Paint Calculator

Estimate paint gallons, door/window deductions, coats, and can counts for room interiors.

Room Length (ft)

Room Width (ft)

Wall Ceiling Height (ft)

Number of Coats

Include Ceiling Painting

Doors Count (3×7 ft standard)

Windows Count (3×5 ft standard)

Paint Coverage per Gallon

Price per Gallon ($)

Paint Surface Formulas & Coverage Guidelines

Paint estimation sums the gross surface area of all four walls, deducts door and window openings, adds optional ceiling area, and multiplies by the required number of coats before converting to standard 1-gallon purchasable cans.

Gross Wall Area: Gross = 2 × (Length + Width) × Height
Net Area per Coat: Net = Gross - Openings + Ceiling Area
Gallons to Purchase (Ceiling): Gallons = ceil((Net Area × Coats) / Coverage per Gal)

Verified Worked Examples

Example: Bedroom 2-Coat Repaint

A 14 ft by 12 ft bedroom with 9 ft ceilings, 1 standard door (21 sq ft), and 2 windows (15 sq ft each).

Net Wall Area:417 sq ft / coat

Total Coverage:834 sq ft (2 coats)

Gallons to Buy:3 gallons (2.38 gal needed)

Gross walls: 2 × (14 + 12) × 9 = 468 sq ft. Openings: 21 + 30 = 51 sq ft. Net: 417 sq ft. 2 coats: 834 sq ft. Gallons: ceil(834 / 350) = 3 gallons.

Frequently Asked Questions

How many square feet does one gallon of paint cover?

A standard gallon of interior wall paint covers between 350 and 400 square feet with one coat on primed, smooth drywall. Rough or unprimed surfaces require more paint.

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