This paint calculator converts your room dimensions into gallons of paint. It computes total wall area from the room’s length, width, and ceiling height, subtracts standard allowances for doors (21 sq ft) and windows (15 sq ft), multiplies by your number of coats, and divides by the industry-standard coverage of 350 square feet per gallon.
One gallon sounds like a lot until the second coat: a typical 12 × 10 ft bedroom needs about 1.6 gallons for two coats, so you would buy two gallons. Buying the right amount in one trip also matters for color consistency — gallons tinted in separate batches can differ slightly, which shows on a large wall.
The Formula
Wall area = 2 × (length + width) × ceiling height. Then:
- Subtract 21 sq ft per door and 15 sq ft per window
- Multiply by the number of coats
- Divide by 350 sq ft per gallon
Example: a 12 × 10 ft room with 8-ft ceilings has 2 × 22 × 8 = 352 sq ft of wall. Subtract one door and two windows (21 + 30 = 51 sq ft) to get 301 sq ft. Two coats is 602 sq ft of coverage, and 602 ÷ 350 = 1.72 gallons — buy two gallons.
How Many Coats Do You Need?
The coat count changes the answer more than any other input:
- 1 coat: repainting with the same (or very similar) color on a sealed surface
- 2 coats: the standard for a normal color change — this is what paint manufacturers assume for their color accuracy
- 3 coats: bare drywall, going from dark to light, or saturated accent colors like deep red, which have weak-hiding pigments
A quality paint-and-primer product can sometimes cover in one fewer coat, but on new drywall a dedicated primer coat is cheaper than an extra coat of finish paint and seals the surface so the topcoat spreads at its rated coverage.
Buying Tips
Practical rules that save a second store trip:
- Round up to whole gallons; use the remainder for touch-ups (label the can with the room name and date)
- A gallon covers about 350 sq ft; a quart covers about 87 sq ft — two quarts cost nearly as much as a gallon, so buy the gallon if you need more than one quart
- Ceilings use dedicated ceiling paint (flatter sheen, spatter-resistant) — calculate it separately, which this tool does
- Trim, doors, and baseboards use semi-gloss and are estimated separately: one quart handles the trim in a typical room
- Textured or porous surfaces (stucco, brick, bare wood) cut coverage to 250 sq ft per gallon or less
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12 × 12 ft room with 8-ft ceilings has 384 sq ft of wall before openings. With one door and two windows (51 sq ft), that is 333 sq ft — about 1.9 gallons for two coats. Buy two gallons of wall paint, plus a quart of semi-gloss for the trim.
How much area does a gallon of paint cover?
Around 350 square feet per coat on a smooth, primed surface — that is the number manufacturers print on the can. Textured walls, bare drywall, and porous surfaces absorb more, dropping real-world coverage to 250–300 sq ft. A quart covers about 87 sq ft.
Do I really need two coats of paint?
For any real color change, yes. One coat almost always shows roller marks and the old color underneath, especially in raking light. The exceptions are repainting in the same color or using high-end paint-and-primer over a well-sealed, similar-colored wall.
Should I subtract windows and doors from the wall area?
Yes — the standard allowances are 21 sq ft per door (3 × 7 ft) and 15 sq ft per window (3 × 5 ft). This calculator subtracts them automatically. For a wall of glass or a large picture window, measure the actual opening instead and subtract it from your wall area.
How much paint do I need for the ceiling?
Ceiling area is simply length × width — a 12 × 10 ft room has a 120 sq ft ceiling, so one coat needs about a third of a gallon. Use dedicated ceiling paint rather than wall paint: it is formulated flatter to hide imperfections and spatters less when rolling overhead.