Paint Calculator
This paint calculator tells you how many gallons of wall paint a room needs. Enter the room's length, width and wall height, the number of doors and windows, how many coats you plan, and the coverage printed on your can. You get the paintable area, the gallons of paint and the number of cans to buy, with or without the ceiling.
Updated
Results
- Paintable areaWalls minus openings, plus ceiling if ticked
- Gross wall areaBefore deducting doors and windows
- Doors and windowsArea deducted
Results are estimates. See the methodology below.
How to use the paint calculator
- Keep Imperial (ft) selected and enter the room's length and width measured along the floor.
- Enter the wall height from floor to ceiling. Standard rooms are often 8 ft; measure if you have 9 ft or vaulted ceilings.
- Enter the number of doors and windows. The calculator deducts a typical size for each.
- Tick Include ceiling if you are painting it with the same paint.
- Choose the number of coats: two is the norm for a color change or new drywall.
- Open Advanced options to change the coverage rate (default 350 sq ft per gallon) and the can size (default 1 gallon; use 5 for a bucket).
How the calculator works
- Wall area = 2 × (length + width) × height
- Openings = doors × 20 sq ft + windows × 15 sq ft
- Paintable area = wall area − openings (+ length × width if the ceiling is included)
- Paint = paintable area × coats ÷ coverage per gallon
- Cans = paint ÷ can size, rounded up to a whole can
The 20 sq ft door and 15 sq ft window allowances are averages. Large patio doors, picture windows or an open archway take out more wall; subtract them yourself by lowering the length, or count them as two openings.
Where the coverage number comes from
Coverage is the area one gallon covers in one coat on a smooth, primed wall. Behr, for example, lists its Premium Plus interior flat at 250–400 sq ft per gallon, depending on the application method and the porosity of the surface. The calculator uses 350 as a middle value; replace it with the figure on your product's label or data sheet. Use the low end of the range for textured walls, fresh drywall or a roller with a thick nap.
Worked examples
Bedroom, 12 × 10 ft with 8 ft walls
One door, two windows, two coats at 350 sq ft per gallon. The walls total 352 sq ft; the openings remove 50 sq ft, leaving 302 sq ft. Two coats need 1.73 gallons, so you buy 2 gallons. Add the ceiling and the area grows to 422 sq ft: 2.41 gallons, or 3 cans.
Living room, 16 × 14 ft with 9 ft walls
Two doors, three windows, two coats with a paint rated at 400 sq ft per gallon. The wall area is 540 sq ft, the openings 85 sq ft, the paintable area 455 sq ft. You need 2.28 gallons: 3 one-gallon cans, or a single 5-gallon bucket if you set the can size to 5 and want leftovers for touch-ups.
Reading the result
The gallon figure is the paint that ends up on the wall. It doesn't include what stays in the tray, the roller and the bucket, so a little extra is normal. If the result lands just above a whole gallon, a quart can be enough to cover the gap; if it lands just below, buy the whole can and keep the rest for touch-ups.
Primer is a separate product. If you prime new drywall or patched areas, run the calculator again with one coat and the primer's own coverage rate. Trim, doors and cabinets usually take a different sheen, so measure them separately.
Frequently asked questions
How much area does a gallon of paint cover?
Most interior wall paints state roughly 250 to 400 sq ft per gallon per coat. The exact number is on the label or the product data sheet, and it drops on rough or porous surfaces.
Do I need one coat or two?
Plan two coats for new drywall, a strong color change or a light color over a dark one. A single coat can be enough when you repaint the same color over a clean, sound finish.
How much paint do I need for a 12 × 12 room?
With 8 ft walls, one door, one window and two coats at 350 sq ft per gallon, the calculator gives 349 sq ft of wall and about 2 gallons. Enter your own openings and coverage for an exact answer.
Should I subtract doors and windows?
Yes. Each standard door is about 20 sq ft and each window about 15 sq ft. In a small room with several openings, skipping the deduction can mean buying a gallon you don't need.
Is it cheaper to buy a 5-gallon bucket?
Prices vary by brand and store, so compare the price per gallon. A bucket only makes sense if the calculator shows you need four gallons or more of the same color and sheen.
My house was built before 1978. Anything to know?
Older homes may have lead-based paint. The EPA notes that sanding or scraping it creates hazardous dust, and paid contractors working on pre-1978 homes must be lead-safe certified.
Sources
This is an estimate based on average opening sizes and the coverage you enter. Always check the coverage and recommended number of coats on the product label.