Paint Calculator
Calculate gallons and liters of paint needed for interior and exterior walls and ceilings, with automatic door/window deductions, multi-coat coverage, and primer.
Calculation Results
Total perimeter wall surface area before deducting door and window openings.
Total square footage subtracted for standard doors (21 sq ft) and windows (15 sq ft).
Actual surface area to be painted in a single coat, including ceiling if selected.
Total painted square footage across all applied coats.
Gallon cans of paint to purchase rounded up to whole factory cans.
Exact liquid paint volume in metric liters.
Primer gallons needed (recommended for fresh drywall, patches, or drastic color shifts).
Estimated cost of topcoat paint based on full gallon can purchases.
How This Calculator Works
Estimate paint requirements for any room in four easy steps:
- Enter Room Dimensions: Measure length, width, and floor-to-ceiling height in feet.
- Specify Openings: Count standard doors (closets, entryways) and windows so their surface area is deducted from the wall paint total.
- Select Coats and Surface: Two coats are standard for durable color consistency. If painting over new drywall or patching compound, also check primer requirements.
- Purchase Cans: Use the gallon and liter outputs when visiting your local paint supply store.
Formula & Calculations
Professional paint estimators calculate room square footage by perimeter wall geometry and subtract standard structural openings:
1. Net Surface Area Formula
Gross Wall Area = 2 × (Length + Width) × Ceiling Height
Net Wall Area = Gross Wall Area - (Doors × 21 sq ft) - (Windows × 15 sq ft)
Total Surface = Net Wall Area + (Include Ceiling × Length × Width)
2. Coverage and Gallon Cans
Total Coverage Area = Total Surface × Number of Coats
Gallons to Buy = ceil(Total Coverage Area / Spreading Rate)
Paint retailers sell paint in whole 1-gallon and 5-gallon containers, so fractions must always round up to the next full gallon.
3. Spreading Rate Standards
- Standard Drywall (350 sq ft/gal): Smooth, previously painted walls in living rooms and bedrooms.
- Textured / Rough Drywall (300 sq ft/gal): Knockdown texture, popcorn ceilings, or fresh unprimed plasterboard.
- Smooth Gloss Enamel (400 sq ft/gal): High-end enamels applied over sanded primer coats.
Step-by-Step Calculation Examples
Standard Bedroom (16ft x 12ft, 9ft Ceiling, 2 Coats)
Repainting bedroom walls with 2 doors and 2 windows, 2 coats of premium latex paint at $45/gallon.
- length: 16
- width: 12
- height: 9
- include_ceiling: 0
- doors: 2
- windows: 2
- coats: 2
- coverage: 350
- cost_per_gallon: 45
- gross_wall_area: 504.0 sq ft
- openings_area: 72 sq ft
- net_surface_area: 432.0 sq ft
- total_applied_sqft: 864.0 sq ft
- gallons_needed: 3 gal
- liters_needed: 9.3 L
- primer_gallons: 2 gal
- estimated_paint_cost: $135.00
Frequently Asked Questions
How many gallons of paint do I need for a 12x12 room?
A standard 12x12 room with 8-foot ceilings has roughly 384 sq ft of gross wall area. Subtracting one door and one window leaves about 348 net sq ft. For two coats (696 sq ft total), you will need exactly 2 gallons of paint (696 / 350 = 1.99, rounded up to 2 gallons).
Do I really need 2 coats of paint?
Yes. Even "one-coat guarantee" paints achieve truer color depth, better sheen uniformity, and greater washability with two coats. A second coat covers microscopic pinholes and roller streaks that inevitably occur during the first pass.
When is a primer coat necessary?
Primer is essential when painting over bare drywall, fresh joint compound patches, bare wood, glossy oil-based paints, or when making a drastic color shift (such as transitioning from dark navy or red to white). Primer seals porous surfaces so the topcoat dries evenly.
How much paint does a standard door or window deduct?
In the architectural painting industry, a standard interior door (30"x80") equals approximately 21 square feet. A standard residential double-hung window (36"x60") equals approximately 15 square feet.
Should I buy 1-gallon cans or a 5-gallon bucket?
If your project requires 4 or more gallons of the same color and finish, purchasing a single 5-gallon pail is usually 15% to 25% cheaper than individual 1-gallon cans. It also eliminates slight color variations between different factory batch cans.
How much extra paint should I keep for future touch-ups?
Keep roughly 1 quart to 1/2 gallon of leftover paint in a sealed, climate-controlled container (never in an unheated garage where it can freeze). Shake or stir well before touching up scuffs or hanging holes in future years.