Estimix

Wall paint: perimeter area, opening deductions, and gallons to order

This is a small-room and touch-up model: four walls, one ceiling height, and a quick deduction for each door and window using typical opening sizes. It does not paint the ceiling or add base, crown, and casing line items. Use the coverage the can label promises for that product, then round up to whole gallons so you are not a quart short on the last wall.

Calculator inputs

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Results (live)

Gross wall area (ft²)
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Adjusted paintable area (ft²)
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Gallons needed (rounded up)
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Estimated material cost (from $/gal × gallons)
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Enter positive room length, width, and wall height, non‑negative door and window counts, at least one coat, and coverage in ft² per gallon to see wall area, paintable area, gallons, and optional cost.

Formula and units

Gross wall area in ft² = 2 × (length + width) × height. Non-paintable openings use fixed averages: 21 ft² per door, 15 ft² per window. Paintable area = max(gross area − count deductions, 0). Work for all coats: total area to cover = paintable area × number of coats. Gallons to buy = the ceiling of (total area ÷ ft² per gallon on the can). If you add a $/gallon, estimated material cost = whole gallons × that price. Ceiling is intentional—you buy by the can, not the theoretical fraction.

Example calculation

A 12 ft by 14 ft room with 8 ft walls is 416 ft² gross. One 21 ft² door and two 15 ft² windows remove 51 ft², so about 365 ft² is paintable. Two coats and 350 ft² per gallon is 730 ÷ 350 ≈ 2.1; you order 3 whole gallons. At $45 per gallon, that is a $135 order band before tax and any primer pass.

Cost explanation

The dollars tie to the rounded-up gallon count and your can price. If the store’s price is per 5-gallon pail, convert mentally or use the single-gallon effective rate so the subtotal is comparable to your line item.

Common mistakes

  • Forgetting the ceiling and built-ins in real bids

    This model is walls only. If the scope is ceiling, trim, and closets, break those out or your gallons will be low on the first coat.

  • Using architect coverage on a heavy nap or repair wall

    Rough drywall or a deep color change can burn more film. Use a conservative spread rate for the first pass.

  • Counting a slider as one 15 ft² window

    The 15 ft² is an average. Picture units and full walls of glass are not 15; field measure for warranty-level jobs.

Frequently asked questions

  • Are 21 ft² and 15 ft² always right?

    They are industry-style averages for quick checks. A full-height glass wall or a pair of 6-0 double doors is not 21—adjust counts or measure openings on bid-grade work.

  • Why is cost gallons × $/gallon and not a theoretical decimal?

    The engine rounds to whole gallons the way a paint desk sells stock. The dollars follow what you would hand to a cashier, not a spreadsheet fraction.

  • Can I add the ceiling in this form?

    Not in this pass—ceiling is a second rectangle with its own length × width. Use a separate run or a full takeoff when ceiling is in the same spec.