How to use this calculator
- Choose a wall input mode. Room dimensions use the four-wall perimeter formula; individual walls let you handle L-shaped rooms, partial walls, or accent walls.
- Add doors and windows. Each opening's area is deducted from the paintable surface. Picking a type pre-fills a standard size you can adjust.
- Set coats, coverage, and waste. Coverage comes from your paint can — typically 350–400 sq ft per gallon. Two coats are standard for most repaints.
- Optionally add a price per gallon or litre to see an estimated total based on the rounded-up container count.
The formula
Paintable area is the gross wall area minus all openings, plus the ceiling if you include it. The purchase quantity is the required paint rounded up to whole containers, because paint is sold in sealed cans.
Worked example
A 14 × 12 ft room with 8 ft walls, 2 doors, 3 windows, 2 coats, 10% waste, coverage 350 sq ft/gal at $42 per gallon:
| Step | Result |
|---|---|
| Gross wall area: 2 × (14 + 12) × 8 | 416 sq ft |
| Openings: 2 × (3 × 7) + 3 × (3 × 4) | − 78 sq ft |
| Net paintable area | 338 sq ft |
| × 2 coats × 1.10 waste ÷ 350 | 2.12 gallons |
| Buy (rounded up) | 3 gallons ≈ $126 |
What coverage rate should I use?
Use the number printed on your paint can — products genuinely differ, which is why the field is editable. Rough textures, bare drywall, and dramatic color changes absorb more paint than the label assumes; if that describes your wall, lower the coverage value or add a coat rather than trusting a generic average.
How much waste should I add?
5–15% covers tray residue, brush build-up, drips, and touch-ups for typical rooms. Use the upper end for textured walls, many corners, or when painting the same color over a strong contrast. See How Much Extra Material Should You Buy? for the reasoning.
Common mistakes
- Forgetting the second coat — the most common cause of running out mid-room.
- Measuring wall height to the top of furniture instead of the ceiling.
- Adding the same window several times instead of using the quantity field.
- Using paint-store coverage for heavily textured or previously unpainted walls.
- Assuming one gallon covers a whole room — most average rooms need two to three gallons for two coats.
FAQ
Does the estimate include the ceiling?
Only when you tick "Include ceiling" in room-dimensions mode. The result panel and shopping list always state whether the estimate covers walls only or walls plus ceiling. Ceiling paint often has different coverage requirements, so treat the ceiling portion as part of the same estimate only for same-product jobs.
Should I really deduct doors and windows?
Yes for standard paint estimates — you do not paint over glass and hardware. The calculator deducts each opening's area from the gross wall area. If openings add up to more than the wall area, the net area is capped at zero and a warning appears so you can double-check your dimensions.
How many gallons do I need for a 12 × 12 room?
With 8 ft walls: gross wall area = 2 × (12 + 12) × 8 = 384 sq ft. Deducting one 3 × 7 ft door (21 sq ft) leaves 363 sq ft. Two coats with a 10% allowance is about 799 sq ft of coverage; at 350 sq ft per gallon that is 2.3 gallons, so buy 3 gallons.
Does the estimate include primer or trim paint?
No. The calculator estimates wall paint (and optionally ceiling paint) only. Primer, sealer, trim, and baseboard paint are separate products with their own coverage — estimate them with the product's stated coverage rate and your measurements.