Interior tools

Paint Calculator

Estimate wall and ceiling paint from measured surfaces, coats, product coverage, and the cans available to you.

Metric & imperial
Adjustable coverage
Measured openings

Room dimensions

Describe the surfaces

Measurement system

Room lengthftRoom widthftWall heightft

Include the ceilingAdds room length × width to the paintable surface.

Doors & windows to subtractft²Enter their combined measured area.Number of coatsFollow the selected product specification.Coverage per gallonft²/galUse the coverage printed on your paint label.Extra allowance%For texture, applicator loss, and touch-ups.

Container sizeThe purchase count is rounded up to this container size.

Optional price per 1 gal container$Uses the supplier price you enter for the current 1 gal container. No live prices are fetched.Currency labellabelDisplay only. BuildMeasure does not convert currencies or exchange rates.

This device

Saved estimates

Save an estimate to keep it here. Data stays in this browser.

Transparent by design

Room paint formula

The engine calculates the four rectangular walls from perimeter and height. A ceiling is optional, while door and window area is measured and subtracted explicitly.

Wall areaA_walls = 2 × (length + width) × heightPaintable areaA_paint = walls + ceiling − openingsPaint volumeV = (A_paint × coats ÷ coverage) × (1 + extra ÷ 100)

Coverage is not universal. Use the spread rate printed on the selected product. BuildMeasure keeps the full calculation unrounded, then rounds only the final container count upward.

Worked example

A 1,000-square-foot wall estimate

A 25 ft × 25 ft room with 10 ft walls produces 1,000 ft² of wall area when the ceiling and openings are excluded. Use one coat, 400 ft²/gal coverage, and 0% extra.

01
Find wall area2 × (25 + 25) × 10 = 1,000 ft²
02
Apply one coat1,000 ft² × 1 = 1,000 coat-ft²
03
Find paint volume1,000 ÷ 400 = 2.5 gal
04
Round containers upwardceil(2.5 ÷ 1 gal) = 3 cans

Verified result2.5 gal; buy 3 one-gallon cans

Coverage is a product input, not a universal constant. Replace 400 ft²/gal with the value printed on the selected coating.

Common questions

Paint calculator FAQ

Inputs you can verify instead of assumptions you cannot see.

How does the paint calculator find wall area?

For a rectangular room, it multiplies the room perimeter—two times length plus width—by wall height. It adds the ceiling if selected, then subtracts the combined measured area of doors and windows.

How much area does one gallon of paint cover?

Sherwin-Williams says a gallon typically covers about 350 to 400 square feet, but coverage changes with the product, texture, porosity, and application method. BuildMeasure starts at 400 square feet per gallon and lets you replace it with the value on your product label.

Should doors and windows be subtracted?

Large openings that will not be painted can be subtracted. Measure and combine their area instead of relying on a standard door or window size.

Does the estimate include multiple coats?

Yes. Paintable area is multiplied by the selected number of coats before it is divided by product coverage.

Why does the calculator add an extra allowance?

The optional allowance helps account for surface texture, paint retained by tools, small losses, and future touch-ups. It is adjustable from 0% to 25%.

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