Interior

For walls: Multiply the perimeter (distance around room) by the height. If using a different paint for door or window trim, subtract the square footage of doors and window frames.

For ceilings or floors: Multiply the length of the room by its width.

To determine how much paint you’ll need, divide the total square footage by 400 since one gallon of paint covers approximately 400 square feet. If using varnish or stain, divide by 200 since one gallon of varnish or stain covers approximately 200 square feet.

Exterior

For siding surface: Multiply the perimeter by the height.

For peak areas: Multiply the height from base of roofline to tip of peak by one-half the base length.

For total square foot area: Add siding surface area and peak areas. For gutter, add 1 square foot for every running foot. If using a different paint for trim, subtract the square footage of doors and frames.

To determine how much paint you’ll need, divide the total square footage by 400.