Material Math Guide
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Paint Calculator: Gallons by Coverage and Coats

Estimate whole gallons from gross area, openings, coats, and product coverage.

Use the coverage on the selected paint and adjust for the actual surface and application.

Enter the total surface area before subtracting openings.

Enter the combined area that will not be painted.

Enter the planned number of finish coats.

Enter coverage from the selected product and application guidance.

Estimate boundary

What this result covers

  • Actual coverage varies with product, surface, application, texture, and condition.
  • Use the result as a purchasing estimate and verify product instructions, site conditions, and local requirements before work begins.

How the calculation works

Formula

((gross area − openings area) × coats) ÷ entered coverage per gallon

Assumptions used

  • Coverage is supplied for the selected paint and surface.

The tool keeps user-entered product values separate from the geometry, then rounds the recommended purchasing quantity up where the material is bought as a whole unit.

Worked example

Project inputs
800 ft² gross, 100 ft² openings, 2 coats, 375 ft²/gal
Purchase result
4 gallons after rounding up.

Enter these values in the calculator to reproduce the estimate. The purchase result uses the same tested calculation logic as the live tool.

Purchasing checklist

  1. Confirm product coverage.
  2. Separate primer if required.
  3. Record color and batch information.

Common mistakes

  • Ignoring additional coats.
  • Using a coverage range as a guarantee.
  • Forgetting texture or condition.

Sources and methodology

These references support the product values, units, or planning boundaries shown on this page. Always confirm the current instructions for the exact material you buy.

  1. Sherwin-Williams — Paint CalculatorChecked 2026-07-11

    A gallon typically covers about 350–400 ft²; product, application, texture, and condition vary.