How to Estimate Paint for a Room
Published 2026-02-15 · Updated 2026-07-14
Estimating paint before a project helps you avoid buying too little, wasting money on too much, or stopping halfway through a room. The key is to calculate paintable wall area, adjust for doors and windows, then account for the number of coats.
Paint estimate formula
Use this formula: gallons needed = paintable area / coverage per gallon x number of coats.
Paintable area usually starts with wall area. For a rectangular room, wall area = room perimeter x wall height. Then subtract door and window areas.
Example paint calculation
A 12 ft by 10 ft room with 8 ft walls has a perimeter of 44 ft. The wall area is 44 x 8 = 352 square feet before subtracting doors and windows.
If one gallon covers 350 square feet and you need two coats, the project needs about two gallons before extra allowance. Textured walls, primer, and color changes can increase the amount.
Common paint estimating mistakes
Do not forget the second coat. Many projects need two coats for even coverage, especially when changing from dark to light colors or painting over patched walls.
Also check the coverage printed on the paint can. Some paints cover 400 square feet per gallon, while primers and specialty finishes may cover less.
Quick Summary
Calculate wall area, subtract openings, divide by paint coverage, multiply by coats, and buy a small extra amount for touch-ups.