How Much Paint Do I Need? Wall Coverage Math
Calculate paint gallons needed: wall area minus doors and windows, ~350 sq ft per gallon, double for two coats — with a worked 12×12 room example.
Buying too much paint is expensive and wasteful; buying too little means an emergency trip mid-project and, often, a slightly different batch. The math is straightforward once you strip out the parts everyone forgets: subtracting openings, planning for two coats, and adjusting for surface texture.
The core formula
Paint needed (gallons) = (Wall area − openings) × coats ÷ coverage per gallon
Standard coverage benchmarks:
- Interior latex on smooth drywall: ~350 sq ft/gallon (~10 m²/L)
- Rough or porous surfaces (new drywall, textured, masonry): ~250 sq ft/gallon
- Second coat on a sealed surface: often stretches to ~400 sq ft/gallon
Most brands quote 350–400 sq ft/gal on the can. Real-world coverage tends toward the lower end.
A worked example: 12×12 ft room, 9 ft ceilings
Step 1 — Wall perimeter × ceiling height. Perimeter = 4 × 12 = 48 ft Wall area = 48 × 9 = 432 sq ft
Step 2 — Subtract doors and windows. - One door: 3 × 7 = 21 sq ft - Two windows: 2 × (3 × 5) = 30 sq ft - Total openings = 51 sq ft - Paintable wall area = 432 − 51 = 381 sq ft
Step 3 — Multiply by coats. Two coats: 381 × 2 = 762 sq ft to cover
Step 4 — Divide by coverage. 762 ÷ 350 = 2.18 gallons
Round up: buy 3 gallons. A 1-gallon can plus a 2-gallon (or three 1-gallon cans) is safer than two, leaving a small stash for touch-ups. Compute your own room with the Paint Calculator and use the Square Footage Calculator for irregularly shaped walls.
Quick estimates by room size
| Room floor size | Wall area (8 ft ceilings) | Gallons for 2 coats |
|---|---|---|
| 10 × 10 | ~320 sq ft | 2 gal |
| 12 × 12 | ~384 sq ft | 2–3 gal |
| 12 × 15 | ~432 sq ft | 3 gal |
| 15 × 20 | ~560 sq ft | 3–4 gal |
| Bathroom (8 × 10) | ~288 sq ft (minus fixtures) | 1–2 gal |
Ceilings are treated separately: ceiling area = room length × width. A 12×12 ceiling = 144 sq ft, well under a gallon for two coats — but you almost always buy a full gallon of ceiling paint anyway.
Primer and surface adjustments
New drywall absorbs 20–30% more paint than sealed drywall — plan on primer *plus* two topcoats, and expect coverage closer to 250 sq ft/gal for the primer coat.
Textured surfaces (orange peel, knockdown, popcorn) have more surface area than the flat measurement suggests. Add 15–25% to your gallons.
Dark to light color change almost always needs a tinted primer plus two coats — often three coats without primer. Don't skip primer to save a can; you'll spend more on the extra topcoat.
Metric conversion
Coverage translates cleanly:
- 350 sq ft/gallon ≈ 8.6 m²/L
- 1 US gallon = 3.785 L
- 1 m² = 10.76 sq ft
For a European spec of 10 m²/L over two coats on a 40 m² room: 40 × 2 ÷ 10 = 8 L (~2.1 US gallons — round up to 3).
Common mistakes
- Forgetting the second coat. Almost every quality paint job needs two.
- Counting only floor area. You paint walls, not floors — perimeter × height is the right measurement.
- Ignoring texture. Textured walls can eat 20% more paint than smooth drywall.
- Not saving a labeled can for touch-ups. Batch codes matter; a "same color" can from the store six months later won't blend perfectly.
FAQ
How much paint do I need for a 12×12 room? About 3 gallons for two coats on 8–9 ft ceiling walls, minus openings. Ceilings need a separate half to full gallon.
How many square feet does a gallon of paint cover? Roughly 350 sq ft on smooth drywall for a single coat, less on textured or unprimed surfaces. Two coats effectively cut coverage per gallon in half.
Do I need primer if I'm using paint-and-primer-in-one? For a same-color repaint on a clean, sealed wall, usually not. For new drywall, stains, patches, or a large color change, use a dedicated primer — the "in-one" products don't seal like a true primer.
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*Skip the arithmetic mistakes: the free Paint Calculator handles openings, coats, and texture in one go.*