According to general guidelines, to paint a house with an area of 1500 square feet, you will generally need approximately 8 to 10 gallons or 30 to 38 liters of exterior paint and 2 gallons of paint for exterior molding for approximately two coats. A one-gallon can of paint will cover up to 400 square feet, which is enough to cover a small room, such as a bathroom. 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. Applying the primer first helps the paint adhere properly to porous or unpainted surfaces, such as new drywall, allows for a uniform application of paint and covers stains. If you're making a drastic color change, a tinted primer of a shade similar to that of paint makes it easier to cover.

Learn more about choosing the right base for your project. 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. 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.