The mathematics of coverage from a Washington, DC area contractor
According to industry standards, if you go to Sherwin-Williams and buy one gallon of paint, it ought to cover 400 square feet of surface. To get a sense of 400 square feet, think of a normal 10 foot by 12 foot bedroom, with 8 foot ceilings. To paint all four walls, you would need 352 square feet of paint; to add the ceiling in, too, you would run out of paint just before the project was completed (the ceiling would bring you to 472 square feet of surface). On a larger room, like a 20×20 living room, that gallon wouldn’t even be enough to cover all the walls, although it would get you more than halfway (wall surface in that living room would equal 640 square feet). So the rule of thumb for interiors is that one gallon of paint ought to cover one small- to medium-sized room, and two gallons are necessary to cover a larger room.
When working with exteriors, you are dealing with larger surfaces, so you need to assume you will be buying larger quantities of paint. An average two-story house is about 20 feet high, and then another twenty feet wide (if not more). Even just a one-sided, 20 x 20 foot facade will take up an entire gallon of paint; more if you’re not careful with it. Six gallons of paint is a standard purchase for a two-story exterior, with additional gallons to cover trim and accents.
How about trim and accents? In some cases, you can get away with only purchasing a quart of paint for trim, but you need to take a close look at the amount of trim on the property before making that assumption. You would be surprised by how quickly that surface area adds up; a quart of paint can only cover 1/4 of what a gallon can, and that only adds up to 100 square feet.
Of course, there are a lot of extra factors that can decrease the coverage you get out of that gallon. While the details of all of these factors are complex, they all fall into three general categories of coverage retardation:
1. Paint Issues,
2. Substrate Issues, and
3. Technique Issues
The quality of your paint will affect the coverage. High quality paint with strong, durable pigments will have greater hiding power, and will therefore be capable of coveraging more surface with less paint. High quality paint will also require fewer coatings to arrive at a thorough covering, and will reduce the materials cost that way.
The substrate will also affect the coverage, which will affect how far you can stretch a gallon. Certain substrates will absorb or repel the coating, necessitating that more paint be brought in to complete the surface.
Finally, your method of application will affect the coverage. If you spill a lot of paint, leave the top off the paint can or roller bucket/tray, or roll wet paint over wet paint, you will end up using a lot more paint than necessary to complete the coverage.
Blue Door Painters is happy to calculate the exact amount of paint you will need for any given project as part of our free estimating service.