How to use
- Enter room dimensions, openings, and coat counts.
- Choose whether to include the ceiling and primer.
- Review exact gallons and rounded gallons to buy.
Input guide
- Default deductions use 20 sq ft per door and 15 sq ft per window.
- Paint coverage should come from the can label when possible.
- Primer coverage can differ from paint coverage.
Common mistakes
- Forgetting the second coat.
- Not adjusting coverage for rough, patched, or dark surfaces.
- Counting ceiling paint with wall paint when using different products.
Assumptions and limitations
- Actual coverage depends on surface, color change, paint type, and application method.
- Door and window deductions are estimates.
- No advice is provided on lead paint, surface hazards, or ventilation.
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Formula
- Wall area = 2 x (length + width) x height.
- Paintable wall area = wall area - door, window, and custom opening area.
- Paint area = paintable wall area x coats + ceiling area x ceiling coats.
- Gallons to buy = paint area / paint coverage, rounded up.
Example
A 12 ft x 10 ft x 8 ft room has 352 sq ft of wall area. After one door and two windows, two coats need 604 sq ft of coverage, or 2 gallons at 350 sq ft per gallon.
FAQ
Why does the calculator round gallons up?
Paint is typically purchased in fixed container sizes, and a small shortage can cause color or finish inconsistencies.
Do I always need primer?
No. Primer depends on surface condition, stains, bare material, and color change. The calculator only estimates quantity if you choose primer.