We all know someone whose garage contains far too many cans of leftover paint, in colors he doesn’t have any earthly use for. We also have friends who start a paint job, run out of paint in the middle, and end up with an uneven finish. These sad fates don’t have to befall you, provided you take the proper precautions. Measuring the space you need to paint isn’t rocket science, and if a stitch in time saves nine, a quick go-round with a measuring tape before you take out the roller prevents plenty of heartache after.

We all know someone whose garage contains far too many cans of leftover paint, in colors he doesn’t have any earthly use for. We also have friends who start a paint job, run out of paint in the middle, and end up with an uneven finish. These sad fates don’t have to befall you, provided you take the proper precautions. Measuring the space you need to paint isn’t rocket science, and if a stitch in time saves nine, a quick go-round with a measuring tape before you take out the roller prevents plenty of heartache after.

As professional painters will tell you, the term “spread rate” refers to the approximate area one gallon of paint – the amount in your typical can – will cover with one coat. In other words, a spread rate of 350 square feet means that a single can puts one coat over roughly that area. When you’re estimating, remember that each kind of surface soaks up paint at its own rate, and that paint sprayers offer a lower spread rate than brushes and rollers. An estimate is just that, so it behooves you to buy a little too much paint rather than too little. Don’t hesitate to talk with a paint store employee; they know how the spread rates of different brands compare.

A wall’s area is equal to its width multiplied by its height – that’s basic geometry. If you’re painting a room with four solid walls, add up the widths of the walls and multiply that distance by the height of the room. Divide that number, in turn, by the paint’s spread rate to determine the necessary number of gallons. Measure woodwork along doors, windows, or baseboards by multiplying the trim’s length by 0.5 feet.

Many painting manuals suggest deducting the area of every window, door, or permanent object from the room’s total surface area. This may seem obvious, yet some tutorials don’t recommend it. They claim that by including the area of doors and windows, you’ll get yourself to buy the appropriate amount of extra paint. It’s impossible to avoid all drips and spills, to say nothing of the loss of paint when you mix two cans, and no job is complete without touch-ups. A rule of thumb for subtracting doors and windows, if you do prefer to do so: Estimate 20 square feet for every door and 15 for each window. (Painting your windows’ trim and sash? Figure on 7.5 square feet for each one.)

The spread rate listed on a can of paint might hold true for smooth surfaces. Rough or cobbled ones, on the other hand, could yield a rate as low as 150 square feet per gallon. If you’re applying primer to drywall, masonry, or wood, you can expect a spread rate of 250 to 300. Surface irregularities or porosity, paint quality, the temperature of the air and/or the surface, and the type of brush you use all influence the spread rate, too.

An example: If a room measures 12 feet by 20 feet, the ceiling is 8 feet high, and there are two windows and a door, start by totaling the widths of the walls (12 + 12 + 20 + 20 = 64 feet). Multiply 64 feet by the height, 8 feet, to get 512 square feet. Subtract the windows (2 x 15 = 30 square feet) and the door (20 square feet) if you like: 512 – 30 – 20 = 462 square feet. Then divide this total by the spread rate, 350 square feet per gallon, to get 1.32 gallons, the amount of paint you need.

Perfect walls don’t exist, so factor in the imperfections. Alcoves or other architectural features cause many rooms to have more than four vertical surfaces. The condition of a surface matters as much as its area when it comes to estimating paint. Cracks and nail holes eat up more paint than you would think; textured walls or ceilings are much the same way. So here’s another rule of thumb: Round up after dividing your square footage by the spread rate. If the remainder is less than a half-gallon, buy a couple extra quarts; if it’s more, purchase an extra gallon.

If you take a few simple measures, your paint can approach its ideal spread rate. By all means, prime and seal bare surfaces; this reduces the effects of abnormalities. In addition, only paint when the air temperature is between 50 and 85 degrees Fahrenheit. Most homes stay within those parameters, but painting exteriors isn’t an all-weather pursuit.

You can measure exterior walls the way you do interior ones; their height runs from the top of the foundation to the roofline. Use a different formula for gables, the triangles beneath a sloping roof. Determine a gable’s height from the base to the roof peak, then multiply it by the width of the base and divide that product by two. Once you’ve computed the areas of the walls and the gables, divide the total by the spread rate to learn how much paint you’ll need.

If you’re running out of paint mid-project, keep some of the paint you have to mix with the paint you buy. This ensures that all of your surfaces turn out evenly colored. Hang onto some of your leftover paint for touch-ups; depending on your future needs, you can either store or get rid of the rest. (To store extra paint, use a rubber mallet to tap the can’s lid back down.) Dry latex paint with cat litter or a commercial paint hardener and put it in the garbage. Leftover oil paint must go to a hazardous waste management site.

Many aspects of do-it-yourself painting can be intimidating, but figuring out the right amount of paint to buy shouldn’t be. If you do your homework and keep in mind the nature of the surfaces you’re working on, you’ll be an ace estimator in no time at all.

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