Do-it-yourselfers building woodworking projects often wonder how to determine the correct size and length of wood screw that should be used for a given project. The answer to this question is more complicated than you might think, even for industry professionals, since it is affected by considerations such as the type of wood being used, the type of wood screw, whether the forces on the joint are lateral (sheer) or vertical, whether joints are reinforced with glue, and whether or not pilot holes are being used.
That being said, there are some general guidelines that can be followed.
How Screws Are Categorized
Screws designed for fastening wood come in two basic forms: steel or stainless steel wood screws, or utility screws (often generically known as "deck screws").
Utility screws are the workhorses of framing carpentry and outdoor woodworking projects. They are available in different types of steel, sometimes corrosion-resistant metals designed to stand up to the chemicals in treated lumber; and they are normally sold by length.
Steel or stainless steel wood screws have thicker bodies and are typically used for precision woodworking and indoor furniture. When you buy wood screws, they are identified both by their length and also by their gauge—which refers to the thickness of the screw shaft. Larger gauge numbers indicate screws with thicker shafts. While utility screws are typically #8 or #10 gauge, steel wood screws are available in a much wider selection of gauge diameters.
The Basic Goal
The idea is to use screws that are long enough and stout enough to hold the two boards together securely—but without being so large that they split the stock receiving the screw, or so long that the tip of the screw pokes through the backside. Drilling a pilot hole bit with a countersink bit before driving the screw is certainly a good idea, particularly on hardwoods that are inclined to splitting.
(Softwoods such as pine or cedar are less susceptible to splitting.) But screws get their power by friction and the pressure of wood fibers gripping the threads. By cutting fibers, a pilot hole can reduce the grip of the wood around the screw threads and thereby slightly lessen the holding power of the screw.
Lateral vs. Withdrawal Loads
Second, the fasteners need to have enough thread driven into the receiving stock to hold fast. The length of the thread that is inserted into the receiving board is really dependent on what forces will be applied to the boards in question when in use. If the forces will apply lateral pressure, consider using a thicker screw than you might otherwise choose. However, if the forces will apply pressure that wants to pull the boards apart (called withdrawal pressure), be sure that ample screw threads are inserted into the receiving board.
In the latter case, always to try to get at least 1 to 1 1/2 inches of thread into the receiving board. This assumes, of course, that this length of screw won't poke through the opposite side of the board. In the event that you need shorter screws to prevent this problem, you'll need to compensate by driving more screws.
Additionally, the number and length of screws that are used will depend on the grain orientation of the receiving board. As a general rule, screws needed for attaching two boards across the grain do not need to be as long as when screws are inserted into end grain. When screwing into end grain, try to get at least two inches of thread into the board.
As a general guideline, you can use #8 diameter deck-type screws for most utility tasks. Use 1-1/4" screws for basic tasks where strength isn't a huge issue or when attaching two 3/4"-thick boards across the grain. Keep some 2-1/2" deck screws for screwing into end grain, plus some beefy 3" screws for attaching 2x by stock into end-grained boards, or when a little extra added oomph is needed. Finally, if there will be a lot of sheer strength needed, you can opt for some 1/4"-diameter lag screws.
TIP: No matter whether inserting screws into end grain or cross-grained boards, many experts and skilled DIYers always opt to drill pilot holes with a special countersink pilot bit that drills a small-diameter hole the length of the screw shaft, with a wider counter-sink portion that will allow the head to be sunk flush with the surface of the wood.
Although most people do not need (or want) more complicated information, it does exist if you are so inclined. The American Wood Council has posted a Connection Calculator that can be used to determine the connection capacity of certain types of fasteners when used in specific connection types with specific members.