the surface, but planning at double is the safe budget. Dark over light or light over dark colors may want three; the calculator's coats dropdown covers both.
How do I account for doors and windows?
Subtract 21 sq ft per standard door and 12 per standard window from wall area before dividing by coverage - or enter exact counts and let the calculator do it. Painting the doors themselves? Add one door's worth back per door painted.
What about primer and ceiling?
Primer covers more per gallon (roughly 400-500 sq ft) but budget it the same way per coat. Ceilings are their own area: length × width, same coverage math - and ceilings drink paint on textured finishes, so round up harder there.