Surfaces calculator

Paint Coverage Calculator

Estimate wall and ceiling paint with coats, doors, windows, and waste included. Switch between US customary and metric units, adjust the assumptions, and keep every step visible.

01 Measure02 Allow03 Order

Project inputs

Use one unit system throughout. Defaults show a realistic example; replace them with measurements from your project.

Unit system

Room lengthExample input — replace with your project value.ftRoom widthExample input — replace with your project value.ftWall heightExample input — replace with your project value.ftDoors & windowsExample input — replace with your project value.ft²Additional wall areaAdd measured wall area from other rooms or sections.ft²Number of coatsExample input — replace with your project value.Coverage per gallon / literUse the coverage printed on the selected product.ft²Include the ceilingWaste allowanceAdd for cuts, breakage, spillage, and uneven site conditions.%

Measure the finished shape. Typical interior paint covers about 350–400 ft² per US gallon, but texture, porosity, color change, and application method affect real coverage.

Estimate matches the inputs shown.

ReviewedNovember 2, 2026

ByBuildGauge Editorial Team

ScopeFormula, defaults & sources

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The calculation

A visible formula, not a black box.

Gallons & liters · Mathematical derivationpaint = ((wall area − openings) + extra wall area + optional ceiling) × coats × waste factor ÷ product coverage

Inputs are converted to a common base unit before calculation. Purchase quantities are rounded only at the final buying boundary.

Example inputDefaultStatus

Room length14 ftReplace with measurement

Room width12 ftReplace with measurement

Wall height8 ftReplace with measurement

Doors & windows60 ft²Planning default — verify

Additional wall area0 ft²Planning default — verify

Number of coats2 Planning default — verify

Coverage per gallon / liter375 ft²Planning default — verify

Include the ceilingfalse Planning default — verify

Waste allowance5 %Planning default — verify

Worked example

Example: a 14 × 12 ft room

With 8 ft walls, 60 ft² of openings, two coats, 375 ft²/gal coverage, and 5% allowance, the unrounded estimate is 1.99 gal and the purchase result is 2 gal.

Use this to understand the workflow—not as a recommendation for your project.

Sources & scope

What supports this calculation.

Manufacturer dataSherwin-Williams Paint Calculator FAQ ↗

Typical 350–400 ft² per gallon coverage and the effect of surface and application conditions.

Mathematical derivationNIST SI conversion factors ↗

Exact length, area, and volume unit conversions; the project formula is derived from standard geometry.

Common mistakes

Check the job, not just the number.

01Leaving the example paint coverage inputs unchanged instead of entering measured project values.
02Treating an editable coverage, size, density, or allowance as universal instead of checking the selected product label, project documents, and local requirements.
03Ordering from the rounded result without reviewing exclusions: Typical interior paint covers about 350–400 ft² per US gallon, but texture, porosity, color change, and application method affect real coverage.

Questions, answered

Paint coverage FAQ

What does the Paint Coverage Calculator estimate?

Estimate wall and ceiling paint with coats, doors, windows, and waste included. The primary output is gallons & liters, with the intermediate assumptions kept visible.

How is the paint coverage result calculated?

The visible formula is: paint = ((wall area − openings) + extra wall area + optional ceiling) × coats × waste factor ÷ product coverage. Inputs are converted to a common base unit before the final purchase boundary is rounded.

Are the default values recommendations?

No. Dimensions are worked examples and product-dependent presets are planning defaults. Replace them with the selected product label, project documents, and local requirements.

Is the result ready to order from?

Use it as a checked planning estimate, then confirm packaging, minimum orders, installation instructions, site conditions, and any code or professional-design requirements.

Project notes

Measure, adjust, and verify.

Details specific to paint coverage planning.

How to measure

Measure each wall's length and finished height, or use room length and width for a simple rectangle. Deduct doors and windows only once. Add the ceiling separately when it will receive the same paint system.

What the result includes

The result combines net wall and optional ceiling area, number of coats, editable product coverage, and a waste allowance before rounding to a practical quarter-gallon or half-liter boundary.

When to adjust allowance

Increase the allowance or reduce expected coverage for rough or porous surfaces, dramatic color changes, sprayed application, many edges, or extensive touch-up stock. Use the selected paint's label instead of the example coverage.

Limitations

Primer, trim paint, doors, cabinets, repairs, texture, and separate colors may need their own calculations. The result does not predict hiding performance or guarantee that mixed batches will match.