Paint Estimator (Rooms with Doors and Windows)
Calculate how much paint you need for any room.
Subtracts door and window areas automatically.
Covers walls, ceiling, trim, and multiple coats.
Accurate paint estimation saves you money — buy too little and you run out mid-room; buy too much and you waste paint. This calculator accounts for wall area, subtracts doors and windows, and applies your chosen number of coats.
The formula:
Paintable Area = (2 × Length × Height) + (2 × Width × Height) − (Doors × 20 ft²) − (Windows × 15 ft²)
Paint Needed (gallons) = (Paintable Area × Number of Coats) ÷ Coverage per Gallon
Standard coverage rates:
| Paint Type | Coverage per Gallon |
|---|---|
| Standard latex wall paint | 350 ft² (32.5 m²) |
| Primer | 200–300 ft² (18.5–28 m²) |
| Semi-gloss / gloss | 300–350 ft² (28–32.5 m²) |
| Ceiling paint | 350–400 ft² (32.5–37 m²) |
Metric equivalents:
- 1 gallon = 3.785 liters
- 1 liter covers approximately 85–90 ft² (8–8.5 m²)
- 1 square meter ≈ 10.76 square feet
Standard door and window sizes:
- Interior door: 20 ft² (1.86 m²) — approximately 6.5 ft × 3 ft (2 m × 0.9 m)
- Standard window: 15 ft² (1.39 m²) — approximately 3 ft × 5 ft (0.9 m × 1.5 m)
Coat recommendations:
- One coat: only for touch-ups over the same color
- Two coats: standard for most repaints
- Three coats: when painting over dark colors with a light shade, or for new drywall (prime + 2 coats)
Tips:
- Always buy 10–15% extra to account for mistakes and future touch-ups.
- Porous surfaces (new drywall, rough masonry) absorb more paint — estimate 20% extra.
- Stirring well before use prevents color inconsistency.
- Buy all paint from the same batch number to ensure color consistency across cans.
- For a standard 12 ft × 12 ft (3.6 m × 3.6 m) room with 9 ft (2.7 m) ceilings: expect 1–1.5 gallons (3.8–5.7 liters) for two coats.
Ceiling note:
Add the ceiling area separately: Length × Width.
Ceiling paint is often sold separately and covers more per gallon.