Paint Calculator
Planning a painting project? Our free paint calculator helps you determine the exact number of paint gallons or liters needed for your room. By factoring in wall dimensions, ceiling area, doors, windows, and number of coats, you avoid running out of paint mid-project or overspending on excess cans.
Project Inputs
Enter your measurements to calculate exact materials
Enter measurements above to view calculation results.
Step-by-Step Calculation Example
Scenario: Painting a standard 12 ft × 15 ft bedroom with 8 ft ceilings, 1 door, 2 windows, and 2 coats of wall paint.
Calculate Perimeter: 2 × (12 + 15) = 54 linear feet.
Calculate Gross Wall Area: 54 ft × 8 ft height = 432 sq ft.
Deduct Openings: 1 door (20 sq ft) + 2 windows (30 sq ft) = 50 sq ft deduction.
Net Paintable Wall Area: 432 - 50 = 382 sq ft.
Factor in 2 Coats: 382 sq ft × 2 = 764 sq ft effective coverage needed.
Calculate Paint Volume: 764 sq ft ÷ 350 sq ft/gallon = 2.18 gallons.
Paint Quantity Formula
Total Paint (Gallons) = [ (Perimeter × Height) - Deductions + Ceiling Area ] × Coats ÷ Coverage Rate Perimeter is calculated as 2 × (Length + Width). Gross wall area is Perimeter multiplied by Height. Openings (doors at 20 sq ft, windows at 15 sq ft) are subtracted from gross area to yield net paintable surface area. This net area is multiplied by the desired number of coats and divided by the industry-standard paint coverage rate of 350 sq ft per gallon.
Formula Variables
Calculation Assumptions
- One gallon of interior paint covers approximately 350 square feet (32.5 m²) on smooth, primed drywall.
- Standard interior doors average 20 square feet (1.86 m²) and windows average 15 square feet (1.39 m²).
- Unprimed, porous, or textured surfaces (like knockdown or popcorn) may require up to 20% more paint.
Frequently Asked Questions
Common homeowner and DIY questionsPaint Calculator
How many square feet does one gallon of paint cover?
A standard gallon of quality interior paint covers between 350 and 400 square feet with one coat on primed, smooth walls. On rough, textured, or unprimed drywall, coverage drops to 250–300 square feet per gallon.
Do I really need 2 coats of paint?
Yes. Even "paint and primer in one" formulas look significantly better with two coats. Two coats deliver uniform color saturation, eliminate roller streaks, and create a durable, washable paint film that lasts years longer.
How much paint do I need for a 12x12 room?
A standard 12x12 foot room with 8-foot ceilings, 1 door, and 1 window requires approximately 1.8 gallons for two coats on the walls. You should buy 2 full one-gallon cans.
Should I buy an extra quart or an entire extra gallon?
If your calculated need is slightly above a whole gallon (e.g. 2.1 gallons), buying 2 gallons plus 1 quart can save money. However, buying 3 whole gallons is often only slightly more expensive and ensures identical factory batch tinting and plenty of leftover paint for future scuffs.
How do I calculate paint for textured walls?
Heavily textured surfaces such as orange peel, knockdown, or stucco absorb more paint. Add 15% to 25% to your base square footage calculation to ensure complete coverage in all crevices.
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