Paint Coverage Explained

Paint coverage is printed on the can. That number, not a rule of thumb, determines how much you buy. Here is what it means and when it does not hold.

What coverage means

Coverage rate is the square feet one gallon covers in one coat under ideal conditions. A 350 sq ft per gallon label means one gallon covers 350 square feet. Two coats means twice the paint for the same area.

The number assumes a properly prepared, smooth surface applied at the recommended temperature and film thickness. Real-world coverage is usually lower.

Surface type changes everything

Surface	Effect on coverage
Fresh primed drywall	Best case — near or above label rate
Previously painted, same colour	Close to label rate
Previously painted, different colour	10–20% worse
Bare wood	15–25% worse — absorbs first coat
Unprimed masonry	20–30% worse — porous
Popcorn or heavily textured ceiling	30–50% worse — surface area is far higher

Why the can is right and the rule of thumb is wrong

The common "one gallon per 400 square feet" rule came from oil-based paint in a dry climate. Modern latex paint, textured surfaces, and two-coat application all break it. Cheap paint with a heavy texture has a lower listed rate, and it is often the paint people are least careful about.

Read the label. It costs five seconds and saves a half-finished room.

Calculating for a room

1. Wall area = 2 × (length + width) × ceiling height.
2. Add ceiling area if painting the ceiling = length × width.
3. Subtract doors (21 sq ft each) and windows (15 sq ft each).
4. Multiply by the number of coats.
5. Divide by the coverage rate on the can.
6. Round up to whole cans — you cannot buy half a can.

A 12 × 8 room with an 8 foot ceiling, one door, two windows, two coats: (2 × 20 × 8) + 96 − 51 = 365 sq ft per coat. Two coats is 730 sq ft. At 350 sq ft per gallon that is 2.09 gallons, so three cans.

Primer, and when you need it

• New drywall — always prime bare joint compound, or the paint will adhere to the compound and peel.
• Bare wood — prime, especially on the end grain and any sapwood.
• Colour change — going from dark to light, or changing sheen, needs primer to avoid bleed-through.
• Repaint, same colour — usually no primer if the surface is sound.
• Water stains — stain-blocking primer, or the stain reappears through the finish.

Buying the right amount

Buy one extra can for a full room. Running out halfway through forces a second trip, and paint left open overnight will skin and often have to be discarded. Half-used cans store reasonably well sealed and will work for touch-ups later — but do not buy paint you will not use for a specific surface. It stores for about two years unopened, much less once opened.

Rough surfaces, cold weather, and heavy roller nap all reduce real coverage below the label number. Budget the extra can for that reason, not out of caution.

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