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
Paint coverage is usually stated on the can, commonly in the range of 250-400 square feet per gallon, but this varies by brand, paint type (flat vs. semi-gloss), and how porous or textured the wall surface is. Always use the coverage rate printed on your specific product rather than assuming a fixed number.
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.
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: 1.09 × 2 ≈ 2.17 gal → round up to 2.5 or 3 gal depending on can sizes available
FAQ
- Do I need a separate calculation for the ceiling?
- Yes — ceiling area is the room's floor-plan area (length × width), a different figure from wall area, and ceiling paint is often a different product than wall paint.
- 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.