About the Paint Calculator
This paint calculator estimates how much paint you need to cover a room. Enter the room length, width and wall height, the number of doors and windows, how many coats you plan to apply and the coverage listed on the can. It returns the paintable area, the exact gallons required, the number of gallon cans to buy and the cost.
It is designed for DIY painters and contractors quoting interior jobs. Turn on the ceiling option to include it in the same total, or run the calculator twice if the ceiling uses a different paint.
Most interior paints cover about 350–400 square feet per gallon per coat on smooth, primed walls. Rough or porous surfaces, fresh drywall and big color changes use more, so two coats is the standard assumption. Doors are deducted as 21 sq ft and windows as 15 sq ft each.
With the default inputs, the paint needed is 1.9 gal. Change any value above to recalculate instantly.
How to use the paint calculator
- 1Measure the room length, width and wall height.
- 2Enter how many doors and windows are in the walls.
- 3Include the ceiling if you are painting it the same color.
- 4Choose the number of coats and enter the coverage from the paint can.
- 5Buy the number of cans shown; keep leftovers for touch-ups.
Formula and method
Wall area is the room perimeter, 2 × (length + width), times the wall height. Standard door (3 × 7 ft = 21 sq ft) and window (3 × 5 ft = 15 sq ft) openings are subtracted, and the ceiling area (length × width) is added if selected.
The paintable area is multiplied by the number of coats and divided by the manufacturer’s spread rate in square feet per gallon to give gallons. Cans are rounded up to whole gallons. Trim, doors themselves and primer are not included — estimate those separately.
- L, W, H
- Room length, width and wall height (ft)
- coverage
- Square feet one gallon covers per coat
Worked examples
12 × 12 ft bedroom, two coats
The walls are 2 × 24 × 8 = 384 sq ft; subtracting one door (21) and two windows (30) leaves 333 sq ft. Two coats is 666 sq ft ÷ 350 = 1.9 gallons, so two gallon cans are enough.
14 × 12 ft living room with ceiling, 9 ft walls
Walls are 2 × 26 × 9 = 468 sq ft, minus 42 for doors and 45 for windows = 381, plus a 168 sq ft ceiling = 549 sq ft. Two coats at 400 sq ft per gallon need 2.75 gallons, so buy 3.
One refresh coat in a 10 × 10 ft room
320 sq ft of wall minus a door and a window leaves 284 sq ft. One coat at 350 sq ft per gallon needs 0.81 gallons — a single gallon can.
Frequently asked questions
How much paint do I need for a 12 × 12 room?+
With 8 ft walls, one door and two windows, a 12 × 12 room has about 333 sq ft of wall. Two coats need roughly 1.9 gallons, so two gallons will do; add about 0.8 gallon for the ceiling.
How many square feet does a gallon of paint cover?+
Most interior paints cover about 350–400 sq ft per gallon on smooth, primed surfaces. Textured walls, bare drywall and masonry absorb more and can drop coverage to 200–300 sq ft.
Do I need two coats of paint?+
Two coats are standard for an even finish and true color, especially when changing colors. One coat can be enough when repainting the same color over a clean, sound surface.
Do I need primer?+
Prime new drywall, patched areas, stains and glossy surfaces, and when making a dramatic color change. Paint-and-primer products can cover some situations, but bare drywall still benefits from a dedicated primer.
How much paint do I need for trim and doors?+
Trim uses far less — a quart often covers the baseboards and casings of an average room, and a quart covers about two sides of a standard door. Estimate trim separately from the walls.