Paint Calculator
Work out exactly how many gallons of paint a room needs, accounting for doors, windows, ceiling height and the number of coats you plan to apply.
8 ft is standard; 9 ft is common in newer homes.
Based on 350 sq ft per gallon per coat, which is typical for interior latex on a primed surface. Deep colors over light walls, or bare drywall, cover less — add a coat if you are making a big color change.
Measure once, buy once
Paint is the project where people most often make a second trip to the store, usually because they estimated from the floor area rather than the wall area.
What matters is the perimeter times the ceiling height — a long, narrow room has considerably more wall than a square room of the same square footage.
Why two coats is the realistic default
One coat is enough only when you are repainting the same color over a finish in good condition. In practice:
- Color change — two coats, minimum.
- Light over dark — two coats plus primer, or a paint-and-primer product and possibly three.
- New or patched drywall — primer first; bare drywall drinks paint.
- Bathrooms and kitchens — two coats of a scrubbable finish.
Set coats to 2 unless you have a specific reason not to. The calculator multiplies the surface area accordingly.
Ceiling height is the input people get wrong
Eight feet is standard in older American homes. Nine and ten feet are common in anything built recently, and that difference alone changes wall area by 12–25%.
Measure it rather than assuming. A room you thought needed two gallons may need three.
Buying practical quantities
Paint comes in quarts and gallons, so the answer gets rounded up — you cannot buy 2.3 gallons.
If the calculation lands just over a whole gallon, buying the extra gallon rather than a gallon plus a quart is often barely more expensive and leaves you with touch-up paint in the exact color. And write the room name and date on the lid; in three years you will not remember which can matched which wall.
Also budget for
- Primer, if you are covering a dark color, bare drywall or stains. Estimate it as its own coat.
- Trim and doors, which use a different finish and are not included here.
- Tape, drop cloths, brushes, a roller and a tray — commonly $40–80 for a room if you are starting from nothing.
Leftover paint should not go in the trash while it is still liquid. Most US counties run household hazardous waste drop-offs, and many paint retailers accept returns for recycling.
How this is calculated
Wall area = 2 × (length + width) × ceiling height Openings = (doors × 21 sq ft) + (windows × 15 sq ft) Ceiling area = length × width (if painting the ceiling) Paintable = wall area − openings + ceiling area Gallons = (paintable × coats) ÷ 350 sq ft per gallon
Frequently asked questions
- How many square feet does a gallon of paint cover?
- About 350 square feet for one coat on a primed, smooth surface. Textured walls, bare drywall and deep colors all reduce that — sometimes to 250 square feet. The 350 figure is a reasonable planning number, not a guarantee.
- Do I need one coat or two?
- Two, in almost every real situation. One coat works only when repainting the same color over a sound existing finish. Any color change, any patched drywall, and any light-over-dark job needs two, and dramatic changes may need primer plus two.
- Should I subtract doors and windows?
- For a normal room the difference is small, but this calculator does it because on a room with several large windows it can be the difference between two gallons and three. It uses standard sizes — roughly 21 square feet per door and 15 per window.
- Should I buy extra paint?
- Buy a little more than you need. Matching a color later is genuinely difficult even with the same formula, because dye lots vary. A leftover quart for touch-ups is cheap insurance, and unopened cans can usually be returned.
- Does ceiling paint differ from wall paint?
- Yes. Ceiling paint is formulated flatter and thicker so it hides imperfections and drips less overhead. If you are painting the ceiling, buy ceiling paint rather than using the wall color.