Paint Calculator
Coats
1
2
Gallons Needed0.9
Total Wall Area352 sq ft
Paintable Area301 sq ft
This paint calculator estimates how many gallons you need to paint a room's walls, accounting for the number of coats and the openings you will not paint. It computes the total wall area from the room dimensions, subtracts standard areas for doors and windows, and divides by a typical coverage of 350 square feet per gallon. The result helps you buy enough paint without overspending.
Formula
gallons = ⌈(paintableArea × coats ÷ 350) rounded up to 0.1⌉
- paintableArea
- 2 × (length + width) × height, minus 21 sq ft per door and 15 sq ft per window
- coats
- Number of paint coats to apply
- 350
- Assumed coverage of one gallon in square feet
- gallons
- Gallons of paint required, rounded up to the nearest tenth
How it works
- Enter the room length, width, and wall height in feet, plus how many coats you plan to apply.
- Enter the number of doors and windows; each door subtracts 21 sq ft and each window 15 sq ft of paintable area.
- The calculator finds wall area as 2 × (length + width) × height, removes the openings, multiplies by coats, and divides by 350 sq ft per gallon, rounding up to the next tenth of a gallon.
Worked example
A 12 ft by 10 ft room with 8 ft walls, 2 coats, 1 door, and 2 windows.
- Wall area: 2 × (12 + 10) × 8 = 352 sq ft.
- Subtract openings: 1 × 21 + 2 × 15 = 51 sq ft, leaving 352 − 51 = 301 sq ft.
- Apply coats: 301 × 2 = 602 sq ft of coverage needed.
- Gallons: 602 ÷ 350 ≈ 1.72, rounded up to 1.8.
About 1.8 gallons for two coats (paintable area 301 sq ft).
Frequently asked questions
- How much area does one gallon of paint cover?
- This calculator assumes 350 square feet per gallon, a common figure for a single coat on smooth, primed walls. Rough, porous, or unprimed surfaces absorb more and cover less, so buy a little extra for them.
- How are doors and windows handled?
- Each door removes 21 square feet and each window removes 15 square feet from the wall area, since you do not paint those surfaces. The paintable area never drops below zero.
- Should I always use two coats?
- Two coats are standard for even colour and durability, especially over a different base colour or fresh drywall. A single coat may suffice when repainting the same colour over a clean, primed surface.
- Does this include ceiling or trim paint?
- No. The estimate covers wall area only. Calculate the ceiling separately as length × width, and account for trim, baseboards, and doors with their own measurements and paint.