For any paint, interior or exterior, the recommended spread rate on the label is usually about 400 sq. ft. per gallon when recoating. This is true whether you are painting exterior (smooth) siding or interior plaster or wallboard. Although it is often possible to roll paint out with a roller, covering perhaps as much as 600 sq. ft. per gallon, it is important to note here that you should try to apply the paint at the rate recommended by the manufacturer. Meeting the intended application or spread rate is critical for proper coverage or "hide" over old paint colors, and to get a paint film thickness that will stand up to future washing. One can hardly blame the manufacturer if the paint doesn't cover old colors, or if it scrubs off with the first washing, if you make the paint coat thinner than suggested.
Especially if you are having colors custom-mixed, it is important to have enough paint to finish the entire job, or at least enough to finish one entire side of the house (if painting the exterior) or to paint an entire room (if painting on the interior.) The problem is that slight color variations may result in mixing. If you stop painting in mid-wall, then begin with a new can of paint, you may get a color shift that will be very noticeable. Try to estimate the job so that you buy enough paint before starting the job.
If you are estimating the amount of area to be painted, measure the length by the height of the walls. Next, measure and subtract out any doors or large windows. If you will be painting the trim around the base, doors, and windows with the same paint you are using for walls, do not subtract the footage of the room openings, because it will take about the same amount of paint to paint the trim and/or window frame and doors as it would take to paint solid walls with no openings.
Trim paint is less critical. Because it is applied over a smaller area, or a series of areas such as multiple windows, you will not notice a color shift from one area to the next. Try to estimate the paint needed as closely as possible, but you can always return for another quart if needed.
An exciting development that is becoming increasingly available is a color computer that will let you plug in a photo of your own house and use computer colorization to see how it would look in any given color. It is difficult for many of us to visualize a color sample on an entire house: color selection by computer lets you actually see the house in the color you're considering before you actually paint the house.
written by Gary Branson
Reprinted with permission. Copyright HouseNet, Inc.