Interior

Paint Calculator: How Much Paint Do I Need?

Estimate the amount of wall paint to buy for a rectangular room. The calculator subtracts typical door and window areas and accounts for multiple coats.

Project measurements

Change the defaults to match your project and the product you plan to buy.

Room length

Room width

Wall height

Doors

Windows

Coats

Paint coverage

sq ft

Paint unit price (optional)

$ / gal

Project result

Your paint plan

See the measured wall area, coated coverage, paint quantity and a whole-gallon purchase plan.

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Planning estimate only. Verify product coverage, package size, supplier specifications and project requirements before purchasing.

Methodology

Formula & assumptions

Gross wall area

2 × (room length + room width) × wall height

Calculates the four rectangular wall surfaces before openings.

Paintable wall area

gross wall area − door allowance − window allowance

Default planning deductions are 20 sq ft per door and 15 sq ft per window.

Paint required

paintable area × coats ÷ coverage per gallon

Replace the default coverage with the value printed on your paint.

Purchase quantity

round calculated paint up to a practical whole-unit purchase

US mode rounds to whole gallons; Metric mode rounds to whole liters for planning.

How to measure this project

Measure the room length and width at floor level, then measure wall height from finished floor to ceiling. Count only doors and windows you intend to leave unpainted. For rooms with large openings, built-ins or partial-height walls, measure those surfaces separately for a tighter estimate.

Before you buy materials

Coverage can fall when you paint porous drywall, masonry, textured surfaces or make a major color change. Primer is not included in the paint quantity. If your product is sold in quarts, 5-gallon pails or other package sizes, use the calculated volume as the target and choose the package mix that meets or exceeds it.

Reference

Paint planning reference

Planning item	Typical starting point	Adjust when
Coverage	350 sq ft/gal	Use the label value for your exact product.
Doors	20 sq ft each	Measure unusually large or small doors.
Windows	15 sq ft each	Measure picture windows or window walls.
Coats	2 coats	Some repaint jobs cover in one; major color changes may need more.

Questions

Frequently asked questions

Does the paint calculator include the ceiling?

No. This version estimates wall paint only. Measure a ceiling as a separate rectangular area if you plan to paint it.

Should I subtract doors and windows?

Yes when they will not be painted. The calculator uses simple planning deductions, and you can improve accuracy by measuring unusually large openings separately.

Why does the calculator round gallons up?

Material planning should not assume you can buy a fraction of a standard gallon. The result shows calculated volume and a practical whole-gallon purchase quantity.

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