Calculate exactly how many gallons of paint you need for walls, ceilings, trim, and exterior surfaces. Accounts for doors, windows, coats, and surface texture — with full cost breakdown.
Painting is one of the most popular and cost-effective home improvement projects — but buying the wrong amount of paint is frustratingly common. Buy too little and you'll make an extra trip mid-project, risking color batch differences. Buy too much and you've wasted money on paint that may dry out before you use it. This guide teaches you the professional method for calculating paint quantities accurately.
The fundamental calculation for any paint project follows three steps: calculate the total surface area, subtract openings (doors and windows), and divide by the paint's coverage rate. Then multiply by the number of coats.
The 400 sq ft per gallon figure is for ideal conditions — smooth, primed drywall with good-quality paint. Real-world coverage varies significantly depending on the surface texture, porosity, and color.
| Surface | Coverage (sq ft/gal) | Notes |
|---|---|---|
| Smooth Drywall (primed) | 350–400 | Best case scenario |
| Lightly Textured | 300–350 | Orange peel, light knockdown |
| Heavy Texture | 250–300 | Popcorn, heavy knockdown |
| Bare Wood | 300–350 | Primer recommended first |
| Concrete / Masonry | 200–300 | Very porous, may need 3 coats |
| Exterior Stucco | 200–250 | Rough surface absorbs paint |
| Metal | 400–500 | Smooth, non-porous |
| Exterior Wood Siding | 300–400 | Depends on weathering |
Two coats is the standard for almost all interior painting. Even when painting the same color, two coats ensure uniform coverage and proper film thickness for durability and washability. There are specific situations that require more or fewer coats:
One coat is acceptable only for touch-ups or when applying the exact same color over a recently painted surface in good condition. Three coats are needed when covering very dark colors with white or light colors, when using colors with poor hiding power (reds, bright yellows, certain oranges), when painting over patched or repaired areas, or when applying over unprimed new drywall. Using a tinted primer close to your topcoat color can often reduce the topcoats needed from 3 to 2.
Exterior paint calculations follow the same logic but with additional considerations. Measure each wall individually (they may have different heights), add gable ends as triangles (½ × base × height), and subtract all windows, doors, and non-painted areas like brick or stone accents. Exterior surfaces are typically rougher, so use 250–350 sq ft per gallon as your coverage rate.
Ceiling area is simply Length × Width of the room. Ceiling paint is typically flatter (ultra-flat or dead-flat sheen) to hide imperfections and is slightly thicker than wall paint. Coverage is usually 350–400 sq ft per gallon. For trim (baseboards, crown molding, door frames, window frames), calculate the linear footage and multiply by the width of the trim in feet. Most trim is 3.5–5.5 inches wide. A standard bedroom typically requires 1 quart of trim paint.
Paint quality significantly affects both coverage and durability. Economy paint ($15–$25/gallon) has lower pigment concentration, requiring more coats and offering less washability. Mid-range paint ($30–$45/gallon) from brands like Behr, Valspar, and Glidden offers good coverage in 2 coats. Premium paint ($50–$80/gallon) from Benjamin Moore, Sherwin-Williams, and Farrow & Ball often achieves better coverage in fewer coats, has superior color depth, and lasts longer. Over the life of a paint job (5–10 years), premium paint often costs less per year than budget paint.
Dedicated primer ($20–$35/gallon) is essential in several situations: new drywall (always prime — mud and paper absorb paint differently), covering stains (use shellac-based primer like Zinsser BIN for water stains, smoke damage, or tannin bleed), dark to light color changes (gray-tinted primer saves topcoats), bare wood (prevents tannin bleed and ensures adhesion), and switching paint types (oil to latex or vice versa). "Paint-and-primer-in-one" products work for minor color changes on previously painted surfaces but are not a substitute for dedicated primer in the situations above.