Paint Calculator

How it works

The calculator multiplies the total length of the walls by their height to get the gross wall area, then subtracts 20 sq ft for each door and 15 sq ft for each window. What is left is the area you will actually paint.

That area is multiplied by the number of coats to get the total area the paint has to cover, and divided by the coverage of one gallon. The result is rounded up to whole gallons, because that is how paint is sold.

Two coats is the normal case for a colour change or a fresh finish; one coat is only realistic when repainting the same colour on a wall in good condition.

Formula

wall area      = total wall length × wall height
openings       = doors × 20 + windows × 15
paintable area = wall area − openings
total area     = paintable area × coats
gallons        = total area ÷ coverage per gallon
gallons to buy = gallons rounded up to a whole number

Example

A 12 ft × 12 ft room has 48 ft of wall. At 8 ft high that is 384 sq ft of wall. One door (20 sq ft) and two windows (30 sq ft) leave 334 sq ft to paint.

Two coats means covering 668 sq ft. At 350 sq ft per gallon that is 1.91 gallons, so buy 2 gallons.

Assumptions and limitations

• Walls only. Ceilings, trim, doors themselves and primer are not included; run the calculator again or add their area to the wall length if you are painting them in the same colour.
• Each door is taken as 20 sq ft and each window as 15 sq ft. A sliding patio door or a picture window is nearer two of each.
• Coverage varies with the paint, the colour change and the surface. Bare drywall, textured or porous walls and painting light over dark all use more than the label figure, so treat 350 sq ft as a sensible default rather than a guarantee.
• The result is rounded up to whole gallons. If you are only a fraction over a whole number, a quart may cover the difference.

Frequently asked questions