Interior Paint and Trim Calculator
Calculate paint, primer, and trim needed for a room. Get gallons per coat, total gallons with waste, and separate calculations for ceiling, walls, and trim in one estimate.
Formula & Reference
| Variable | Formula | Units |
|---|---|---|
| Interior Paint and Trim Calculator | Gallons = Paintable Area / 350 sq ft per gallon | gallons per coat |
Step-by-Step Examples
15x12 ft room, 9-ft ceilings, 2 doors, 3 windows, 2 wall coats.
- Wall area = (15+12)x2x9 = 486 sq ft
- Minus doors (2x21=42) + windows (3x15=45) = 399 sq ft
- Wall paint: 399x2/350x1.10 = 2.51 gal
- Ceiling: 180/350x1.10 = 0.57 gal
- Total: 3.08 gal -> buy 4 gallons (walls) + 1 qt (ceiling)
20x16 ft, 9-ft ceilings, 1 door, 4 windows, 2 coats.
- Net wall: 648-21-60 = 567 sq ft
- Wall paint: 567x2/350x1.10 = 3.56 gal
- Ceiling: 320/350x1.10 = 1.01 gal
- Total: 4.57 gal -> buy 5 gallons
1,800 sq ft, 9-ft ceilings, 15 doors, 20 windows.
- Rough estimate: total wall area ~2,400 sq ft net
- Ceiling: 1,800 sq ft
- Total: (2,400x2+1,800)/350x1.10 = 20.8 gal
- Buy 21 gallons for 2-coat walls and 1-coat ceiling
Real-World Applications
Common Mistakes to Avoid
'I need to paint 3 bedrooms' is not enough information. Room sizes vary hugely. Always measure square footage for accurate paint quantities.
Switching from a dark color to a light one (or vice versa) often requires a coat of primer plus 2 finish coats = 3 coats total. Factor this into your gallon count.
Quality paint (Sherwin-Williams, Benjamin Moore, Behr Premium) covers better (fewer coats), lasts longer, is more washable, and often works out cheaper per square foot over time despite higher per-gallon cost.