Surfaces & finishes · free tool
Paint Calculator
How much paint do you need? Enter your room, subtract doors and windows, and get the gallons for your coats — plus primer and cost.
Paint Calculator
How much paint you need — gallons, primer & cost.
1 Your space
2 Paint to buy
Estimates only — coverage varies with surface, colour and application; buy a little extra for touch-ups. See how we calculate · Updated monthly
Method · how it works
How to calculate how much paint you need
Wall area, minus your openings, times the number of coats, divided by what a gallon covers. Here is exactly what the calculator does.
Find the wall area
Add up the walls: 2 × (length + width) × height for a rectangular room.
Subtract openings
Take out doors (about 20 ft²) and windows (about 15 ft²) you won't paint.
Multiply by coats
Most walls need two coats — so multiply the paintable area by 2.
Divide by coverage
Divide by what a gallon covers (about 400 ft²) and round up to full gallons.
A 12 × 12 ft room, 8 ft ceilings
Less 1 door + 1 window = 349 ft²
349 × 2 coats = 698 ft²
698 ÷ 400 = 1.75 gal
2 coats, doors & windows out
Sources: manufacturer coverage (Sherwin-Williams, Behr) · PCA coverage standards (350-400 ft²/gal)
Coverage checked against manufacturer specs · Updated monthly
Estimates only. Coverage changes with surface texture, colour and how you apply it — buy a little extra so you have touch-up paint from the same batch.
Paint · by room size
How much paint for a room?
A quick guide at 2 coats, 8 ft ceilings, with doors and windows subtracted. For your exact room, use the calculator above.
Small, and broken up by fixtures — one gallon usually does it.
~5 x 8 ftNursery, office or small bedroom at two coats.
10 x 10 ftThe most common room size — two gallons for walls.
12 x 12 ftLarger walls push you into a third gallon.
14 x 16 ftGreat room or big primary bedroom at two coats.
18 x 18 ftOne extra gallon covers most room ceilings in one coat.
L x WGoing light-to-dark or dark-to-light? Add a coat (or a primer) — and always keep a little for touch-ups.
Coverage
How much does a gallon of paint cover?
A gallon covers roughly 350–400 sq ft per coat — but the surface makes a big difference. Match the rate to your walls in the calculator.
Previously painted, smooth drywall — paint goes furthest here.
A safe average for most interior walls in good shape.
Knockdown, orange-peel, bare or older surfaces soak up more.
Primers cover less — plan about 200–300 sq ft per gallon.
Spraying, rolling and brushing all cover differently, and deep or bright colours may need an extra coat. When in doubt, round up — leftover paint is handy for touch-ups.
Coats & primer
How many coats — and do you need primer?
Coats and primer are the two things that most change how much paint you buy, so the calculator lets you set both.
Coats
Two is the standard
Primer
When to add a coat of primer
Beyond the walls
Ceilings, trim & exterior
The wall estimate is the core, but a full project often adds a ceiling, trim, or an exterior. Here's how to figure each.
Ceilings
L x WA ceiling is just length × width — toggle it on in the calculator. Most take one coat of flat ceiling paint; about a gallon per average room.
Trim & doors
linear ft x heightEstimate trim by the perimeter (linear feet) times the trim height, converted to square feet. Doors and windows take surprisingly little — a quart often does a room.
Exterior
porous = moreMeasure walls the same way, but rough siding, stucco and masonry soak up much more — drop your coverage toward 250–300 and buy extra. Add gables as triangles.
For big or oddly shaped surfaces, work out the area first with the square footage calculator, then bring the total back here as a wall area.
Answers
Paint calculator FAQs
How much paint do I need for a 12x12 room?
About 2 gallons for the walls at two coats — a 12 x 12 room with 8 ft ceilings has roughly 349 sq ft of paintable wall after doors and windows.
How much does a gallon of paint cover?
Roughly 350 to 400 sq ft per coat on smooth walls. Textured or porous surfaces drop to about 300, and primer covers around 250.
How many coats of paint do I need?
Two coats for most walls. Use one for a same-colour refresh, and three for dramatic colour changes or deep, bright colours.
Do I need primer?
Yes for bare drywall or wood, big colour changes, and covering stains. Over a similar existing colour with a quality paint, you often can skip it.
Do I subtract doors and windows?
For accuracy, yes — deduct about 20 sq ft per door and 15 sq ft per window. The calculator does it from simple counts.
How much paint per square foot?
About 1 gallon per 350–400 sq ft per coat — so roughly 1 gallon covers 175–200 sq ft at two coats.
How much paint for a ceiling?
A ceiling is length x width, usually one coat — about a gallon for an average room. Toggle "Include ceiling" to add it.
How do I measure an irregular or tall wall?
Break it into rectangles and triangles, find each area, and add them — our square footage calculator helps, then enter the total as wall area.