How much paint do I need for a room?
Short answer
Measure the walls, subtract the doors and windows, then divide by how far a gallon goes. A standard 12 by 12 room with 8 ft ceilings, painted in two coats, works out to about 2 gallons. Most gallons cover 350 to 400 square feet, so your number depends on room size, how many openings you have, and whether you do one coat or two. These are estimates for shopping, not exact amounts.
Paint quantity formula
Wall area = 2 x (length + width) x height; Paintable area = wall area - (20 x doors) - (15 x windows); Paint (gallons) = (paintable area x coats) / coverage per gallon
- •length and width are the room's floor dimensions in feet
- •height is the floor-to-ceiling measurement in feet
- •doors and windows are the counts of each, at about 20 and 15 sq ft each
- •coats is 1 or 2, usually 2 for an even finish
- •coverage per gallon is 350 to 400 sq ft; use 350 to be safe
Start with the wall area
Paint is measured by area, so the first job is finding how many square feet of wall you have. You do not measure each wall separately unless the room is an odd shape. Walk the perimeter and use the ceiling height.
Measure the length and width of the floor in feet, add them, double that for the full perimeter, then multiply by ceiling height.
- Wall area = perimeter x ceiling height = 2 x (length + width) x height
- Length and width are the floor dimensions in feet
- Height is floor to ceiling, usually 8, 9, or 10 feet
Subtract doors and windows
You do not paint glass or a door slab, so that area comes off the total. Instead of measuring every opening exactly, most estimates use round figures that are close enough for buying paint: about 20 square feet per standard door and 15 square feet per average window.
Add up your openings, subtract them from the wall area, and what is left is your paintable area. For oversized windows or double doors, measure those individually and use the real number instead of the round one.
Account for coats and coverage
A gallon of interior paint typically covers 350 to 400 square feet on a smooth, previously painted wall. Textured walls, deep colors, and fresh drywall pull that toward the low end, so planning around 350 keeps you from running short.
Most rooms need two coats for an even finish, especially when changing color or covering patched spots. Multiply your paintable area by the number of coats before dividing by coverage.
- Paint (gallons) = (paintable area x number of coats) / coverage per gallon
- Use 350 square feet per gallon as a safe planning figure
- Plan on two coats unless you are refreshing the identical color
A worked example: 12 x 12 room, 8 ft ceilings
Here is the full calculation for a common bedroom size: a room 12 feet by 12 feet with 8 foot ceilings, one door, and one window.
Perimeter is 2 x (12 + 12) = 48 feet. Wall area is 48 x 8 = 384 square feet. Subtract one door at 20 and one window at 15, which removes 35 square feet, leaving 349 square feet of paintable wall. For two coats, double that to 698 square feet. At 350 square feet per gallon, that is about 2 gallons.
That two-gallon figure is an estimate, not a guarantee. Real usage shifts with roller technique, wall texture, and how thickly you apply each coat, so treat it as a shopping target.
Should you include the ceiling?
The wall formula above leaves out the ceiling. If you are painting it too, calculate it separately by multiplying length times width. For the 12 by 12 example, the ceiling is 144 square feet, well under one gallon.
Keep the ceiling on its own line because it usually gets a different product. Flat ceiling paint hides imperfections and resists spatter, and ceilings often stay white while walls take color. Bundling the two makes it easy to buy the wrong amount of the wrong finish.
Common mistakes
Most paint-buying errors fall into a few predictable traps. Knowing them ahead of time saves a return trip and keeps you from overspending.
- Forgetting the second coat: buying for one coat, then finding the color looks patchy, is the most common way to run out halfway through.
- Ignoring primer on new drywall: bare drywall soaks up the first coat and needs primer or a self-priming paint, and primer is a separate purchase, not part of your color-coat total.
- Underbuying to save money: coming up short mid-wall means a second can that may be tinted slightly differently, leaving a visible seam.
- Overbuying by ignoring the openings: skipping the door and window subtraction inflates the total and leaves you with cans you do not need.
- Assuming full coverage on deep colors: strong reds, blues, and dark shades often need an extra coat, so plan for it rather than discovering it on the wall.
Frequently Asked Questions
Do I count the ceiling in my paint total?+
Only if you are painting it, and it gets its own calculation. The perimeter-times-height formula covers walls only. For the ceiling, multiply length by width: a 12 by 12 room is 144 square feet, which is under one gallon. Keep it as a separate line because ceilings usually take flat ceiling paint in a different color or finish than the walls, so lumping the square footage together makes it easy to buy the wrong amount of the wrong product.
Does one gallon really cover 350 to 400 square feet?+
That is the range printed on most cans for a smooth, previously painted surface. You land near 350 on textured walls, deep or bold colors, and porous surfaces like fresh drywall that soak up paint. Rolling wastes more than spraying. Plan around 350 square feet per gallon so you are not caught short mid-wall, and treat 400 as a best case you may not hit.
Do I need primer, and does it change how much paint I buy?+
New drywall, bare patches, and big color jumps (dark to light especially) call for primer or a paint-and-primer product. Primer is a separate purchase with its own coverage, so budget for it on top of your color coats rather than counting it against them. On raw drywall the first coat soaks in fast, so priming seals the surface and stops you from burning through extra finish paint to get an even color.
How much paint for two coats versus one?+
Double the paintable area before dividing by coverage. In the worked example, 349 square feet becomes 698 for two coats, which is about 2 gallons at 350 square feet per gallon. One coat of that same room is roughly 1 gallon, but a single coat rarely looks even over patched or previously colored walls. Plan for two coats unless you are refreshing the exact same color.
Should I round up when the math lands between sizes?+
Round up to the nearest practical container. If your total is 1.3 gallons, buy two gallons, or a gallon plus a quart, rather than stretching one and hoping. Running out mid-project means a second store trip and the risk that the new can is tinted slightly differently, which can leave a visible seam. Leftover paint also comes in handy later for touch-ups on scuffs and nail holes.
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