Paint Calculator: How Much Paint Do I Need?
Multiply the room's perimeter by the wall height, subtract doors and windows, multiply by the coats and divide by the paint's coverage. A 12 × 12 ft room with 8 ft walls, a door and two windows is 333 sq ft; two coats at 350 sq ft per gallon needs 1.9 gallons, so buy 2.
Paint to buy
2 gallons
Exact amount
1.9 gal
Wall area
333 sq ft
Total to cover
666 sq ft
Primer over new drywall, bare wood or a big color change usually needs its own coat.
Show the math
- 1
Wall area = perimeter × height
2 × (12 + 12) × 8 = 384 sq ft
The perimeter is the total length of all four walls.
= 384 sq ft
Read the steps as text
- Wall area = perimeter × height. 2 × (12 + 12) × 8 = 384 sq ft The perimeter is the total length of all four walls.
- Subtract doors and windows. 384 − 1 × 21 − 2 × 15 = 333 sq ft Using a standard door of 21 sq ft and window of 15 sq ft.
- Multiply by 2 coats. 333 × 2 = 666 sq ft Two coats is standard for an even finish, especially over a different color.
- Divide by the paint's coverage. 666 ÷ 350 sq ft per gallon = 1.9 gal Check the can: most interior paint covers 350–400 sq ft per gallon (about 9–10 m² per liter) on smooth walls. Rough or porous surfaces need more.
Working out the wall area
For a rectangular room, the total wall area is the perimeter (twice the length plus twice the width) times the wall height. A 12 × 12 ft room with 8 ft walls has 2 × (12 + 12) × 8 = 384 sq ft of wall.
Doors and windows don't get painted, so their area comes off: the calculator uses a standard 21 sq ft door (3 × 7 ft) and 15 sq ft window (3 × 5 ft), or 1.9 m² and 1.4 m² in metric. Very large windows or sliding doors are worth subtracting separately.
Coats, coverage and how much to buy
Most interior paint covers 350 to 400 sq ft per gallon (about 9 to 10 m² per liter) on smooth, primed walls. Textured, porous or freshly plastered walls soak up more. Two coats is standard for even color, especially over a different shade.
In US units the result is rounded up to the gallons and quarts you can actually buy. Three quarts cost about as much as a gallon, so the calculator suggests the gallon instead. Leftover paint is handy for touch-ups.
Frequently asked questions
- How much paint do I need for a 12 × 12 room?
- With 8 ft walls, a door, two windows and two coats, about 1.9 gallons, so 2 gallons. Add the ceiling and you'll need about 3 gallons in total.
- How many square feet does a gallon of paint cover?
- Typically 350 to 400 sq ft per coat on smooth walls. Check the can, since coverage varies by product and surface.
- Do I need primer?
- Usually on new drywall, bare wood, stains or when making a big color change. Primer is a separate coat with its own coverage figure.
- Should I paint the ceiling the same color?
- Ceilings are usually flat white ceiling paint. Tick 'Paint the ceiling too' to include its area, but buy ceiling paint separately if it's a different product.