Paint Quantity Calculator
Subtracting Openings Beats Guessing a Deduction
Wall paint calculations often use a fixed per-door or per-window deduction, but real rooms have doors and windows of every size — a floor-to-ceiling window removes far more paintable area than a small bathroom window, and treating them the same overstates or understates the paint needed. This calculator instead takes your actual measured door and window area and subtracts it directly from the gross wall area.
The Formula
Gallons Needed = (Paintable Area × Coats) / Coverage per Gallon
Coverage per gallon is fixed at 350 sq ft, a standard trade assumption for one coat on a primed, smooth wall.
Where This Calculation Matters
- Rooms with large glazing — a room with floor-to-ceiling windows or sliding glass doors has significantly less paintable area than its gross wall dimensions suggest.
- Accent walls and feature painting — when only part of a room is being painted, entering the exact wall area for that section (rather than the whole room) keeps the gallon estimate precise.
- Color changes needing full coverage — going from a dark to a light color, or vice versa, typically requires the full 2-coat default rather than a single touch-up coat.
- Budgeting a multi-room repaint — running each room's wall and opening measurements through the same formula and summing the gallons avoids both an under-order and excess leftover paint.
Gallons Needed by Room Size
At 2 coats and 350 sq ft/gallon coverage, accounting for door and window area:
| Wall area | Door/window area | Paintable area | Gallons needed |
|---|---|---|---|
| 400 sq ft | 60 sq ft | 340 sq ft | 2 |
| 600 sq ft | 80 sq ft | 520 sq ft | 3 |
| 800 sq ft | 100 sq ft | 700 sq ft | 4 |
How to Use This Calculator
- Enter the Total Wall Area (sq ft) to be painted, before deductions.
- Enter the Door/Window Area to Subtract, measured from the actual openings.
- Enter the Number of Coats (defaults to 2).
- Select Calculate to see the gallons needed, rounded up to a whole gallon.
Related Calculations
Choosing a scheme before you shop for paint? See the Room Color Scheme Calculator, or plan the room's furniture around the new color with the Interior Design Space Planner.