BuildProjectCalc
Painting guide

How Much Paint Do I Need?

Estimate paint from wall area, openings, coats, and the coverage rate printed for the selected product.

The short answer

Add the paintable surface area, subtract openings that will not be painted, multiply by the number of coats, and divide by the coverage rate for the selected product. Keep walls, ceilings, trim, primers, and different coatings as separate estimates when their products or coat counts differ.

Use the Paint Calculator

Build a paintable-area estimate

  1. Measure each surface

    For a wall, multiply length by height. Add separate walls rather than assuming every room is a perfect rectangle.

  2. Subtract excluded openings

    Deduct doors, windows, or other areas only when they will not receive that coating.

  3. Apply the coat count

    Multiply net area by the planned number of coats. Primer and finish paint may require separate calculations.

  4. Use the product coverage rate

    Divide coated area by the coverage shown for the actual product and surface conditions.

Paint quantity formula

Net area = gross surface area − excluded openings
Coated area = net area × number of coats
Paint volume = coated area ÷ product coverage rate

Coverage is product- and surface-dependent. Treat any default as an editable planning assumption and check the selected product documentation.

Worked example: four walls with openings

A 12-by-10-foot room has 8-foot walls. One 3-by-7-foot door and two 3-by-4-foot windows are excluded. The example uses two coats and an assumed coverage of 350 square feet per gallon.

  1. Wall perimeter: (12 + 10) × 2 = 44 ft
  2. Gross wall area: 44 × 8 = 352 ft²
  3. Excluded openings: (3 × 7) + 2 × (3 × 4) = 45 ft²
  4. Net wall area: 352 − 45 = 307 ft²
  5. Two-coat area: 307 × 2 = 614 ft²
  6. Estimated paint: 614 ÷ 350 = 1.75 gal

Result: The arithmetic produces about 1.75 gallons under the explicit 350-square-foot coverage assumption. Package sizes, application loss, texture, color change, and product instructions may affect the practical purchase quantity.

Details that deserve separate inputs

Ceilings
Calculate ceiling area separately when it uses a different product, coverage rate, or coat count.
Interior and exterior surfaces
Do not merge surfaces that use different coatings or preparation systems.
Primer
Primer has its own coverage and coat assumptions; it should not be silently included in finish-paint gallons.
Small openings
Skipping deductions may intentionally provide some allowance, but make that choice explicit.

Paint estimate questions

Is one coverage number reliable for every paint?

No. Product formulation, surface porosity, texture, application method, and color change can all affect coverage. Use the selected product information.

Should gallons be rounded?

Compare the calculated volume with the package sizes actually available. The calculator reports the estimate; purchasing decisions should reflect product packaging and project conditions.