Interior paint quantity starts with paintable wall area, not floor area. Calculate perimeter times wall height, subtract only meaningful openings, multiply by coat count, then divide by the selected coating's stated coverage and round to purchasable containers.
Interior finish gallons = net wall area × coats ÷ selected product coverage. Calculate wall area from room perimeter and height, subtract meaningful openings, and keep primer separate.
Published: September 26, 2026
Last reviewed: September 26, 2026
Published by BuildYard. The guide reflects BuildYard's calculation methodology and editorial standards and does not replace project-specific professional advice.
For a rectangular room, gross wall area = 2 × (length + width) × height.
Measure unusual wall shapes, stair walls, and vaulted zones separately.
Doors and large windows can reduce paintable area, while small openings may not materially reduce purchase quantity.
Keep trim and doors separate when they use another coating.
Total coat area = net paintable area × number of coats.
Primer is a separate product and should not be counted as a finish-paint coat unless the product system explicitly says otherwise.
Gallons = total coat area ÷ stated coverage per gallon, rounded for the containers actually sold.
Texture, porosity, application method, and color change can alter realized coverage.
Coverage figures are planning inputs rather than guarantees for every substrate.
Use current product data and field conditions before purchase.
Estimate deck stain for boards, stairs, fascia, and rails using developed area, coat count, product coverage, and wood condition.
Read guide →Paint & FinishEstimate fence coating from run length, height, painted sides, developed board and rail area, coat count, and product coverage.
Read guide →Paint & FinishConvert baseboard, casing, fascia, and molding linear feet into developed paint area, then calculate coating quantity by coats and coverage.
Read guide →Example uses 42 sq ft of openings, two finish coats, and 350 sq ft/gal planning coverage.
| Step | Calculation | Result |
|---|---|---|
| Gross walls | 2 × (14+12) × 8 | 416 sq ft |
| Net walls | 416 − 42 | 374 sq ft |
| Two coats | 374 × 2 | 748 sq ft |
| Paint | 748 ÷ 350 | 2.14 gal |
Current manufacturer data, supplier specifications, drawings, field measurements, applicable codes, permit requirements, and qualified professional guidance can take priority over a general-purpose planning estimate.