A gallon of interior paint covers approximately 350-400 square feet on smooth drywall, but that number drops to 250-300 square feet on textured surfaces, new drywall, or when making a dramatic color change. Most rooms need two coats, which effectively doubles your paint needs. This calculator accounts for wall dimensions, doors, windows, number of coats, and surface texture to give you an accurate gallon count. The most common mistake is buying too little — running out mid-wall creates visible lap marks even with the same batch of paint. Buy an extra quart as insurance.
Paint Coverage Calculator
How to Use This Calculator
- Enter the room dimensions — length, width, and ceiling height
- Subtract door and window openings to avoid paying for paint you will not use
- Select one or two coats — two coats is recommended for most color changes and always for new drywall
- Choose your surface type — smooth drywall, textured, or previously unpainted for accurate coverage estimates
- View the total gallons needed with a small buffer for touch-ups
How It Works
This paint coverage calculator uses established formulas to provide accurate results.
The basic rule:
- Wall Area = Perimeter × Ceiling Height
- Paintable Area = Wall Area − (Doors × 21 sq ft) − (Windows × 15 sq ft)
- Gallons = (Paintable Area × Coats) ÷ 350 sq ft per gallon
Material prices and building codes vary by region. Get local quotes before purchasing.
Tips & Considerations
- Primer is not optional on new drywall, bare wood, or when covering dark colors with light. Skipping primer means 3+ coats of expensive finish paint instead of one primer coat and two finish coats.
- Paint coverage ratings assume ideal conditions. Textured walls, porous surfaces, and dramatic color changes can reduce coverage by 30-40%.
- One gallon of paint costs $25-$50 but covers a 12x12 room in two coats. Labor is 85% of a professional paint job cost, so buying better paint is always worth it.
- Keep at least one quart of each color for touch-ups. Store it lid-down in a climate-controlled space — it lasts 2-5 years when sealed properly.
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12x12 room with 8-foot ceilings has about 384 square feet of wall space. After subtracting a door and two windows (about 51 sq ft), you have roughly 333 square feet of paintable area. With standard 2 coats, you need about 1.9 gallons, so buy 2 gallons. One gallon covers approximately 350 square feet per coat on smooth walls.
Do I really need two coats of paint?
Two coats are recommended for most paint jobs and are essential when changing colors, especially going from dark to light. Two coats provide better coverage, richer color, better durability, and a more uniform finish. You can sometimes get away with one coat when painting the same color for a touch-up, or when using a high-quality paint-and-primer-in-one over a similar shade.
Is expensive paint worth it?
Generally yes. Premium paints ($45-$65 per gallon) have higher concentrations of pigment and resin, meaning better coverage in fewer coats, better adhesion, more durability, and truer color. Budget paints ($25-$35) often require an extra coat, negate any savings. For most rooms, spending an extra $30-$50 on better paint saves you time and produces a noticeably better result that lasts years longer.
Should I use primer before painting?
Use primer when painting over new drywall, bare wood, stains, or dark colors. Primer seals porous surfaces, blocks stains, and provides a consistent base for topcoat adhesion. When painting over a similar existing color in good condition, a paint-and-primer combo product is usually sufficient. Tinting primer toward your final color improves coverage and reduces the number of topcoats needed.
How long should I wait between coats?
Latex (water-based) paint typically needs 2-4 hours between coats, while oil-based paint needs 24 hours. Check the paint can for specific recoat times. Applying the second coat too early can cause lifting, streaking, or poor adhesion. In humid conditions or cool temperatures, extend drying time. The paint should feel completely dry to the touch and not tacky before applying the next coat.