Exterior House Paint Calculator

How it works

The walls of a house, unrolled, form one long rectangle: the distance around the house times the wall height. Each gable end adds a triangle above the eaves line, half its width times its height.

Doors and windows are subtracted using an area for each. What is left is multiplied by the number of coats and divided by the coverage of one gallon, then rounded up to whole gallons.

Coverage depends heavily on the siding. Smooth siding takes paint close to the label rate; rough-sawn wood, stucco and masonry have far more surface and absorb more. Exterior paint data sheets quote a range and state the actual spread rate depends on surface texture and porosity: Benjamin Moore lists 300–400 sq ft/gal for Element Guard flat (763, 2026 data sheet) and 250–350 for Aura exterior satin (N631, 2026 data sheet), while its Ultra Spec Masonry FIL (452, 2026 data sheet), made for block and concrete, lists 100–200 on cinder block and 200–250 on concrete.

Formula

wall area      = perimeter × wall height
gable area     = gables × ½ × gable width × gable height
openings       = doors × area per door + windows × area per window
paintable area = wall area + gable area − openings
paint (gal)    = paintable area × coats ÷ coverage for the siding
gallons to buy = paint, rounded up

Example

A 40 × 30 ft house has a 140 ft perimeter. With 10 ft walls that is 1,400 sq ft, and two 30 ft wide gables 8 ft high add 2 × ½ × 30 × 8 = 240 sq ft. Two doors at 20 sq ft and ten windows at 15 sq ft take away 190 sq ft, leaving 1,450 sq ft.

Two coats on smooth siding at 350 sq ft per gallon is 2,900 ÷ 350 = 8.29 gallons, so 9 gallons to buy. On rough siding at 200 sq ft per gallon it is 14.5 gallons, so 15.

Assumptions and limitations

• Walls are flat and of one height all round. A house with several wall heights can be calculated in parts and the results added.
• Each gable is a triangle. Dormers, bay windows, chimneys, soffits, fascia and trim are not included.
• Door and window areas are typical sizes you can change. Garage doors are much larger than a 20 sq ft entry door; a 16 × 7 ft garage door is 112 sq ft.
• Coverage figures are typical and editable. The product's technical data sheet gives its own spread rate, which the manufacturer states depends on surface texture and porosity.
• Primer, caulk and the extra paint used by spraying are not included.
• The result is an estimate and not a substitute for the paint manufacturer's instructions.

Frequently asked questions