Once walls are covered with finished drywall, the studs disappear, but there is more than one reason to find them again. The most secure way to hang a heavy object is to screw a hanger into a stud, and driving screws into the studs is the most secure way to stabilize bookshelves and cabinets. Builders use tricks to locate studs, but if you don't want to rely on tricks, use a stud finder.
The Knock-Knock Method
The simplest way to locate a stud is to tap on the wall with your knuckles or a small hammer. The tapping makes a hollow sound between studs, and this sound gradually deepens as you approach one. When you tap directly on a stud, you'll hear an easily distinguishable, dull thud. This method is only approximate because it doesn't allow you to distinguish between the center and edges of the stud. If you need to locate the center, drive a finish nail into three or four strategic spots within a distance of 1 1/2 inches, which is the thickness of a stud. You can easily patch these small holes with spackling compound.
One of the benefits of an imperfect drywall finishing job is that it makes it easier to find studs. All you have to do is look for the small indentations left by the nails or screws holding the drywall to the studs. If the drywall finisher was skillful enough to avoid leaving these trails, use a tape measure to measure distances that are multiples of 16 inches from a corner or from a wall switch, which is always attached to a stud. That's the standard stud spacing. Once you know approximately where to look, you may be able to spot a screw or nail more easily.
Use a Stud Finder
You can buy a stud finder at any hardware or building supply outlet, and if you need to find studs often, it may be a tool worth having. It's an electronic device that detects the density difference between empty space and the space occupied by a stud. It can find metal studs as easily as wooden ones, though a heavy magnet will allow you to easily locate metal studs as well. To , simply place it flat on the wall and move it horizontally until the meter indicates a stud or the light illuminates, depending on the type you have. Some stud finders are able to distinguish the center of a stud from the edges, while others -- usually those with a light -- are less sensitive.
Measuring distance may not be much help when looking for studs in a plaster wall. Plaster walls are generally found in older homes, and builders didn't use the same spacing standards when plastering was common. You also don't want to excavate plaster by driving nails into it because plaster is a brittle material that chips, and it isn't as easy to repair as drywall. You may have success with the knocking test, but the most reliable way to find studs in a plaster wall is to use a high-sensitivity stud finder capable of distinguishing studs from the surrounding lath. A stud finder capable of detecting metal may also help by pinpointing where lath is nailed to the studs.