Paint Calculator

Estimate how many gallons of paint you need for any room. Enter the room dimensions (or total wall area), the number of coats, the area of doors and windows to subtract, and your paint's coverage. A 10% waste allowance is included.

📅 Updated: 2026-08-15 ✍️ Author: RenoFig Research Team 📊 Data: Sherwin-Williams / BEHR coverage standards ✅ Free & No Sign-Up

Calculator Inputs

Use this if your walls are not simple rectangles (e.g., vaulted or with many openings).
~21 sq ft per door, ~15 sq ft per window. Leave 0 to paint everything.
Default 350 (typical 1-coat interior). Primer ≈ 300–350.

Results

Total Wall Area—
Paintable Area (after openings)—
Base Gallons (no waste)—
Gallons Needed (+10% waste)—
Whole Gallons to Buy—

* Estimate only. Texture, unprimed drywall, dark-to-light color changes, and heavy spray loss increase usage. Always round up to whole gallons.

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Formula & How It Works

The calculator converts your room size into paintable wall area, then divides by coverage and scales by coats:

StepFormula
Wall area2 × (Length + Width) × Height
Paintable areaWall area − Doors/Windows area
Base gallonsPaintable area × Coats ÷ Coverage
Total gallonsBase × 1.10 (10% waste)

Standard coverage: 1 gallon of interior paint covers ~350–400 sq ft per coat (Sherwin-Williams and BEHR published guidance). Primer covers ~300–350 sq ft/gallon. Ceilings and textured/unprimed surfaces need more.

Frequently Asked Questions

A gallon of interior paint covers about 350–400 sq ft per coat for a primed, smooth wall (Sherwin-Williams / BEHR standards). Primer covers roughly 300–350 sq ft per gallon. We default to 350 sq ft/gallon and let you adjust it.
A 12x12 room with 8-ft ceilings has about 384 sq ft of wall (2 x (12+12) x 8). Subtract doors/windows (~60 sq ft) and you have ~324 sq ft. One coat at 350 sq ft/gal needs ~0.93 gal; with two coats and a 10% waste allowance plan on about 2.0 gallons.
Yes. Painters typically add 10% for spillage, overlap, and touch-ups. Buy whole gallons (round up) and keep one for future repairs — colors can be hard to re-match later.

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Data sources & retrieved: 2026-08-15 — Sherwin-Williams "Paint Calculator & Estimator" coverage guidance (≈350–400 sq ft/gal per coat); BEHR published paint coverage (≈350–400 sq ft/gal); typical door area ≈21 sq ft and window area ≈15 sq ft.

Disclaimer: This tool is for educational estimation only. Actual paint needs vary with surface texture, primer, color change, and application method. It does not constitute a quote or professional advice.