Interior paint calculator

Calculator result and formula

Paint purchase: 3 gallons

Formula: (2 × (L + W) × H − door deductions − window deductions) × coats ÷ coverage

Formula ID: PAINT-INTERIOR-002 · Version 2.0.0 · Published · Last reviewed Aug 26, 2026.

Inputs

• Room length: Finished room length Default: 16.
• Room width: Finished room width Default: 12.
• Ceiling height: Finished floor-to-ceiling height Default: 8.
• Doors: Standard door openings to deduct Default: 1.
• Windows: Standard window openings to deduct Default: 1.
• Door deduction: Editable area deducted per door Default: 20.
• Window deduction: Editable area deducted per window Default: 15.
• Number of coats: Coats of selected finish paint Default: 2.
• Coverage per gallon: Use coverage stated on the selected product label Default: 375.

How this calculator works

This calculator measures all four interior room walls, deducts editable door and window areas, then applies selected coats and product-label coverage before whole-gallon rounding.

1. Measure finished room length, width and ceiling height.
2. Confirm or edit door and window deductions using actual opening areas when needed.
3. Use the coverage stated on the selected interior or exterior paint product label.

Worked example: Example: a 16 ft × 12 ft room at 8 ft high has 448 ft² of gross wall area. One 20 ft² door and one 15 ft² window leave 413 ft²; two coats at 375 ft² per gallon round to 3 gallons.

Assumptions and limitations

• Gross wall area uses 2 × (room length + room width) × ceiling height.
• Door and window deductions are editable planning assumptions, not a substitute for measured openings.
• Coverage must use the selected paint product label; the purchase quantity rounds up to whole gallons.

Common mistakes to avoid

• Confirm the measurement basis and units before using the result.
• Keep waste or allowance visible and adjust it for the actual job conditions.
• Confirm supplier packaging, delivery increments, specifications, and final site requirements before ordering.

Calculator disclaimer

This planning calculator does not determine structural design, code compliance, installation requirements, or guaranteed pricing. Confirm final requirements with the appropriate responsible professional, authority, manufacturer, and supplier.

Formula review and sources

Internal QA complete · independent external review pending · 2 source references.

• Benjamin Moore: room, doors and windows paint inputs
• Glidden: paint coverage guidance

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