Paint Calculator
Find how many gallons of paint a room needs from its size, doors, windows, and coats, using standard 350 sq ft coverage.
Learn how it works: How Much Paint Do I Need?Assumes 350 sq ft of coverage per gallon, per coat. Doors are counted at 21 sq ft and windows at 15 sq ft.
Enter the room size to find the gallons of paint.
How to use the paint calculator
Enter the room length, width, and wall height, then set how many doors and windows the room has so their area can be left out. Choose the number of coats, and tick the ceiling box if you plan to paint it too. The gallon total updates as you go.
The wall height defaults to eight feet, the most common ceiling height in homes, and coats default to two. The result rounds up to whole gallons, since paint is sold by the can and a little left over is better than running dry mid-wall.
How much wall does a gallon of paint cover
A gallon of interior wall paint covers about 350 square feet in one coat, the figure most manufacturers print on the can. Rough, porous, or previously unpainted surfaces drink up more, so treat 350 as a smooth- wall ceiling rather than a guarantee.
The calculator multiplies the paintable area by the number of coats before dividing by that coverage. A 12 by 12 foot room with eight-foot walls, one door, and one window has about 348 square feet to paint, so two coats land just under two gallons.
Why doors and windows are subtracted
You do not paint the glass or the door slab in a wall paint estimate, so their area comes off the total. The tool uses 21 square feet for a standard three by seven foot door and 15 square feet for a typical window, which keeps the math honest without fiddly measuring.
If your openings are much larger or smaller, adjust the counts to get a closer figure. A wall of patio doors, for example, removes far more area than a single interior door, so add door counts to approximate the opening.
Primer and the case for two coats
Two coats are standard for an even finish, especially over a new color or a patched wall. A single coat often leaves the old shade ghosting through, which is why the calculator defaults to two and most pros plan for the same.
Bare drywall, raw wood, or a drastic color change calls for primer first. Primer is its own product with its own coverage, so estimate it as a separate one-coat job rather than folding it into the topcoat gallons.
Frequently asked questions
- How much paint do I need for a 12x12 room?
- A 12 by 12 foot room with eight-foot walls has about 384 square feet of wall. After subtracting one door and one window, roughly 348 square feet remain, so two coats need close to two gallons at 350 sq ft per gallon.
- How many square feet does a gallon of paint cover?
- About 350 square feet per coat on a smooth, sealed wall. Rough or unpainted surfaces absorb more and cover less, so use 350 as a guide and buy a little extra for touch-ups.
- Do I need one coat or two?
- Two coats are the standard for an even, durable finish, particularly when changing color or covering repairs. The calculator defaults to two, and you can set it to one for a simple refresh in the same shade.
- Should I include primer in the estimate?
- Primer is a separate product with its own coverage, so estimate it as an extra single coat over bare drywall, raw wood, or a big color change. It is not counted in the topcoat gallons here.
- Does the calculator include the ceiling?
- Only if you tick the ceiling box, which adds the floor area to the paintable total. Leave it unchecked to estimate walls alone, which is the common case when the ceiling is staying white.