Buying too little paint means a mid-project store run and a possible dye-lot mismatch. Buying too much wastes money. The good news: calculating paint is straightforward once you know the formula and coverage rates. Use our paint calculator for an instant estimate, or follow the steps below.
The Basic Formula
Paint needed (gallons) = Net Wall Area × Number of Coats ÷ Coverage per Gallon
Net wall area is the total wall surface minus doors, windows, and other openings. The standard coverage rate is 350 square feet per gallon for a smooth, previously painted wall. Here is how to calculate each part.
Step 1: Measure the Room
- Measure wall length and height. Use a tape measure to record each wall's length and the ceiling height in feet. For a rectangular room, perimeter = 2 × length + 2 × width.
- Calculate gross wall area. Multiply perimeter by ceiling height. Example: 12×12 room with 8 ft ceilings → perimeter 48 ft × 8 ft = 384 sq ft.
- Subtract openings. Deduct approximately 20 sq ft per standard door and 15 sq ft per average window. For large openings (sliding glass doors, bay windows), measure and subtract the actual area.
- Add the ceiling (optional). Ceiling area = length × width. A 12×12 room ceiling is 144 sq ft. Use flat or matte ceiling paint, which covers 350–400 sq ft per gallon.
Step 2: Choose Coverage Rate
Coverage varies by surface type. Use this table to pick the right rate:
| Surface | Coverage per Gallon | Notes |
|---|
| Smooth repainted wall | 350–400 sq ft | Best case |
| New drywall (unprimed) | 250–300 sq ft | Prime first |
| Textured walls | 250–300 sq ft | Orange peel, knockdown |
| Brick or concrete | 200–250 sq ft | Porous, use masonry primer |
| Bare wood | 250–300 sq ft | Always prime first |
| Wallpaper (painted over) | 200–250 sq ft | Use primer-sealer |
Step 3: Decide Number of Coats
- One coat: Only for repainting the same color in good condition.
- Two coats: Standard for color changes, new drywall, and dark-to-light transitions. This is the safe default.
- Three coats: Drastic color changes (red to white, dark to very light), or staining new drywall.
Quick Reference: Paint by Room Size
| Room Size | Ceiling Height | Walls (2 coats) | + Ceiling | Est. Cost |
|---|
| 10×10 ft | 8 ft | ~1.6 gal | +0.3 gal | $50–$100 |
| 12×12 ft | 8 ft | ~2.0 gal | +0.4 gal | $60–$130 |
| 14×16 ft | 9 ft | ~3.1 gal | +0.6 gal | $100–$200 |
| 16×20 ft | 9 ft | ~4.2 gal | +0.9 gal | $140–$280 |
| 20×24 ft | 10 ft | ~6.5 gal | +1.4 gal | $220–$430 |
Cost estimates assume $30–$65 per gallon for interior latex paint. Primer is $20–$40 per gallon.
Primer: When and How Much
Primer is not always necessary, but it is critical in these situations:
- New drywall: Use PVA drywall primer, ~300 sq ft per gallon.
- Stains or water damage: Use stain-blocking primer (oil-based or shellac-based like BIN or Kilz).
- Drastic color changes: Especially dark to light — primer prevents bleed-through.
- Porous surfaces: Bare wood, brick, and unpainted masonry all need primer.
For repainting a similar color on smooth walls, a paint-and-primer-in-one product works well and saves a step.
Common Mistakes
- Forgetting the ceiling. Ceiling paint is formulated differently (flatter, spatter-resistant). Don't use wall paint on ceilings.
- Not accounting for texture. Textured walls use 15–25% more paint than smooth walls.
- Buying from different lots. Even the same color can vary between production runs. Buy all paint at once.
- Ignoring trim. Baseboards, doors, and window trim need separate semi-gloss or satin paint. Calculate trim separately.
- Rounding down. Always round up to the nearest gallon or half gallon. Touch-ups need leftover paint.
FAQ
How much paint for a 12×12 room?
A 12×12 room with 8 ft ceilings needs approximately 2 gallons for two coats on walls, plus 0.4 gallons for the ceiling. In metric: 7.6 liters for walls + 1.5 liters for ceiling.
How many square feet per gallon of paint?
One gallon covers 350–400 sq ft on smooth surfaces. Textured or porous surfaces cover 250–300 sq ft. Primer covers approximately 300 sq ft per gallon.
Do I need primer before painting?
Primer is needed for new drywall, stained surfaces, drastic color changes, and porous surfaces like bare wood. For repainting similar colors, paint-and-primer-in-one products work well.
How many coats of paint should I apply?
Two coats are standard for even coverage and durability. Dark colors, new surfaces, or drastic color changes may require three coats. Allow 2–4 hours drying time between coats of latex paint.
Calculate It Instantly
Skip the math — use our free paint calculator to estimate gallons, liters, and material cost in seconds. It accounts for doors, windows, ceilings, and coverage rate automatically.
Related: wallpaper calculator · drywall calculator · square footage calculator