Does this deduct doors or windows?

This straight-wall tool does not deduct openings. Use the room paint calculator or subtract significant openings deliberately.

Is one gallon always enough for 375 ft²?

No. Product coverage, surface condition, color change and application method affect actual coverage. Use the product label and field judgment.

Painting calculator

Wall paint calculator

Straight-wall paint coverage, coats, allowance and whole-gallon purchase quantity.

Calculator result and formula

Paint purchase: 1 gallons

Formula: Wall length × wall height × coats × (1 + waste) ÷ stated coverage

Formula ID: PAINT-WALL-001 · Version 1.0.0 · Published · Last reviewed Aug 26, 2026.

Inputs

Wall length: Finished wall length Default: 20.
Wall height: Finished wall height Default: 8.
Number of coats: Coats of selected finish paint Default: 2.
Coverage per gallon: Use coverage stated on the selected product label Default: 375.
Waste factor: Allowance for cuts, touch-ups and application loss Default: 5.

How this calculator works

This calculator converts one rectangular wall, the selected number of coats, product-label coverage and a visible allowance into a whole-gallon paint purchase quantity.

Measure finished wall length and height.
Enter the number of finish coats and coverage shown on the selected paint label.
Keep the allowance visible for touch-ups, cut-ins and actual job conditions before whole-gallon rounding.

Worked example: Example: a 20 ft × 8 ft wall has 160 ft². Two coats with 5% allowance need 336 ft² of coverage; at 375 ft² per gallon, the purchase quantity rounds to 1 gallon.

Assumptions and limitations

Wall area uses finished length × finished height.
Coverage must use the selected paint product label.
Waste is applied before whole-gallon purchase rounding.

Common mistakes to avoid

Confirm the measurement basis and units before using the result.
Keep waste or allowance visible and adjust it for the actual job conditions.
Confirm supplier packaging, delivery increments, specifications, and final site requirements before ordering.

Calculator disclaimer

This planning calculator does not determine structural design, code compliance, installation requirements, or guaranteed pricing. Confirm final requirements with the appropriate responsible professional, authority, manufacturer, and supplier.

Formula review and sources

Internal QA complete · independent external review pending · 1 source reference.

Glidden: paint coverage guidance

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