Paint Calculator
Determine gallons of paint needed for interior rooms, deducting doors and windows, for 1 or 2 coats.
- 2 × gallons interior wall paint (1 coat)
- 396 sq ft net paintable wall surface (openings deducted)
Standard Project Worked Example (No JavaScript Required)
Baseline Scenario: 12×15 ft room with 8 ft ceilings: Gross 432 ft², minus 1 door (21 ft²) and 1 window (15 ft²) = Net 396 ft². 1 coat @ 350 ft²/gal = 1.13 → 2 gal. 2 coats = 2.26 → 3 gal.
Our online calculator updates dynamically in real-time as you type your project measurements. If JavaScript is disabled, use the formula reference below.
How This is Calculated
Gross Wall Area = 2 × (Length + Width) × Ceiling Height. Net Paintable Area = Gross Area - (Doors × 21 sq ft) - (Windows × 15 sq ft). Gallons per coat = Net Area / 350 sq ft per gal. Total Gallons = ceil(Gallons per coat × Coats).
Sourced Reference Constants
- All material yields and bag volumes are derived from official manufacturer technical data sheets.
- Buy quantities are strictly rounded UP to full purchasable units (bags, boxes, sheets, gallons, pavers).
- Waste allowances are user-adjustable to fit complex room alcoves, diagonal patterns, or soil conditions.
Frequently Asked Questions
How many square feet does one gallon of paint cover?
One gallon of interior wall paint covers approximately 350 to 400 square feet with one coat on primed, smooth drywall.
How do I deduct doors and windows from wall area?
Deduct 21 square feet per standard interior door and 15 square feet per standard window from your total gross wall square footage.
Do I always need two coats of interior paint?
Two coats are strongly recommended for long-term durability and color depth, though one coat may suffice when refreshing an identical paint color.