How Much Paint Do I Need? Coverage, Coats and Gallons

How many square feet does a gallon cover?

Read the coverage range for the exact paint, sheen and substrate. For example, the cited Benjamin Moore Super Hide product sheet lists approximately 400–450 ft² per gallon at the recommended film thickness. That is a product specification, not a promise that every paint covers the same area.

Coverage is normally for one application at the specified film thickness. If a project needs two coats, one gallon at a 400 ft²-per-gallon rate supplies two coats over about 200 ft² before losses. Do not divide coverage by the coat count and then multiply the area by coats again.

Measure walls instead of using floor area

For a rectangular room, gross wall area is 2 × (length + width) × wall height. Subtract measured openings that will not receive that paint. Add a ceiling only when it will use the same product and is included in the estimate.

A 12 × 12 ft room with 8 ft walls has 384 ft² of gross wall area. If the unpainted openings total 42 ft², the paintable wall area is 342 ft². The floor and ceiling are each 144 ft², which is a different measurement.

Worked example: two coats in a 12 × 12 room

For the 342 ft² net wall area, two coats require 684 ft² of coat coverage. At 400 ft² per gallon, that is 1.71 gallons. An example 10% allowance increases it to 1.881 gallons, so two 1-gallon cans cover the calculated purchase quantity.

At a lower example rate of 350 ft² per gallon, the same room and allowance need 684 ÷ 350 × 1.10 = 2.149… gallons. That crosses the two-gallon threshold and needs three 1-gallon cans, or another available can combination. The change comes from product coverage, not room size.

Compare coverage before rounding the cans

Whole-container rounding can make a small change in theoretical gallons change the shopping list. Keep the calculated quantity and purchased quantity separate so the remaining paint is visible.

Estimate each paint product separately

Wall colors, trim, ceilings and primer may use different products and coverage rates. Do not pool all their fractional gallons into a single purchase. A gallon of ceiling paint cannot automatically replace a shortfall in tinted wall paint.

Check whether the store sells quarts, gallons or larger containers for the selected product and base. Compare complete container prices and plan any desired touch-up reserve explicitly. A general allowance and a separate reserve can otherwise count the same extra paint twice.

Use an allowance for a stated reason

The 10% used here illustrates how an allowance affects an order. Choose it based on the surface, application method and project needs; follow the product’s preparation and coverage instructions. A rough or absorbent surface can change coverage, and the needed coat count can differ from an initial plan.

• Measure all surfaces receiving the same paint.
• Use measured openings rather than assuming every door and window is identical.
• Confirm the product coverage and coat count separately.
• Apply an allowance once, then compare available container combinations.

Common questions

Sources and product references

• Benjamin Moore — Super Hide product coverage

  Product example listing approximate coverage of 400–450 ft² per gallon at the recommended film thickness. The calculation examples also test other clearly labeled coverage inputs.

• NIST — SI units: volume

  Reference for volume units. Coverage tables and worked examples on this page are Calcstead calculations, not product performance guarantees.