Primer Calculator

How it works

Primer is estimated the same way as paint: the area to prime, times the number of coats, divided by how many square feet one gallon covers. What changes is the coverage. A primer's first job is to seal the surface, so on bare, porous or rough material much of it soaks in and each gallon covers less than it would on smooth, sealed board.

Choose the surface to pick which coverage figure is used. Both are editable and start from typical values; the spread rate on the technical data sheet of the primer you buy is the better figure. Benjamin Moore's data sheets, for example, give a range and add that the actual rate depends on surface texture and porosity.

For patched walls, spot priming covers only the repaired areas. With spot priming chosen, only the share of the wall entered as patched is counted. Primer data sheets call for one or two coats; Benjamin Moore describes its Fresh Start High-Hiding primer (046, 2025 data sheet) as its go-to for significant color change and notes two coats may be required over severe stains.

The result is shown both as whole gallons and as whole quarts, since a small spot-priming job often needs less than a gallon.

Formula

area to prime   = wall area                         (whole area)
area to prime   = wall area × share patched ÷ 100   (spot priming)
total area      = area to prime × coats
primer needed   = total area ÷ coverage for the chosen surface
gallons to buy  = primer needed, rounded up to whole gallons
quarts to buy   = primer needed × 4, rounded up to whole quarts

Example

400 sq ft of new drywall, one coat, at a typical 350 sq ft per gallon on a smooth surface needs 400 ÷ 350 = 1.14 gallons: 2 gallons, or 5 quarts.

The same 400 sq ft with 25% patched and spot-primed is 100 sq ft, or 0.29 gallon: 2 quarts.

Assumptions and limitations

• Coverage figures are typical values and editable. A primer's technical data sheet gives its own spread rate, which manufacturers state depends on the texture and porosity of the surface.
• The porous-surface default of 250 sq ft/gal is a typical allowance, not a figure from a specific product. Very rough masonry such as cinder block can take far more: Benjamin Moore lists 100–200 sq ft/gal on cinder block for its Ultra Spec Masonry FIL (452, 2026 data sheet).
• Spot priming counts only the share of the area you enter; feathering primer past the edge of each patch adds a little more.
• Whether a surface needs priming, and which primer suits it (stain-blocking, bonding, masonry), is set by the product's label and data sheet, not by this calculator.
• The result is an estimate. It is not a substitute for the manufacturer's instructions.

Frequently asked questions