How Many Gallons of Paint Do I Need?
Buying too much paint wastes money, while buying too little results in mid-project runs to the hardware store and mismatched color batches. To get it right, you need to calculate the "net paintable area."
Try our Paint Calculator to do this automatically.
Step 1: Calculate Gross Wall Area
Measure the total perimeter of the room (add the lengths of all walls) and multiply by the ceiling height.
Perimeter (40 ft) × Height (8 ft) = 320 sq ft
Step 2: Subtract Doors and Windows
You won't be painting the doors and windows. Subtract their area:
- Standard Door: ~21 sq ft
- Standard Window: ~12 sq ft
If the room has 1 door and 2 windows (21 + 12 + 12 = 45 sq ft), your net paintable area is 320 - 45 = 275 sq ft.
Step 3: Factor in Coverage and Coats
A high-quality gallon of interior paint typically covers 350 to 400 square feet for one coat.
If you are applying 2 coats, you need to cover 275 × 2 = 550 sq ft.
550 ÷ 350 = 1.57 gallons
You will need to buy 2 gallons of paint (or 1 gallon and 3 quarts).