How Much Paint Do I Need? Room-by-Room Calculator Guide

Buying too much paint wastes money, and buying too little means an extra trip to the store and a potential color mismatch between batches. This guide shows you how to calculate the exact amount of paint you need for any room, so you buy the right quantity the first time.

The Basic Paint Coverage Formula

One gallon of paint typically covers approximately 350 to 400 square feet of smooth wall surface with one coat. The exact coverage depends on the paint quality, surface texture, and application method.

The formula is:

Gallons Needed = (Total Paintable Area × Number of Coats) / Coverage per Gallon

Step 1: Measure Your Walls

For each wall, multiply the width by the height to get the square footage. Then add all walls together.

Example — a 12×14 foot room with 8-foot ceilings:

• Two walls: 12 × 8 = 96 sq ft each = 192 sq ft
• Two walls: 14 × 8 = 112 sq ft each = 224 sq ft
• Total wall area: 416 sq ft

Step 2: Subtract Doors and Windows

Subtract areas you will not paint:

• Standard door: approximately 21 sq ft (3 ft × 7 ft)
• Standard window: approximately 15 sq ft (3 ft × 5 ft)

If the example room has one door and two windows: 416 − 21 − 30 = 365 sq ft of paintable area.

Step 3: Factor In Number of Coats

Most paint jobs require two coats for even coverage and true color. You may need additional coats if:

• Painting a light color over a dark color (consider using a tinted primer first).
• The surface is textured or porous (bare drywall, stucco, brick).
• Using a deep or saturated color, which may require 3 coats for full opacity.

For our example with two coats: 365 × 2 = 730 sq ft of coverage needed.

Step 4: Calculate Gallons

Using the standard 350 sq ft per gallon: 730 / 350 = 2.09 gallons. Round up to 3 gallons to ensure you have enough for touch-ups and to account for any waste from rollers and trays.

For smaller areas like a bathroom or accent wall, you may only need 1 quart to 1 gallon.

Special Considerations

• Ceilings: Measure length × width of the room. Ceiling paint is typically thicker and may cover slightly less per gallon.
• Trim and doors: A quart of trim paint covers about 75 to 100 linear feet of standard trim.
• Primer: If using primer, calculate it separately — primer coverage is similar to paint at about 300 to 400 sq ft per gallon.

Save Money on Your Next Paint Project

Use our paint calculator to get an instant estimate for your room dimensions, or explore all our construction calculators to plan your home improvement projects accurately and avoid costly overbuying.

Accurate measurement before you head to the paint store saves money, reduces waste, and ensures your project goes smoothly from the first coat to the last.