How much paint? Calculating the amount of paint you need for a project

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Completing a painting project on your own can save you money and give you a feeling of accomplishment. Yet one common mistake – misestimating the amount of paint you need – that can undercut your sense of achievement. Disposing of extra paint can be a pain (remember to recycle paint) , and making an emergency trip to the paint store mid-project is no fun, either. Luckily, a little planning can prevent a lot of headaches later in the process.
Professional painters and painting Web sites provide various estimates for the “spread rate” of one gallon of paint, the amount that a single can typically holds. A spread rate of 350 square feet means that one can will put a single coat over roughly that area. A key principle when estimating paint-to-wall ratios is that different surfaces soak up paint at different rates.
A smooth surface might obey the 350-square-feet-per-gallon rule; a rough or cobbled one, however, may absorb paint at a rate of just 150 square feet per gallon. Similarly, primer on uncoated drywall, masonry, or wood has a typical spread rate of 250 to 300. Many other factors can affect spread rate, including surface irregularities or porosity, paint quality, the air and surface temperature, and the type of brush (nylon or polyester is best).
Many brands of house paint list a spread rate on the can; you’ll want to use that number in making your estimations. Remember that airless paint sprayers are less efficient, in terms of spread rate, than brushes and rollers. Paint-to-area estimates are always inexact, so it pays to err on the side of buying more paint rather than less. When in doubt, consult a paint store employee for information on the differences between brands.
As you may remember from basic geometry, a solid wall’s area is equal to its width multiplied by its height. In a room with four solid walls, you can simply add up the widths of the walls and multiply the total by the height of the room. Once you know the surface area of the walls, divide that number by the spread rate of your paint to determine how many gallons you’ll need.
Many how-to guides suggest subtracting the area of each window, door, or immovable object – which can all be measured with the length-by-width formula – from your estimate of the total surface area. While this step makes intuitive sense (you’re not painting the windows, so why include them?), some sources advise against it. They reason that including the surface area of doors and windows provides you with roughly the right amount of extra paint. After all, no painter completely avoids spills, drips, and the loss of paint when two cans are mixed, and touch-ups are always necessary. (If you’re painting a door, of course, its area must be part of the room estimate.) Other DIY manuals simply recommend adding 10 percent to your surface-area total in order to compensate for mishaps.
Certain kinds of surfaces are likely to need a second coat of paint, which doubles the amount you’ll need to buy. Previously painted walls, especially in dark colors, almost certainly require two layers; unfinished walls behave similarly. For light walls, one coat may suffice. If you’re painting moldings, or even the ceiling, those surfaces must also be considered.
While painting the exterior of a house isn’t radically different from working on the interior, it presents some unique challenges. For one thing, exterior surfaces are much more likely to need multiple coats. Due to its roughness, edges, and undersides, siding is especially likely to require extra paint.
You can measure an exterior wall as you would an interior one, with the proviso that height is measured from the top of the foundation to the roofline. Gables, the triangular areas beneath a sloping roof, demand a different formula. Determine a gable’s height from the base to the roof peak, then measure the base width. Multiply the two figures, then divide the product by two. Once you’ve totaled the area of the walls and the gables, divide by the spread rate to find the volume of paint you need.
The perfect wall is a myth, so it’s wise to prepare for imperfections. For one thing, alcoves or other architectural features complicate matters; many rooms have more than four vertical surfaces to paint. You can measure the area of any surface, but its condition can be just as significant. Cracks and nail holes in a wall absorb more paint than an area estimate will suggest, and the same goes for textured walls or flocked ceilings. It’s therefore best to round up after you divide your estimated square footage by the spread rate.
Even the Internet’s many paint calculators (you enter the room’s dimensions, they spit out a number of gallons) are only so precise. Fortunately, you can take a few steps to get your paint as close as possible to its ideal spread rate. First, prime and seal bare surfaces to mitigate irregularities. Then find a time to paint when the air/wall temperature is between 50 and 85 degrees Fahrenheit. If you’re painting interior walls, that shouldn’t be a challenge.
If you find yourself running out of paint, save a bit of what’s left to mix with the new paint you purchase; that way, you’ll finish the job in a consistent color. If you end up with extra paint when the project is done, save some for touch-ups and then dispose of the rest. Latex paint can be dried (with cat litter or a commercial paint hardener) and put in the trash; oil paint must be taken to a hazardous waste management site, of which King County has several (http://www.lhwmp.org/home/HHW/disposal-locations.aspx).

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