How to Calculate Paint: A Complete Guide

Buying too much paint wastes money. Buying too little means a second trip and the risk of a mismatched color if the batch changes. Estimating paint is straightforward once you know the coverage rate and the area you need to coat.

The Basic Paint Formula

Most interior latex paints cover approximately 350 square feet per gallon on smooth surfaces. The formula is:

Gallons Needed = (Total Wall Area - Openings) x Number of Coats / Coverage Rate

For example, a 12 ft x 14 ft room with 8-foot ceilings:

1. Wall perimeter: (12 + 14) x 2 = 52 linear feet
2. Total wall area: 52 x 8 = 416 sq ft
3. Subtract a door (21 sq ft) and two windows (30 sq ft): 416 - 51 = 365 sq ft
4. Two coats at 350 sq ft/gallon: (365 x 2) / 350 = 2.09 gallons

Buy 3 gallons to have a small reserve for touch-ups.

Coverage Rates by Paint Type

• Interior latex (flat, eggshell, satin): 350 to 400 sq ft per gallon on smooth drywall.
• Interior semi-gloss and gloss: 300 to 350 sq ft per gallon.
• Exterior latex: 250 to 350 sq ft per gallon depending on surface texture.
• Primer: 200 to 300 sq ft per gallon. New drywall and bare wood absorb more.
• Spray application: Uses 20 to 30 percent more paint than brushing or rolling due to overspray.

Always check the coverage rate printed on the can. Premium paints often cover more per gallon than budget options.

Accounting for Doors, Windows, and Trim

Subtract openings to avoid overestimating. Use these standard sizes if you prefer not to measure every opening: standard door (21 sq ft), standard window (15 sq ft), sliding glass door (40 sq ft). If painting trim separately, calculate those areas independently — a six-panel door takes about a quart for two coats.

When You Need This Calculator

• Homeowners repainting a bedroom who want to buy the right number of gallons without overbuying. If you are renovating the whole room, the Tile & Flooring Calculator can help estimate flooring materials too.
• Landlords preparing a rental unit who need estimates for multiple rooms quickly.
• DIYers tackling an exterior job who need to account for large walls and different surface textures.

Common Mistakes to Avoid

• Forgetting multiple coats. One coat rarely provides full coverage. Plan for two minimum, and three when covering dark colors with a lighter shade.
• Skipping primer on new surfaces. Bare drywall, patches, and raw wood absorb paint unevenly. If you are hanging new sheets, use the Drywall Calculator first, then primer creates a consistent base.
• Using smooth-wall rates for textured surfaces. Knockdown, orange peel, and popcorn ceilings can use 20 to 50 percent more paint.
• Not buying all your paint at once. Colors vary slightly between batches. Buy everything in one trip and ask the store to box your gallons together.

Pro Tips

• Keep a touch-up supply. Set aside a quart for future repairs. Label it with the room, color name, and date.
• Calculate the ceiling too. Multiply room length by width and add it to your total.
• Tint your primer. Tinting primer close to the finish color improves coverage and can save an extra topcoat.
• Buy in fives. Five-gallon buckets cost less per gallon and ensure color consistency.

Frequently Asked Questions

How much paint do I need for a 12x12 room?

A 12 ft x 12 ft room with 8-foot ceilings has about 384 sq ft of wall area. Subtract a door and windows (about 50 sq ft) for 334 sq ft of paintable surface. At two coats, you need roughly 2 gallons.

Do I need to prime before painting?

Primer is recommended for new drywall, repaired surfaces, bare wood, and dramatic color changes. If repainting over a similar color on a clean, intact surface, a paint-and-primer-in-one product may be sufficient.

How many coats of paint do I need?

Two coats is standard. You may need three when painting light over dark, or when using reds, yellows, and oranges that have lower hiding power.