The base formula
For a rectangular room:
- Wall area = perimeter × ceiling height = 2 × (L + W) × H
- Paintable area = wall area × (1 − openings fraction)
- Gallons (raw) = paintable area × coats ÷ coverage per gallon
- Gallons (ordered) = round up to whole gallons
For a 12 × 14 ft room with 8 ft ceilings, 10% openings, 2 coats, 350 ft²/gal:(2 × (12+14) × 8) × 0.90 × 2 ÷ 350 = 416 × 0.90 × 2 ÷ 350 ≈ 2.14 gal → 3 gal
Coverage rates
Manufacturer-stated coverage assumes a smooth substrate at the recommended film thickness with no spray loss. Real-world coverage is usually 10-20% lower than the claim. Use these realistic defaults:
- Smooth drywall (Level 4-5 finish): 350 ft²/gal, one coat
- Light orange-peel texture: 300 ft²/gal
- Heavy texture / knock-down: 250 ft²/gal
- Popcorn ceiling: 200-225 ft²/gal
- Stucco, concrete block, masonry: 175-225 ft²/gal
- Bare wood (raw substrate): 250-300 ft²/gal
Two coats vs three coats
Default to two coats for any color change. The first coat is the base; the second is the hide. With most modern interior latex paints, two coats over a primed surface achieves Level 4 finish (full color hide, no streaking).
Three coats are warranted for:
- Deep saturated colors (red, navy, forest green) over white
- White over a previously dark color (without tinted primer)
- High-traffic commercial areas where wash-down durability matters
- Critical light areas (Level 5 finish requirements)
Primer logic
Primer is the substrate-conditioning coat that goes on before finish paint. Whether to prime depends on the substrate:
- New drywall: 1 coat of drywall primer. The paper face is absorbent; without primer the first coat of paint flashes (uneven sheen) over the joint compound vs the paper. PVA primer is the standard product.
- Stained or patched walls: 1 coat of stain-blocking primer over the affected areas only. Shellac-based primers block the worst stains (smoke, water rings, knot bleed).
- Drastic color change: 1 coat of tinted primer matched to the topcoat. Reduces coats of finish paint needed from 3 to 2.
- Glossy surfaces being recoated:1 coat of bonding primer over satin/semi-gloss surfaces being repainted. Without primer the new finish doesn't stick.
- Recoats on flat/eggshell in good condition: No primer. Just two coats of finish.
Primer coverage is usually 200-250 ft²/gal — denser product, lower coverage than topcoat.
Openings allowance
Doors and windows reduce paintable wall area. Two ways to handle:
- Subtract by area — measure each opening (door ≈ 20 ft²; window 10-30 ft²) and subtract from the gross wall area. Most accurate.
- Lump-sum percent — apply a flat percentage deduction. 10% for a typical residential room, 15% for rooms with large windows, 5% for windowless garages. Faster but less accurate.
The calculator on this site uses the lump-sum approach; for precise commercial bids with a door-and-window schedule, use the per-opening subtraction in BidScreen XL.
Ceilings and trim
Run separate calcs for ceiling paint (ceiling paint is usually different product than wall paint — flatter, scrubbable for cleaning) and trim paint (semi-gloss enamel for durability).
For trim quantities, a rule of thumb: 1 quart of trim paint covers one average-sized room (door casings, window casings, base, and crown if any). Bid by the room or by linear foot of trim; crews track per-room usage easily.
Ordering
Round the raw gallon count up to whole gallons. Paint stores don't sell fractional gallons — and you don't want to run out mid-job. A 5-gallon bucket is cheaper per gallon than 5 individual gallons; if you need 4 or more gallons, order one 5-gallon. If you need 10 or more, order multiple 5-gallons.
Note the batch number on the can or 5-gallon. Mixing batches in the same wall can show as a streak in the finished paint. Box-and-strain across multiple gallons before starting a wall to guarantee consistent color.