Work Out How Much Paint to Buy
This paint calculator estimates how many gallons you need to paint a room's walls, and optionally the ceiling. Enter the room's length, width and ceiling height, the number of doors and windows, the number of coats and the coverage per gallon listed on the paint can. It subtracts door and window openings automatically and shows the total area, gallons needed, gallons to buy (rounded up) and quarts.
How Paint Is Calculated
Wall area = 2 × ceiling height × (length + width) − (doors × 21 sq ft) − (windows × 15 sq ft). The ceiling area (length × width) is added if included. Total area × coats ÷ coverage per gallon = gallons needed.
Example: a 12 ft × 14 ft room with 9 ft ceilings, 1 door and 2 windows has about 417 sq ft of wall. Adding the 168 sq ft ceiling and applying 2 coats gives 1,170 sq ft. At 350 sq ft per gallon, that is about 3.3 gallons, so buy 4.
Painting Tips
- Most interior paints cover about 350–400 sq ft per gallon. Check the label for your exact product.
- Plan on two coats for color changes and new drywall, which should be primed first.
- Textured or very porous surfaces can use 10–20% more paint.
- Buy all your paint at once and mix (box) the cans together for consistent color.
Frequently Asked Questions
How much area does a gallon of paint cover?
Typically 350–400 square feet for one coat on smooth, primed walls. Rough, porous or unprimed surfaces absorb more paint and reduce coverage.
Do I really need two coats of paint?
Usually, yes. Two coats give even color, better durability and hide roller marks. One coat may be enough only when repainting the same color with a high-quality paint.
Should I buy quarts or gallons?
Gallons are much cheaper per square foot. Buy quarts only for small touch-ups or when you need a little more than a whole number of gallons.