Paint Calculator
One gallon of paint covers about 350 square feet per coat on smooth walls. Enter your area and coats to get gallons — rounded up, because paint stores close.
Paint Calculator
About this paint calculator
Running out of paint mid-wall is how you end up with a visible lap line that haunts you for years. The fix is simple arithmetic: wall area × coats ÷ coverage per gallon, rounded up. Paint cans print their coverage — typically 350–400 sq ft per gallon on smooth, primed drywall — and this calculator uses your number or the 350 default.
Two coats is the honest default for most repaints. One coat works only when refreshing the same color with quality paint; three coats (or a primer coat plus two) is the reality when going from dark to light. Primer gets its own calculation if you are painting bare drywall or making a big color jump — run the numbers once for primer, once for topcoat.
Buy all your paint at once, and from the same batch if possible. Tint varies slightly between mixes, and a wall finished from two different gallons can show a faint line in raking light. One spare gallon for future touch-ups is cheap insurance.
Worked example: painting a 12×12 bedroom
A 12×12 ft room with 8-ft ceilings: perimeter 48 ft × 8 ft = 384 sq ft of gross wall area. Subtract one door (~21 sq ft) and one window (~15 sq ft): 348 sq ft of paintable wall. Two coats: 348 × 2 = 696 sq ft. At 350 sq ft per gallon: 696 ÷ 350 = 1.99, rounded up to 2 gallons.
Two gallons covers the walls with a little left for touch-ups — which is exactly what you want, since touching up with a different batch later never quite matches. Painting the ceiling too? That is another 144 sq ft × 2 coats ÷ 350 = 0.82, so one more gallon of ceiling paint.
Buying tips
Coverage ratings assume smooth, previously painted walls. Bare drywall, brick, and stucco drink 25–50% more paint on the first coat — prime raw surfaces first and the topcoats behave. For drastic color changes (dark to light especially), budget three coats, not two; the calculator lets you set that directly.
Buy all the gallons for a room at once and “box” them — mix the cans together in a 5-gallon bucket. Factory color drifts slightly between cans, and boxing is how pros eliminate the visible banding where one can ended and the next began.
How to use
- Measure each wall (length × height) and add them up; subtract large windows/doors if you want a tighter number.
- Enter the total square feet and choose your coats.
- Adjust coverage per gallon from the can if it differs from 350.
- Press Calculate — buy the rounded-up gallon count, all at once.
Good to know
- Textured, brick, or unprimed porous surfaces drink far more paint — add 20–30%.
- Ceilings need flat ceiling paint; the math is the same but buy a separate product.
- Sprayers waste more paint to overspray than rollers; pros add ~15% for spray application.
- One gallon covers about 350 sq ft per coat on smooth, previously painted walls — but bare drywall, brick, or stucco can drink 25–50% more on the first coat. Prime raw surfaces.
- Buy all gallons for a room at once and “box” them (mix together) — factory color varies slightly between cans, and boxing eliminates visible banding.
Frequently asked questions
How much paint do I need for a 12x12 room?
Four 12×8 walls = 384 sq ft. Two coats at 350 sq ft/gal = 2.2 gallons → buy 3 gallons. Subtract ~20 sq ft per door and ~15 per window if you want it tighter.
How many square feet does a gallon of paint cover?
About 350–400 sq ft per coat on smooth primed walls. Porous or textured surfaces drop that to 250–300.
Is one coat of paint enough?
Only for refreshing the same color with good paint. Color changes and bare drywall need primer plus two coats for an even finish.
How do you calculate paint for a room?
Add up wall areas (length × height), multiply by coats, divide by the can's coverage per gallon, and round up.
Should I buy extra paint for touch-ups?
Yes — one spare gallon (or at least a quart) stored properly lasts years and saves you from an impossible color match later.
How many gallons of paint do I need for a 12x12 room?
Walls are 384 sq ft (4 × 12 × 8); minus about 36 for a door and window = 348. Two coats = 696 sq ft ÷ 350 per gallon ≈ 2 gallons. Add 1 more gallon if you are painting the ceiling. Enter your room’s numbers above for an exact figure.
Does primer count as a coat?
No — primer is its own step. On bare drywall, fresh wood, or drastic color changes, prime first, then apply two topcoats. Skipping primer on raw drywall means the topcoat soaks in unevenly, and you will need a third coat of the expensive paint anyway.
Why do I need two coats if the first coat looks fine?
The first coat seals and bonds; the second builds uniform color and sheen. One-coat jobs flash — they look patchy in angled daylight — and touch-ups on a single coat never blend invisibly. Two coats is also what every paint warranty assumes.
How do I measure a room for paint?
Measure the perimeter in feet and multiply by wall height for gross wall area, then subtract about 21 sq ft per door and 15 per average window. A 12×12 room with 8-ft ceilings: 48 × 8 = 384 sq ft gross, about 348 net.