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: PVA 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 — PVA primer seals raw paper and joint compound so finish paint applies uniformly
- 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.
Frequently asked questions
How many gallons of paint for a 12x12 room?
Wall area: 48 ft perimeter × 8 ft = 384 sq ft, minus 1 door + 1 window (36 sq ft) = 348 sq ft. At 350 sq ft/gal for 2 coats: 2.0 gallons → buy 3 gallons (buffer for cutting-in + touch-ups). For the ceiling (144 sq ft): 1 gallon. Total: 4 gallons.
How much paint do I need for 1,000 square feet of walls?
At 350 sq ft per gallon × 2 coats: 1,000 × 2 ÷ 350 = 5.7 gallons → order 6 gallons of finish paint. If primer is also needed (new drywall or major color change): add 1,000 ÷ 250 = 4 gallons of primer. In practice, buy one extra gallon above your calculation as touch-up stock — 1,000 sq ft of walls is a significant amount of surface and cutting-in alone uses more than you expect.
Do I need primer before painting new drywall?
Yes — always. Raw drywall paper absorbs paint unevenly and causes "flashing" (alternating dull and shiny patches) that no amount of finish coats will fix. Use PVA drywall primer, then 2 coats of finish. Skipping primer on new drywall is one of the most expensive painting mistakes.
What is the real-world coverage per gallon of interior paint?
Label says 400 sq ft/gal. Real-world: 350 sq ft/gal on primed smooth drywall (safe default); 300–350 on textured walls; 200–250 on popcorn ceilings; 150–250 on stucco or masonry. Use the manufacturer number only for spotless smooth surfaces.
How much paint for an entire house interior?
Rule of thumb: total wall area (all rooms combined) ÷ 350 × 2 coats. For a 1,500 sq ft single-story house with 8 ft ceilings, total wall area runs ~1,800–2,200 sq ft → 10–13 gallons of finish. Add 1 gallon of ceiling paint per 400 sq ft of ceiling. Add primer if painting new drywall throughout.
Related guides
Paint typically goes up after drywall is finished and before flooring is installed. Related material calculations:
- How to Calculate Drywall — the walls you'll be priming and painting, including sheet count and joint compound
- How to Calculate Flooring — goes in after paint cures, so it won't catch drips from your roller
- How to Calculate Framing Lumber — the wall structure behind the drywall you're painting
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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