Interior Paint Calculator
Our interior paint calculator provides accurate paint gallon estimates based on room perimeter, ceiling height, coat counts, surface texture, and standard US door and window cutouts.
Formula
Total Wall Area = 2 × (Length + Width) × Height - Deductions; Gallons = (Net Area × Coats × Waste) ÷ Coverage
1. Gross Wall Area = 2 × (Length + Width) × Wall Height 2. Deductions = (Doors × 21 sq ft) + (Windows × 15 sq ft) 3. Net Paintable Area = Gross Wall Area - Deductions 4. Total Coated Area = Net Area × Number of Coats 5. Raw Gallons = Total Coated Area ÷ Coverage Rate (375 or 275 sq ft/gal) 6. Gallons with Waste = Raw Gallons × (1 + Waste %) 7. Purchase Gallons = Ceil(Gallons with Waste)
Inputs
- Room Length
- Room Width
- Ceiling Height
- Number of Coats
- Standard Doors
- Standard Windows
- Wall Surface Type
Worked example
A 14 ft by 12 ft room with 9 ft ceilings, 2 standard doors, and 2 standard windows, receiving 2 coats of premium latex paint on smooth walls.
Outcome: Gross wall area is 468 sq ft. Deducting 72 sq ft for doors and windows leaves 396 net sq ft. Two coats equals 792 sq ft of coverage. With 10% waste (871.2 sq ft effective), exact need is 2.32 gallons, resulting in a recommended purchase of 3 single gallons (or 2 gallons plus 1 quart).
How many square feet does one gallon of paint cover?
One US gallon of quality interior paint covers approximately 350 to 400 square feet of smooth, primed drywall for one coat. On textured walls or unprimed surfaces, coverage drops to 250 to 300 square feet.
Should I buy 5-gallon buckets or 1-gallon cans?
If your project calls for 4 or more gallons, purchasing a 5-gallon bucket is typically 15% to 25% cheaper per gallon and ensures a single consistent color blend (dye lot) across your entire room.
Do I always need two coats of paint?
Yes, for optimal durability, washability, and true color representation, two coats are standard industry practice. A single coat is only sufficient for exact color match touch-ups over an undamaged existing surface.