How to Calculate Paint Needed for a Room
Wall area, minus openings, divided by real-world coverage
Wall area, not floor area
Paint coverage is based on wall (or ceiling) surface area, not the room's floor area. For a rectangular room, add up the perimeter (2 × (length + width)) and multiply by the wall height to get total wall area.
Subtract doors and windows
Standard doors and windows take up real wall area that doesn't get painted the same way (or at all, if you're not painting trim). Subtract their combined area from the total wall area before calculating paint quantity — a room with several large windows can see a noticeably lower paint requirement than the raw wall area suggests.
Coverage rate varies by product and surface
Coverage (the spread rate) is printed on the can and in the product's technical data sheet, usually as square feet per gallon for one coat. It varies with the product and with how smooth and sealed the surface is, so use the figure for your paint. The Paint Calculator starts at 350 ft² per gallon; treat that as a starting value and replace it with your product's rate.
Multiply by the number of coats
Two coats are standard for most interior painting, especially over a different existing color or bare drywall/primer. Multiply the paint-area-divided-by-coverage figure by the number of coats to get the total paint volume needed.
Ceilings, primer and trim
A ceiling is the room's floor-plan area (length × width), worked out separately from the walls; the Ceiling Paint Calculator handles it. Primer is a coat of its own: add it to the coat count, or work it out separately at the primer's own spread rate. Trim and doors are separate from the wall area.
Round up to cans
Paint is commonly sold in gallons and quarts, and a quart is a quarter of a gallon. Round up to cans whose total is at least the calculated amount: 2.17 gallons is covered by 2 gallons plus 1 quart (2.25 gallons), or by 3 gallons if you want spare paint for touch-ups. At 350 ft² per gallon, a quart covers about 87 ft² per coat.
Worked Example
A 12×14 ft room, 8 ft ceilings, one door (21 ft²) and one window (15 ft²), coverage 350 ft²/gal, 2 coats.
Perimeter: 2 × (12 + 14) = 52 ft
Gross wall area: 52 × 8 = 416 ft²
Net wall area: 416 − 21 − 15 = 380 ft²
Paint per coat: 380 ÷ 350 ≈ 1.09 gal
Total for 2 coats: 380 × 2 ÷ 350 = 2.17 gal
Buy 2 gallons + 1 quart (2.25 gal, at least the 2.17 gal needed), or 3 gallons for spare touch-up paint
FAQ
- Do I need a separate calculation for the ceiling?
- Yes. Ceiling area is the room's floor-plan area. For the 12 × 14 ft room above, that is 168 ft²; two coats at 350 ft² per gallon is 168 × 2 ÷ 350 = 0.96 gallons, so 1 gallon.
- Why does my actual paint use differ from the calculation?
- Textured surfaces, a porous unprimed wall, and application method (roller nap size, spray vs. roll) can all shift real-world coverage away from the can's stated rate. Treat the calculation as a solid starting estimate, and buy from a source that allows easy returns of unopened cans.
- How much paint do I need for a 12×12 room?
- With 8 ft walls, one door and one window as in the example: 2 × (12 + 12) × 8 = 384 ft², minus 36 ft² = 348 ft². Two coats at 350 ft² per gallon is 1.99 gallons, so 2 gallons. Use your own product's spread rate if it differs.