About this calculator
A paint calculator estimates gallons from wall area, coats, and coverage rate, with optional waste factor.
Who should use it: DIY painters and contractors ordering materials.
When to use it: Use it after measuring paintable area and checking the can’s coverage label.
Paint area and cans
Start from wall rectangle area, subtract door and window openings, scale by coats and a waste margin, then divide by the can’s labeled coverage and round up to whole cans. Volume estimates treat coverage as sq ft per gallon (imperial) or sq m per liter (metric).
Step-by-step
Choose units
Use feet or meters consistently for height, width, openings, and coverage.
Enter wall size and count
Typical bedroom: 8 ft height, 12 ft width, 4 walls.
Subtract openings
Enter door and window counts, or override opening areas for oversized doors.
Set coats, coverage, and waste
Two coats are common; use the can label for coverage and keep ~10% waste.
Optional price
Enter price per can to see an estimated material cost.
Worked example: 8×12 ft room, 4 walls, 2 coats
Two doors and two windows, 350 sq ft/gal coverage, 10% waste, $35 per can.
- Gross = 8 × 12 × 4 = 384 sq ft
- Openings = 2×21 + 2×15 = 72 sq ft → net = 312 sq ft
- With coats = 312 × 2 = 624; with 10% waste = 686.4 sq ft
- Cans = ceil(686.4 ÷ 350) = 2; cost = 2 × $35 = $70
About 2 cans (~1.96 gal / 7.4 L) and roughly $70 in paint.
How to interpret the result
Textured or porous surfaces lower coverage — buy a little extra for touch-ups.
Key definitions
- Coverage per can
- How much wall area one can covers in one coat, in sq ft or sq m matching your unit (often listed per gallon).
- Waste margin
- Extra paint reserved for texture, cutting in, and touch-ups — typically about 10%.
- Paintable area
- Net wall area after openings, multiplied by the number of coats.
Common use cases
- Estimating interior wall paint for a single room
- Comparing one-coat vs two-coat material needs
- Rough DIY budget for paint cans
- Converting coverage between gallons and liters
Tips
- Always confirm coverage on the product label — flat paints and textured walls often cover less.
- Prime new drywall or drastic color changes; primer is not included in this estimate.
- Buy an extra quart for touch-ups if your calculation is very close to a whole can.
Common mistakes
Forgetting a second coat
Fix: Set coats to 2 (or more) whenever the label or color change requires it.
Using floor square footage instead of wall area
Fix: Paint needs wall height × width × walls, not room floor area alone.
Ignoring waste and openings
Fix: Subtract doors/windows and add a waste percent so you do not under-buy.
Limitations
- Color changes and primers can require extra coats.
- Ceiling and trim may need separate estimates.