Paint Calculator
A 12×14 ft room with 9 ft ceilings needs about 2 gallons for two coats (walls only). Add ceiling: 2.5 gallons total. Budget: $80-175.
Paint Calculator
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How to Calculate Paint
The formula computes net paintable wall area:
wall_area = (2 × (length + width) × height) - (doors × 21) - (windows × 15)
gallons = wall_area / coverage_per_gallon × number_of_coats
Standard coverage: 350 sq ft/gallon for flat, 300 sq ft/gallon for semi-gloss.
| Surface | Coverage (sqft/gal) | Coats Needed |
|---|---|---|
| Smooth drywall (primed) | 350-400 | 2 |
| Textured drywall | 250-300 | 2 |
| New drywall (unprimed) | 200-250 | 1 primer + 2 |
| Wood trim/doors | 300-350 | 2 |
| Brick / masonry | 150-200 | 2 |
| Stucco (exterior) | 150-250 | 2 |
| Previously painted (same color) | 350-400 | 1-2 |
| Drastic color change | 350-400 | 2-3 |
| Feature | Area to Subtract |
|---|---|
| Standard door (3×7 ft) | 21 sq ft |
| Standard window (3×5 ft) | 15 sq ft |
| Sliding glass door | 40 sq ft |
| Fireplace opening | 12-15 sq ft |
Common Mistakes:
- Calculating for one coat when you need two — almost every paint job needs two coats
- Forgetting primer on new drywall — paint soaks into unprimed drywall, requiring 50% more paint
- Not subtracting doors and windows — overbuying by 10-15%
- Buying flat paint for kitchens and bathrooms — use semi-gloss or satin for moisture resistance and cleanability
- Mixing cans from different batches — pour all cans into a bucket and stir ("boxing") for consistent color
Frequently Asked Questions
How much paint do I need for a 12x12 room?
A 12×12 room with 8 ft ceilings has about 350 sq ft of wall area after subtracting one door and two windows. You need 2 gallons for two coats. Add the ceiling (144 sq ft) and you need a third gallon.
How many square feet does a gallon of paint cover?
One gallon covers 350-400 square feet on smooth, primed surfaces with one coat. For practical purposes, plan on 175-200 square feet per gallon with the standard two coats. Textured walls reduce coverage to 250-300 sqft per coat.
Do I need primer before painting?
Yes, on new drywall, bare wood, stains, or when making drastic color changes (dark to light). Paint-and-primer combos work for repainting similar colors on previously painted surfaces. Separate primer costs $20-30/gallon but saves money by reducing the number of paint coats needed.
How much does it cost to paint a room?
DIY: $50-200 for paint and supplies (roller, tape, drop cloth). Professional: $300-800 per room depending on size, prep work, and paint quality. Premium paint ($50-75/gallon) costs more upfront but covers better — often saving a coat.
Should I buy extra paint?
Always buy 10-15% more than calculated. Keep leftover paint in the original labeled can for touch-ups — wall paint gets scuffed within months of painting. Most stores accept returns of unopened gallons within 30 days.
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