How to Calculate Paint Coverage
The gallon-per-square-foot math is simple. The part that wrecks estimates is forgetting that coverage depends on surface condition, and that primer, doors, and multi-coat jobs all change the numbers. Here's how pros calculate paint for a room or a whole house in about 5 minutes.
The core formula
That's it. The rest is knowing which numbers to plug in.
Step-by-step for an interior room
Measure perimeter and height
Walk the room and add up all wall lengths. A 12 ft × 15 ft bedroom has a perimeter of (12+15) × 2 = 54 linear feet. Multiply by ceiling height (say 9 ft): 54 × 9 = 486 sq ft of gross wall area.
Subtract openings
- Standard door: 21 sq ft (3 ft × 7 ft)
- Standard window: 15 sq ft (3 ft × 5 ft, can adjust)
- Closet door: 15 sq ft (bifold or sliding)
For a bedroom with 1 door + 1 window: 486 − 21 − 15 = 450 sq ft of paintable wall.
For small rooms or when openings are less than 15% of wall area, it's acceptable to skip this subtraction and let the extra serve as cutting/waste buffer.
Divide by coverage per gallon
Manufacturer labels say 400 sq ft/gal, but real-world coverage depends on surface:
- Primed smooth drywall: 350–400 sq ft/gal
- Previously painted walls (similar color): 400 sq ft/gal
- Textured drywall (orange peel, knockdown): 300–350
- Popcorn ceiling: 200–250
- Raw drywall (first coat of primer): 200–300
- Rough wood siding / cedar shakes: 200–300
- Smooth wood siding / trim: 350–400
- Stucco / masonry: 150–250
Use 350 sq ft/gal as a safe interior default.
450 ÷ 350 = 1.29 gallons per coat.
Multiply by number of coats
- Same color, refresh: 1 coat
- New color over similar tone: 2 coats
- Drastic color change (dark over light, light over dark): primer + 2 coats
- New drywall: primer + 2 coats
For 2 finish coats on our bedroom: 1.29 × 2 = 2.58 gallons.
Round up and add buffer
Order 3 gallons of finish paint. If primer is needed: 1.29 gallons (primer covers less, ~250 sq ft/gal → 1.8 gallons), round up to 2 gallons of primer.
Total: 3 gallons finish + 2 gallons primer = 5 gallons.
Ceilings
Ceiling area = length × width. Our 12 × 15 room ceiling = 180 sq ft.
Ceiling paint at 350 sq ft/gal × 1 coat (same-color refresh) = 0.51 gal → 1 gallon.
Ceilings with heavy texture or popcorn need 2 gallons for the same area.
Exterior estimation
Same formula, rougher surfaces. For a two-story house:
- Measure perimeter at ground level
- Multiply by average wall height (gable-end walls average the peak height)
- Add a 10% waste factor for cutting around trim, windows, and siding laps
- Use 250–300 sq ft/gal for rough siding, 350 for smooth
Trim is calculated separately in linear feet: assume 1 gallon covers roughly 350 linear feet of standard 4-inch trim per coat.
Primer — when you really need it
- New drywall or bare wood: yes, required
- Stains (water, smoke, ink): yes, stain-blocking primer
- Drastic color change: tinted primer saves a finish coat
- Glossy surface being repainted: bonding primer
- Same color refresh on painted wall: no primer needed
Common mistakes
- Trusting the label coverage. Label numbers are laboratory conditions — smooth surface, thick application, ideal temperature. Real-world is 15–30% less.
- Forgetting the second coat. 90% of interior paint jobs need 2 coats. Estimating for 1 is the #1 reason people run out mid-project.
- Not subtracting openings on exterior jobs. A typical 2,500 sq ft house exterior has 10–15% windows and doors. Subtract 250 sq ft of openings and save yourself a gallon.
- Buying paint from different lots. If you need 3 gallons, buy 3 at the same time from the same lot — color matching between lots is imperfect. The store can combine them into a 5-gallon bucket (called boxing) to eliminate any variation.
Run the numbers — the calculator does the rest
BuildCalc Pro's paint calculator handles walls, ceilings, doors, windows, primer, and multi-coat factoring in one form. Includes rough surface and exterior coverage modes.
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