How this calculator works
The math is the same formula painters use on the job:
- Wall area = room perimeter × wall height. For your room: 2 × (length + width) × height.
- Subtract openings. A standard door is about 21 sq ft; a standard window about 12 sq ft. Paint doesn't go there.
- Multiply by coats. Two coats is the professional standard — one coat only works when repainting the same color.
- Divide by coverage. One gallon of quality paint covers about 350 sq ft on a smooth, primed surface. We round up to the next quart, because running out of paint mid-wall is how visible lap marks happen.
Tips before you buy
- Buy all your paint at once and have the store shake the cans together (or box them yourself) — tint can vary slightly between cans.
- Textured or unprimed walls drink more paint. Coverage can drop to 250–300 sq ft per gallon; add 15–25% for rough surfaces or bare drywall.
- Dramatic color changes need primer (or a third coat). Going from dark to light without primer usually costs more in extra paint than the primer would.
- Keep the leftover quart. Label it with the room and date — future touch-ups will thank you.
Planning to paint walls, trim and ceiling the same color? Read our guide to color drenching small rooms first — it changes what finishes you should buy.