Paint Calculator
Estimate gallons of interior or exterior paint from room dimensions or direct wall area. Deducts doors and windows and applies coats and waste.
Assumptions
- Default coverage is 350 ft² per gallon per coat (≈32.5 m²/gal) for quality latex on smooth surfaces.
- Default door deduction is 21 ft² (≈1.95 m²); window deduction is 15 ft² (≈1.39 m²) each.
- Two coats are assumed unless you change the setting.
- Does not include ceiling area unless included in wall area entry.
- Primer is calculated separately if coverage differs.
Paintable area = wall area − openings (all converted to m² internally). Gallons = (paintable area × coats ÷ coverage rate) × (1 + waste%). Results round up to the nearest quarter gallon. Coverage and openings use the area units you select (ft² or m²).12 × 10 ft room, 8 ft ceilings, 2 coats
- Perimeter = 2×(12+10) = 44 ft. Wall area = 44 × 8 = 352 ft². Minus 1 door (21) and 2 windows (30) = 301 ft² paintable. Two coats on 350 ft²/gal: (301×2)÷350 = 1.72 gal → 1.75 gal with quarter-gallon rounding, plus waste. Metric equivalent: enter lengths in m and openings/coverage in m² for the same physical result.
How to measure
Measure room length and width along the floor. Measure ceiling height at a corner. Count doors and standard windows, or enter custom opening areas with matching ft² or m² units for large patio doors or picture windows.
Common mistakes
- Using floor square footage instead of wall surface area.
- Forgetting the second coat on new drywall or bold color changes.
- Using manufacturer best-case coverage on textured or porous walls.
- Entering metric opening areas while leaving the unit on ft² (or the reverse).
Frequently asked questions
Do I need primer?
New drywall, stains, or dark-to-light color changes usually need primer. Calculate primer as a separate coat with its coverage rate.
What coverage rate should I use?
Check the can label. Smooth walls often 350–400 ft²/gal (≈32–37 m²/gal); textured walls may be 250–300 ft²/gal. Switch the coverage unit to match the label.
Should I include the ceiling?
Add ceiling square footage to your wall area, or calculate the ceiling as a separate rectangle.
Why round to quarter gallons?
Retail paint is sold in quart increments above 1 gallon. Rounding avoids coming up short mid-project.