Rooms from 10 x 10 to 20 x 15 ft need 2 to 3 gallons of paint for two coats on 8 ft walls. The rule of thumb our paint calculator starts from: one gallon covers about 350 sq ft per coat on smooth, previously painted walls, so divide your paintable wall area by 350 and multiply by the number of coats. A 12 x 10 ft bedroom with 8 ft walls works out to about 1.7 gallons, so you would buy 2.
Enter your room below and the calculator subtracts doors and windows, adds the ceiling if you want it, and rounds up to whole gallons.
How to measure a room for paint
You only need a tape measure and a few minutes. Measure to the nearest half foot and round up.
- Measure the room length and width. Run the tape along the floor from wall to wall in both directions. For an L-shaped room, measure every wall and add them up, then enter a length and width that add up to half that total (for 50 ft of wall, 15 and 10 works). Figure the ceiling separately in that case.
- Measure the wall height. Go from the floor to the ceiling, not to the top of the baseboard. Do not assume 8 ft: a 9 ft wall has 12.5 percent more surface than an 8 ft wall.
- Count the doors and windows. The calculator removes about 20 sq ft for each door and 15 sq ft for each window. A 36 x 80 in door is exactly 20 sq ft. Count a double closet door or a wide slider as two doors.
- Decide on the number of coats. Plan on two coats for most jobs. Plan on more if you are covering a dark color with a light one, or the other way around.
- Find the coverage on your paint. Look for a square-feet-per-gallon figure on the can or the product data sheet. If the label gives a range, use the low end for rough, textured, or porous walls.
- Decide whether the ceiling is included. If you are painting it with the same product, tick the ceiling box. If you are using a separate ceiling paint, run the numbers for it on their own.
The formula (with a worked example)
Paint estimates come down to two steps: find the paintable area, then turn that area into gallons.
- Wall area = 2 x (length + width) x height, minus 20 sq ft per door and 15 sq ft per window
- Paintable area = wall area, plus length x width if you are painting the ceiling
- Gallons = paintable area x coats / coverage per gallon, then round up to buy
Here is the calculator's default room worked out by hand: 12 ft long, 10 ft wide, 8 ft walls, 1 door, 2 windows, 2 coats, and paint that covers 350 sq ft per gallon.
- Perimeter: 2 x (12 + 10) = 44 ft
- Gross wall area: 44 x 8 = 352 sq ft
- Openings: (1 x 20) + (2 x 15) = 50 sq ft
- Paintable wall area: 352 - 50 = 302 sq ft
- Gallons: 302 x 2 coats / 350 = 1.73 gallons
The calculator shows "You need about 1.7 gallons. Buy 2 gallons." with a paintable area of 302 sq ft. If you tick the ceiling box, it adds 12 x 10 = 120 sq ft, for 422 sq ft in total. Two coats of that is about 2.4 gallons, so the answer becomes 3 gallons.
How many gallons of paint by room size
The chart below uses the same formula and assumptions as the calculator: 8 ft walls, one door, two windows, two coats, and 350 sq ft per gallon. Use it for a quick sanity check before you head to the store.
| Room size | Wall area | Walls only | Walls + ceiling |
|---|---|---|---|
| 10 x 10 ft | 270 sq ft | 1.5 gal (buy 2) | 2.1 gal (buy 3) |
| 12 x 10 ft | 302 sq ft | 1.7 gal (buy 2) | 2.4 gal (buy 3) |
| 12 x 12 ft | 334 sq ft | 1.9 gal (buy 2) | 2.7 gal (buy 3) |
| 14 x 12 ft | 366 sq ft | 2.1 gal (buy 3) | 3.1 gal (buy 4) |
| 16 x 14 ft | 430 sq ft | 2.5 gal (buy 3) | 3.7 gal (buy 4) |
| 20 x 15 ft | 510 sq ft | 2.9 gal (buy 3) | 4.6 gal (buy 5) |
Notice the rows that land just over a whole number, such as 2.1 gallons. Sherwin-Williams estimates a quart covers about 100 sq ft, so when the extra is small, a gallon count plus one quart may be enough instead of another full gallon.
Paint coverage per gallon by product
The coverage number is the biggest lever in the estimate, and it varies by product and surface. Here is what manufacturers and PaintCare publish:
| Product or guidance | Coverage (one coat) | Notes |
|---|---|---|
| PaintCare general guidance | 300 to 400 sq ft per gallon | Suggests 350 as a planning number when unknown |
| Sherwin-Williams general guidance | 350 to 400 sq ft per gallon | Varies with surface and application method |
| Behr Ultra and Premium Plus Interior | 250 to 400 sq ft per gallon | Less on porous or textured surfaces |
| Benjamin Moore Regal Select Matte | 400 to 450 sq ft per gallon | Listed on the product spec table |
| Glidden interior paint | Up to 400 sq ft per gallon | On primed, smooth, non-porous surfaces |
| Primer (Sherwin-Williams guidance) | 200 to 300 sq ft per gallon | Plan primer separately from paint |
| One quart of paint | About 100 sq ft | Handy for touch-ups and small add-ons |
Our calculator defaults to 350 sq ft per gallon because it sits in the middle of these ranges. If your label says 400 and your walls are smooth and already painted, type 400. If you are painting textured plaster or bare drywall, go lower.
What changes how much paint you need
Wall texture and porosity
Smooth, sealed walls let paint go the farthest. PaintCare notes that rough textures can take up to 50 percent more paint. Behr likewise lists porous, rough, and textured surfaces as ones that take more paint. On heavily textured walls or bare masonry, lower the coverage number before you calculate.
Color changes
Going from a similar shade to another is the easy case. Dark-to-light and light-to-dark changes can take three or even four coats, and PaintCare points out that some vivid colors, especially reds, oranges, and purples, can take more. A tinted primer as the first coat often cuts the number of finish coats; PaintCare specifically suggests a gray primer under red.
New or unpainted drywall
Bare drywall is both porous and previously unpainted, two of the conditions Behr flags as needing more paint. Prime it first, and budget primer at its own coverage rate. Sherwin-Williams puts primer at about 200 to 300 sq ft per gallon, so a room that needs 302 sq ft of walls will need roughly one to one and a half gallons of primer on top of the paint.
Trim, doors, and ceilings
Trim is often painted in a different sheen or color, so estimate it separately. Behr suggests multiplying the total length of your trim by 0.5 ft to get its approximate square footage. If you need the length, our baseboard calculator gives you the linear feet for a room.
Application method
Sherwin-Williams notes that paint requirements vary with the application method as well as the surface type and condition. If you plan to spray, check the product data sheet for its application notes and round your estimate up rather than down.
Common mistakes to avoid
- Using floor area instead of wall area. A 12 x 10 room has 120 sq ft of floor but over 300 sq ft of wall. Multiply the perimeter by the height.
- Forgetting the second coat. A single-coat estimate is half the paint you will actually use on most jobs.
- Taking the best-case coverage number. The top of a label's range assumes ideal surfaces. Use the low end on rough, patched, or unpainted walls.
- Measuring to the top of the baseboard. Measure floor to ceiling. The small overestimate from the baseboard strip is a buffer, not a problem.
- Ignoring big openings. A sliding patio door or a bank of windows removes far more than 20 or 15 sq ft. Count them as multiple doors or windows.
- Buying exactly the decimal. Round up. A little left in the can is what you will use for touch-ups later, and PaintCare suggests leftover paint can also serve as a base coat on another project.
Frequently asked questions
How many square feet does a gallon of paint cover?
Most interior paints cover about 300 to 400 sq ft per gallon for one coat, and 350 sq ft is a sensible planning number when you do not know the product. Some premium paints list more (Benjamin Moore Regal Select Matte lists 400 to 450 sq ft), while rough or porous walls get less. Always check the label or the product data sheet.
How much paint do I need for a 12 x 12 room?
With 8 ft walls, one door, and two windows, a 12 x 12 room has about 334 sq ft of paintable wall. Two coats at 350 sq ft per gallon come to about 1.9 gallons, so buy 2 gallons. Add the ceiling (144 sq ft) and the total rises to about 2.7 gallons, so buy 3.
Do I really need two coats of paint?
Two coats is the standard plan for even color and a uniform sheen, and it is what our calculator assumes by default. A fresh coat of the same color over sound paint can sometimes get by with one. Big color changes can take three or even four coats, which is why a tinted primer is often worth it.
How much paint do I need for a ceiling?
Multiply the room length by the width to get the ceiling area, then divide by the coverage on the can and multiply by the coats. A 12 x 10 ft ceiling is 120 sq ft, so two coats at 350 sq ft per gallon use about 0.7 gallon. Tick "Paint the ceiling too" in the calculator to add it automatically.
How much paint do I need for one accent wall?
A 12 ft wall that is 8 ft high is 96 sq ft. Two coats is 192 sq ft of coverage. Sherwin-Williams puts a quart at about 100 sq ft, so two quarts would be cutting it close; one gallon leaves room for a third coat or touch-ups.
Should I subtract doors and windows?
Yes, for anything bigger than a small room it makes a real difference. Our calculator removes about 20 sq ft per door and 15 sq ft per window. If you have a large patio door, picture window, or built-in shelving, measure that opening and treat it as extra doors or windows.