How to Use This Calculator
- Enter room dimensions: Measure floor length, floor width, and ceiling height in feet.
- Count openings: Input the number of standard doorways and windows to subtract non-painted surfaces.
- Select coat coverage: Two coats ensure uniform color saturation and wipeable finish durability.
Formula and Calculation Steps
Gross Wall Area = 2 × (Length + Width) × Ceiling Height
Deductions = (Doors × 21 sq ft) + (Windows × 15 sq ft)
Net Wall Area = Gross Wall Area − Deductions
Gallons Needed = ⌈ (Net Wall Area × Coats) ÷ 350 ⌉Worked Examples and Reference Table
| Room Size (9' Ceiling) | Net Wall Area | 1 Coat | 2 Coats |
|---|---|---|---|
| 10 ft × 10 ft (Small Bedroom) | 309 sq ft | 1 Gallon | 2 Gallons |
| 12 ft × 14 ft (Standard Bedroom) | 417 sq ft | 2 Gallons | 3 Gallons |
| 16 ft × 20 ft (Living Room) | 597 sq ft | 2 Gallons | 4 Gallons |
| 20 ft × 24 ft (Open Great Room) | 741 sq ft | 3 Gallons | 5 Gallons |
Frequently Asked Questions
How many square feet does one gallon of paint cover?
One standard US gallon of interior wall paint covers approximately 350 to 400 square feet on smooth, primed walls. Textured walls or unprimed drywall absorb more paint, reducing coverage to ~250–300 sq ft.
How do you account for doors and windows?
Standard interior doors subtract roughly 21 square feet each. Standard residential windows subtract approximately 15 square feet each from total wall surface area.
Should I apply one coat or two coats?
Two coats are strongly recommended for durable sheen, uniform color depth, and complete hiding—especially when transitioning between light and dark shades.
How do I calculate paint needed for ceilings?
Ceiling area is simply room length multiplied by room width. Divide that ceiling square footage by 350 sq ft per gallon.
What should I do with leftover paint?
Store remaining paint in an airtight, labeled container in a temperature-controlled area for future spot touch-ups. Avoid freezing temperatures.