How the paint calculator works
Paint is bought by the gallon, but it is used by the square foot. So the calculator finds the wall area you will actually paint, multiplies it by the number of coats, and divides by how far one gallon spreads.
Wall area = 2 × (length + width) × height − doors − windows
Gallons = wall area × coats ÷ coverage per gallon
Each door counts as 21 sq ft (3 by 7 feet) and each window as 15 sq ft (3 by 5 feet). If your doors or windows are much bigger, such as a patio slider, add an extra door or two to the count.
Worked example
A 14 by 12 foot bedroom with 8 foot walls has a perimeter of 52 feet, so 416 sq ft of wall. One door and two windows remove 51 sq ft, leaving 365 sq ft. Two coats at 350 sq ft per gallon need 2.09 gallons, which means 2 gallons plus 1 quart. If the leftover is more than a quarter gallon, the calculator rounds up to another full gallon, because two or more quarts usually cost more than a gallon.
Measuring your room
- Measure the length and width of the floor along the walls, ignoring baseboards.
- Measure wall height from floor to ceiling. Standard ceilings are 8 feet; many newer homes have 9 feet.
- Count doors and windows on the walls you are painting. Closets with doors count as a door.
- For a room with angled or uneven walls, add up the widths of all the walls, divide by two, and enter that as the length with a width of 0. The calculator then uses your exact total wall length.
What affects how much paint you use
| Surface | Effect on coverage |
|---|---|
| Smooth, previously painted wall, similar color | Close to the label's top figure |
| Textured walls (orange peel, knockdown) | Uses noticeably more; lower the coverage to about 300 |
| New, unprimed drywall | Soaks up paint; prime first |
| Dark color to light color | Expect a third coat or primer |
| Roller with thick nap on smooth walls | Holds more paint; use 3/8 in nap for smooth walls |
Before you buy
- Buy all the paint for one color at the same time and ask the store to mix it together ("box" it) so every can matches.
- Keep a little leftover in a sealed container, labeled with the color name and room, for touch-ups.
- Ceilings are usually painted in flat white ceiling paint, so the calculator lists it separately from wall paint.
- Trim and doors need a separate semi-gloss paint; one quart covers the trim in most rooms.
Common questions
How much paint do I need for a 12x12 room?
A 12 by 12 foot room with 8 foot walls has 384 sq ft of wall. Take off one door and one window (about 36 sq ft) and two coats need roughly 2 gallons at 350 sq ft per gallon. Add a gallon if you are also painting the ceiling.
How many square feet does a gallon of paint cover?
Most interior paint labels say 250 to 400 sq ft per gallon for one coat. Smooth, primed walls get close to the top of that range; textured, porous or unpainted drywall uses more. This calculator uses 350 by default, and you can change it.
Do I really need two coats?
Usually yes. One coat often looks patchy, especially with a color change. Two coats give even color and the durability the manufacturer rates the paint for.
Should I buy a 5-gallon bucket?
If you need more than about 4 gallons of one color, a 5-gallon bucket is normally cheaper per gallon and keeps the color consistent across walls.
When do I need primer?
Prime new drywall, patched areas, stained walls, glossy surfaces, and big jumps from dark to light colors. Clean walls already painted a similar color often need no primer.