Paint quantity calculator
Work out how much paint a room needs from its size, the number of coats and the paint's coverage, then turn it into cans and cost.
Assumes smooth interior walls and one-room walls only; ceiling and trim not included.
How this calculator works
Paint quantity is wall area divided by coverage, multiplied by the number of coats. Wall area is the perimeter of the room (2 × length + 2 × width) times the ceiling height, minus the area of doors and windows that will not be painted. A standard interior door is about 20 sq ft including the frame and a window about 15 sq ft, which is why the default allowance is 40 sq ft.
Coverage matters more than people expect. Most paints state 350–400 sq ft per gallon on smooth, primed drywall, but porous or textured surfaces absorb much more, and a drastic colour change may need three coats rather than two. Buy the next whole can up from the calculated amount: leftover paint is useful for touch-ups, whereas a shortfall means a second trip and a possible colour-batch mismatch. Ceiling paint and primer are separate calculations — add the ceiling area if you are painting it too.
The formula
wall area = 2 × (length + width) × height − openings
gallons = wall area × coats / coverage per gallon
cans = ceil(gallons)
cost = cans × price per gallonFor a 12 × 15 ft room with 8 ft walls: 2 × 27 × 8 = 432 sq ft, minus 40 sq ft of openings = 392 sq ft. Two coats at 350 sq ft/gallon needs 2.24 gallons, so buy 3 cans.
Worked example
A 12 × 15 ft room with 8 ft walls has 392 sq ft of paintable wall after openings. At 2 coats and 350 sq ft per gallon you need 2.24 gallons, so buy 3 cans — about $120.00 at 40 per gallon.
Common mistakes
- Forgetting the second coat. One coat of paint over a different colour rarely covers evenly; two coats is standard and some colours need three.
- Using ceiling square footage instead of wall area. Paint quantity depends on wall area — perimeter times height — not floor area.
- Not subtracting openings. Doors and windows can remove 10–15% of the wall area, which is often most of a can.
Frequently asked questions
How much paint do I need for a 12x15 room?
With 8 ft walls that is about 392 sq ft of paintable wall. Two coats at 350 sq ft per gallon needs 2.24 gallons, so buy 3 gallons.
How many square feet does a gallon of paint cover?
Usually 350–400 sq ft per gallon on smooth, primed drywall. Textured or porous surfaces and colour changes reduce coverage.
Do I need to add primer to the calculation?
Primer is a separate coat with its own coverage, usually similar to paint. Add it as an extra coat when painting bare drywall, a strong colour or a repaired surface.
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