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CONSTRUCTION CALCULATOR

Paint Calculator

Estimate paint for four room walls after deducting standard doors and windows, applying the number of coats, and using the coverage printed for your product.

Calculator

Measurement system
Use ft² per gallon in imperial mode or m² per litre in metric mode.
YOUR RESULTS
Paint required1.505 gal
Whole gallons or litres
2
Net wall area
301 ft²
Area across all coats
602 ft²
Door and window area
51 ft²

Paint volume equals net wall area times coats divided by the product coverage entered. Whole containers round up.

Understanding your result

Gross wall area is the room perimeter multiplied by height. Standard opening areas are deducted, coats multiply the net area, and product coverage converts area to gallons or litres.

To check the rectangular area of a wall or opening separately, use the Square Footage Calculator before subtracting openings or applying paint coverage.

The formula

Paint = ((2 × (length + width) × height) − openings) × coats ÷ coverage

Each imperial door deducts 21 ft² and each window 15 ft². Metric equivalents use the same physical areas. Whole-container guidance rounds the calculated volume up.

A 12 ft by 10 ft room

With 8 ft walls, one standard door, and two standard windows, net wall area is 301 ft². Two coats cover 602 ft²; at 400 ft² per gallon, that is 1.505 gallons, so plan for two whole gallons.

How to use this calculator

  1. Choose units and enter room length, width, and wall height.
  2. Enter standard opening counts, coats, and the product's stated coverage.
  3. Calculate paint volume, whole containers, and the area behind the estimate.

Use the product label for coverage

Paint coverage changes with product, surface texture, color change, application method, and porosity. Replace the default with the manufacturer's figure.

Primer and finish paint can have different coverage. Calculate them separately when both are required.

Opening deductions are standardized estimates

Actual doors and windows vary, while this calculator uses fixed conventional areas for quick planning.

For unusually large glass walls, double doors, or built-in cabinets, calculate the net wall area directly with measured opening sizes.

Assumptions & limitations

What this calculation assumes

  • The room is rectangular with four walls of equal height.
  • Doors and windows use standard fixed areas.
  • Coverage uses square feet per gallon or square metres per litre as labeled.

What to keep in mind

  • Ceilings, trim, primer, and surface texture are not modeled.
  • Real coverage can differ from the product estimate.
  • Paint may be sold in container sizes other than one gallon or one litre.

Common questions

Does the estimate include the ceiling?

No. The formula covers the four walls only.

Why should I change the coverage value?

The can label reflects the selected product; the default is only a planning example.

Are doors and windows measured exactly?

No. Each count uses a standard area, so measure unusual openings separately.

Sources & further reading