This tip might fall under the umbrella of a more general "don't be a cheapskate" category, along with the next couple of pages. While it might seem like the paint industry is running an expensive racket -- going on about how you have to use primer, multiple coats and fancy-pants exterior paint -- don't become so jaded that you decide to scam the system by buying the least amount of paint possible.
The problem with that is twofold. One is pretty obvious: Someone is required to make a trip to the hardware store, in a panic, every time you run out of paint. The hapless person gets the wrong color. You yell. The person cries. The house -- half the color of "Summer Leaves," half the color of "Evergreen Dew" -- is a monument to hurt feelings.
The other problem is that even if you do get the right color, you're still not guaranteed a uniform hue. Buying the right amount of product, from the beginning, allows you to "box" the paint. That's a fancy way of saying "pour all the cans together." Because each shade can vary by bucket, you'll get a nice, even tone if it's all mixed together [source: DIY Network].
So just remember: For every 400 square feet (37 square meters) of area, buy 1 gallon (almost 4 liters) of paint. Don't skimp.