Running out of paint two-thirds of the way through the last wall is the classic weekend disaster — and the fix is never as simple as buying another gallon, because a second batch can shift slightly in color.
Here is how to get the number right the first time.
The basic formula
Wall area = perimeter of the room × ceiling height. Then subtract openings.
- Standard door: about 21 sq ft
- Standard window: about 15 sq ft
- Sliding glass door: about 45 sq ft
Divide the result by the coverage rate, then multiply by the number of coats.
Real coverage rates
Cans say 350–400 square feet per gallon. That is a laboratory number, achieved on a sealed, smooth, primed surface by someone not trying to hide a color change. Real-world rates:
| Surface | Realistic coverage per gallon |
|---|---|
| Smooth, previously painted drywall | 350–400 sq ft |
| Light orange-peel texture | 300–350 sq ft |
| Knockdown or heavy texture | 250–300 sq ft |
| New, primed drywall | 300–350 sq ft |
| Bare, unprimed drywall | 200–250 sq ft (prime it instead) |
| Rough cedar or stucco exterior | 150–250 sq ft |
| Smooth exterior siding / fiber cement | 300–350 sq ft |
The four adjustments
- Color change severity. Light over light: two coats. Dark over light, or any strong saturated color: primer tinted toward the finish color, plus two coats. Deep reds, bright yellows and some oranges genuinely need three finish coats because the pigments are transparent.
- Texture. Every step up in texture costs you 15–25% coverage. Texture increases actual surface area — a knockdown wall has substantially more square footage than its dimensions suggest.
- Porosity. Old flat paint, fresh patches and joint compound all drink paint. Spot-prime patches or your finish coat will look dull in those spots — that is called flashing, and it is why professionals prime repairs even on a repaint.
- Application method. Rolling is the most efficient. Brushing uses a bit more. Spraying without back-rolling can use 20–40% more material because of overspray and heavier film build.
Worked example
Perimeter = (12 + 14) × 2 = 52 ft
Wall area = 52 × 9 = 468 sq ft
Subtract openings = 468 − 21 − 30 = 417 sq ft
Coverage on light texture ≈ 325 sq ft/gal
Two coats: 417 × 2 = 834 sq ft ÷ 325 = 2.6 gallons
Buy 3 gallons. The ceiling, if you are painting it, is 168 sq ft — one more gallon covers it in two coats.
Trim, doors and ceilings
- Baseboard and casing: one gallon covers roughly 250–350 linear feet in one coat. Most whole-house trim packages take 2–3 gallons for two coats.
- Doors: figure about 8 doors per gallon, both sides, two coats.
- Ceilings: use the room's floor square footage. Flat ceiling paint covers well but is thirsty on the first coat over old ceilings.
How much to buy
Three practical rules:
- Round up, always. A partial extra gallon costs $50–$90. Running out costs you a trip, a color-match risk, and a visible lap line.
- Buy it all at once. Paint is mixed in batches. If you need more than about 4 gallons of one color, ask the store to box it — combine everything in a 5-gallon bucket and stir — so any batch-to-batch variation disappears.
- Keep and label the leftovers. Write the room, the color name, the product line and the sheen on the lid with a marker. In three years that quart is the difference between an invisible touch-up and repainting a wall.