According to general guidelines, to paint a house with an area of 3500 square feet, you will generally need approximately 14 to 15 gallons or 55 to 60 liters of exterior paint and 4 gallons of paint for exterior molding for approximately two coats. The standard-sized doors measure approximately 20 square meters. Add up how many of each there are in your project space and subtract those square feet from the square feet of wall painting. Divide the new final number by 400 to get the best estimate of how many gallons of paint you'll need to apply a coat to walls and ceiling (minus the doors and windows you won't be painting).
A room's skirting boards, windows, doors and ceiling moldings are usually painted with a different finish or color than the walls and ceiling, so you can calculate your painting needs for moldings separately. Per gallon, so you'll need a little more primer than paint. Take the total area above and divide that number by 300 to get the minimum gallons of primer you'll need for a layer. One gallon of any BEHR paint is enough to cover 250 to 400 square feet of surface with a single layer.
Anyone who has done their first interior painting project has been left in the paint aisle wondering how many cans they need to take home.