How this paint calculator works
The wall area equals the room perimeter (2 × length + 2 × width) multiplied by the wall height. Subtract the area of doors and windows you will not paint, then multiply by the number of coats. Divide that total by the paint coverage per gallon — about 350 square feet for most interior paints — to get the gallons you should buy.
A typical door is around 21 square feet and a standard window around 15 square feet; count them and enter the combined number. For ceilings, add the floor area (length × width) to your total before calculating. If you are changing from a dark color to a light one, plan a primer coat as well.
Save money, buy smart
- One gallon covers roughly 350 sq ft per coat on smooth walls.
- Buy in 5-gallon buckets when total coverage exceeds about 10 gallons — the per-gallon price drops.
- Keep 1 extra gallon of each color for touch-ups; paint color codes can change between batches.