🎨 🎨 Paint Calculator: How Much Paint Do You Need?

Buying too little paint means a frustrating mid-project trip to the store — and risking a dye-lot mismatch with a different batch of custom color. Buying too much wastes money. Getting the amount right requires a simple formula that anyone can do with a tape measure and a few minutes of math.

The Paint Calculation Formula

Gallons of paint needed = (Paintable wall area ÷ Coverage per gallon) × Number of coats

Standard coverage rates:

• Interior wall paint: 350–400 sq ft per gallon (use 350 for rough or porous surfaces)
• Primer: 200–300 sq ft per gallon
• Ceiling paint: 400 sq ft per gallon (ceilings are smoother and more uniform)
• Trim paint: 400 sq ft per gallon

Step-by-Step: Calculate Paint for a Room

Step 1: Measure total wall area

Add up the length of all four walls, multiply by the ceiling height:

Total wall area = (Wall 1 + Wall 2 + Wall 3 + Wall 4) × Ceiling height

Or for a rectangular room: (2 × length + 2 × width) × ceiling height

Step 2: Subtract doors and windows

Subtract the areas you won't paint:

• Standard door: subtract 20 sq ft per door
• Standard window: subtract 15 sq ft per window

Step 3: Divide by coverage and multiply by coats

Divide paintable area by 350 (conservative coverage), then multiply by number of coats.

Full worked example

Room: 12 ft × 15 ft, 8 ft ceilings, 2 doors, 2 windows, 2 coats

• Perimeter: (12 + 12 + 15 + 15) = 54 ft
• Total wall area: 54 × 8 = 432 sq ft
• Subtract doors: 2 × 20 = 40 sq ft
• Subtract windows: 2 × 15 = 30 sq ft
• Paintable area: 432 − 40 − 30 = 362 sq ft
• One coat: 362 ÷ 350 = 1.03 gallons
• Two coats: 1.03 × 2 = 2.06 gallons → buy 2 gallons + 1 quart

How Many Coats Do You Need?

Ceiling Paint Calculation

Ceiling area = Room length × Room width

Example: 12 × 15 = 180 sq ft

At 400 sq ft per gallon: 180 ÷ 400 = 0.45 gallons → buy 2 quarts for one coat

Ceilings typically only need one coat unless going from a dark color to white or painting new drywall.

Trim Paint Calculation

Trim includes baseboards, door frames, window frames, and crown molding. Measure total linear feet of trim, multiply by 0.5 ft (approximate trim width) to get square footage.

Example: 60 linear feet of trim × 0.5 ft = 30 sq ft. At 400 sq ft/gallon: 30 ÷ 400 = 0.075 gallons → 1 quart is more than enough.

Paint Can Sizes and Quantities

• 1 quart (0.25 gallon): Covers ~100 sq ft. Best for accent walls, trim, or small rooms.
• 1 gallon: Covers ~350–400 sq ft. Standard for one coat in a bedroom or bathroom.
• 5-gallon bucket: Most economical for large projects or whole-house painting. Always buy in the same batch (same lot number) for color consistency.

Pro Tips to Avoid Running Short

• Always round up: If your calculation says 2.1 gallons, buy 3 gallons. Partial cans aren't sold at the paint counter.
• Buy 10% extra: Factor in touch-ups, drips, and roller waste. For custom colors especially, you can't guarantee an exact match from a new batch later.
• Keep batch numbers consistent: When buying multiple cans of the same color, ensure all cans share the same "lot" or "batch" number on the lid to avoid subtle color variations.
• Porous surfaces absorb more: Textured walls, stucco, or unpainted drywall absorb significantly more paint. Use 300 sq ft/gallon instead of 350–400 for these surfaces.