Studs behind drywall or plaster are typically 16 or 24 inches apart, but they can be randomly spaced as well -- and you can encounter doubled studs around windows and doors. You'll want to pin down their exact location -- and in fact, their centers -- if you want to drive nails or screws to hang heavy objects from the stud or conversely avoid them to create a wall niche. The simplest stud finders entail only a magnet, while more complex models can find studs, metal items such as pipes and live electrical wiring.
Magnetic Stud Finders
Pros don't hesitate to use the super-simple magnetic stud finder, which never needs batteries and fits easily into a tool pouch.
Place the smooth back of the stud finder, about the size of two fingers, against the wall, and gently sweep it in an S shape over the surface until you feel a slight tug, representing the magnet detecting a screw or nail head.
Once the stud finder detects a fastener head, let go of it. Its powerful rare earth magnet allows it to stick to the wall, swinging into a vertical position.
Mark the stud location by putting a pencil mark above the yellow centerline arrow at the top of stud finder. A rotating vial in the middle of the tool allows you to also calibrate plumb and horizontal.
If you want to locate multiple studs without marking the wall, particularly if it's freshly painted, you can map your whole wall quickly with a similar device that releases multiple magnets. Move the stud finder slowly over the wall, so that it can release the target magnets over the studs.
Working With Plaster
Either type of magnetic stud finder should work on typical drywall or plaster walls. For thicker walls, try the magnetic stud finder along the baseboard; its nails should track with studs rising from the floor plate to the top plate.
Typical Electronic Models
An electronic stud finder works quite differently, sending out a weak electrical field to measure changes in density as you move it. These need to be calibrated over an area of wall that doesn't have a stud behind it, as they can only detect increases in density. Major makers include Zircon, Ryobi, Craftsman, Black & Decker, Stanley and Bosch.
Lay the stud finder flat on the wall. Press and hold the "On" button, which may be right under your thumb pad, on the left edge of the device; modern stud finders are typically ergonomically designed for ease of use. This calibrates the stud finder so it can detect the wallboard or plaster thickness, and begin looking for thicker areas backed by a stud. You may have to also press a "Mode" button, giving you a choice of scanning for "Wood" or "Metal" or through "Deep" material. The "Deep" option can look for studs through 1 1/2 inches of wall material, such as thick plaster.
Move the stud finder slowly back and forth horizontally until it gives a "beep" or its LED lights light up, or both, indicating it is passing the edge of a stud.
If the stud finder is unresponsive, you have accidentally attempted to calibrate it on top of a stud instead of on a free area of wall. Move the stud finder 6 inches to the left or right and start over.
Mark the stud edge with a pencil; a thin groove or marking channel on the top centerline of the stud finder will make placing the mark easy.
Recalibrate the stud finder on the wall at the other side of the stud. Bring the device slowly toward the second edge of the stud, and make a second mark when it beeps or flashes. Your two marks should be very close to exactly 1 1/2 inches apart.
High-End Electronic Models
If you want more functionality, you can work with a stud finder that provides a digital display of not only the edges but the center of the stud. Pause and mark the edge of the stud, and then slowly keep moving the stud finder in the same direction until the device displays "center," which should be 3/4 inch from the edge.
Models may also detect live AC wires with a digital readout, by providing a LED readout of a bolt icon or text reading "Live A/C."