How much paint do I need?
For a room, wall area is 2 x (length + width) x wall height. A 12 x 12 room with 8 ft walls, 2 coats, and 350 sq ft per gallon needs about 2.19 gallons before a separate buffer.
Coverage changes by surface
Porous drywall, patched areas, textured walls, dark color changes, exterior surfaces, and rough wood can reduce real coverage per gallon.
Estimate rooms separately
Separate rooms, ceilings, trim, doors, cabinets, decks, and accent walls when coats, color, surface type, or coverage differs.
Stain and sealer coverage examples
Examples use broad planning coverage. Rough, dry, or weathered wood may need more stain or sealer.
| Surface | Measured area | Estimated gallons |
|---|---|---|
| Deck boards | 300 sq ft | 2 gallons |
| Deck rails | 220 sq ft | 2 gallons |
| Fence face | 600 sq ft | 3 gallons |
| Wood siding | 1,200 sq ft | 6 gallons |
Before you calculate
- Measure boards, rails, posts, stairs, siding, pergolas, and fence faces separately.
- Use lower coverage for rough, dry, weathered, or very porous wood.
- Check whether the product coverage is for one coat or the full recommended system.
Common mistakes
- Using deck floor area only when rails, stairs, or posts are also being stained.
- Ignoring wood porosity, sanding, previous coatings, or second coats.
- Combining transparent stain, solid stain, and sealer under one coverage number.
Formula
units = ceil((area * (1 + wastePercent / 100)) / coveragePerUnit)
Assumptions
- Coverage changes with wood species, age, porosity, roughness, previous coatings, and application method.
- Rails, pickets, posts, deck boards, stairs, and both fence sides should be measured consistently.
- Use the exact stain or sealer label for final coverage and coat assumptions.
Example
Estimated fence sealer needed (gallons): 3 gallons
How to estimate paint gallons
- Measure the paintable surface area in square feet, subtracting only major openings when appropriate.
- Enter paint coverage per gallon from the product label and include the number of coats when the project needs more than one coat.
- Add waste or touch-up allowance for texture, color changes, overspray, trim edges, and small parts.
- Divide adjusted paint area by coverage per gallon and round up to whole gallons or containers; the default example returns 3 gallons.
- Check primer, prep, masking, trim paint, and sprayer or roller needs separately from paint quantity.
Before you buy materials
- Round up to full gallons or containers and keep extra for touchups.
- Use the selected stain or sealer label as the final coverage source.
FAQ
How do I calculate wall area for painting?
Use wall area = 2 x (room length + room width) x wall height. Then multiply by coats and divide by coverage per gallon.
How much paint for a 12 x 12 room?
With 8 ft walls, 2 coats, and 350 sq ft per gallon, a 12 x 12 room needs about 2.19 gallons before subtracting openings or adding a separate buffer.
Should I subtract windows and doors?
For quick room estimates, many people leave openings in as a buffer. For detailed takeoffs, subtract large openings and calculate trim separately.
Does this include ceilings or trim?
No unless you add those surfaces to the area. Ceilings, trim, doors, cabinets, and decks often need separate paint or stain estimates.
Related calculators
This calculator is for planning estimates only. Verify final quantities with product labels, project conditions, and a qualified professional when accuracy matters.