Paint Calculator
This paint calculator answers “how much paint do I need” for any room: enter length, width and ceiling height to get gallons and liters for one or more coats. It subtracts standard door and window areas, rounds up to the next quart as your buffer, and can add ceiling paint and a rule-of-thumb trim estimate.
Paint needed
1.75gallons
- In liters6.6 L
- Paintable area301 sq ft
- Exact before round-up (your buffer)1.72 gal
Formulas last reviewed on .
How the gallons are calculated
The calculator measures your paintable wall area, multiplies by coats, and divides by how far a gallon spreads:
gallons = paintable × coats ÷ coverage, rounded up to the next ¼ gallon
Two times length-plus-width is the room perimeter; times height, that’s your wall area. Each door credits back 21 sq ft (a 3 × 7 slab) and each window 15 sq ft, since you cut in around them instead of rolling over them. The ceiling, if toggled on, adds length × width.
The default coverage of 350 sq ft per gallon is the honest middle of manufacturers’ 250–400 claims — drop toward 250 for bare drywall, masonry or heavy texture, and push toward 400 only for smooth, previously painted walls. It’s editable because the surface matters more than the brand. Results round up to the next quarter gallon, which is both how paint is actually sold and your built-in buffer, and every result shows liters alongside gallons (1 gal = 3.785 L).
Worked example: two coats in a 12 × 10 bedroom
Say you’re repainting a 12 × 10 ft guest bedroom with 8-foot ceilings from beige to white — two coats, one door, two windows, ceiling and trim staying as they are. These are the calculator’s default inputs, so the result box above shows this exact room.
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft.
- Opening credits: one door (−21) and two windows (−30) leave 301 sq ft of paintable wall.
- Two coats: 301 × 2 = 602 sq ft of spreading to do.
- Divide by coverage: 602 ÷ 350 = 1.72 gallons exactly.
- Round up to the quart: 1.75 gallons — about 6.6 liters.
In the store that’s one gallon plus three quarts, but quarts cost nearly gallon money per ounce, so most people rightly buy two gallons. The quarter-gallon round-up already covers roller and tray losses; the honest 1.72 figure is shown separately so you can see how close to the line you are before deciding.
How much paint do I need for a 10×10 to 20×20 room?
Gallons below are for walls only with 8-foot ceilings, one door, two windows and 350 sq ft per gallon coverage — this calculator’s defaults — rounded up to the quart the way the tool sells it back to you.
| Room size | Paintable area | 1 coat | 2 coats |
|---|---|---|---|
| 10 × 10 ft | 269 sq ft | 1 gal | 1.75 gal |
| 10 × 12 ft | 301 sq ft | 1 gal | 1.75 gal |
| 12 × 12 ft | 333 sq ft | 1 gal | 2 gal |
| 12 × 16 ft | 397 sq ft | 1.25 gal | 2.5 gal |
| 14 × 16 ft | 429 sq ft | 1.25 gal | 2.5 gal |
| 16 × 20 ft | 525 sq ft | 1.5 gal | 3 gal |
| 20 × 20 ft | 589 sq ft | 1.75 gal | 3.5 gal |
Two coats never quite doubles the one-coat figure once rounding settles out, but it’s close — budget as if it does. If your room falls between rows, run your real numbers above rather than interpolating: the quart round-up means a foot of extra wall can tip you into the next size up.
Frequently asked questions
How many coats of paint do I need to cover a dark color?
Going light over dark, plan on a tinted primer coat plus two finish coats — entering 3 coats in the calculator approximates that total nicely. Deep reds and navy are the stubborn ones; a gray-tinted primer under the new color kills them faster than extra topcoats do. Two coats is the right default for ordinary color changes, and one-coat claims on the can only hold when you’re repainting close to the same color on a smooth wall.
Do I need to buy primer separately from wall paint?
For bare drywall, fresh joint compound, stains or a drastic color change — yes, a dedicated primer is cheaper and works better than extra finish coats. Estimate it by running this calculator at 1 coat with coverage around 300 sq ft per gallon, since primer spreads a little thinner. Paint-and-primer-in-one products are fine for repainting a similar color on sound, previously painted walls, which is exactly the situation where you didn’t really need primer anyway.
How many square feet does a gallon of paint really cover?
Labels say 350–400 sq ft, and that’s achievable on smooth, sealed, previously painted walls with a proper 3/8-inch roller nap. Real-world first coats on bare drywall, skim coats or textured surfaces land closer to 250–300, which is why this calculator lets you edit coverage from 250 to 400 instead of hard-coding the marketing number. When in doubt, use 350 for repaints and 300 for anything thirsty — the quart round-up absorbs small misses.
How much paint do I need for baseboards, doors and trim?
The trade rule of thumb is about one quart per 250 linear feet of standard trim per coat, and that’s what the trim toggle applies — using your room perimeter as a stand-in for baseboard length. A typical bedroom’s baseboard alone is 40–50 linear feet, so a quart of trim enamel usually handles the baseboards, casings and a door with room to spare. Count each paneled door as roughly 20 sq ft per side if you’re painting several.
Should I include the ceiling in my wall paint estimate?
Only if it’s getting the same paint — the toggle adds length × width to the area, which is right for a color-drenched room done wall color throughout. Most ceilings, though, get dedicated flat ceiling paint, which hides roller marks far better than wall eggshell. In that case leave the toggle off and run the calculator a second time at your ceiling’s area using the override field, 1 or 2 coats depending on the stains.
These results are planning estimates only — verify quantities with your supplier and local building code before you buy or build. See our terms of use.