A wood screw is a small screw made of brass, stainless steel or bronze that is specially designed to hold two pieces of wood together. Wood screws come in gauge sizes from 0 to 24, with each of these numbers indicating the screw's maximum head diameter, shank diameter, root diameter and threads per inch. Determining what size screw is best for your project is a matter of taking a few simple measurements.
Measure the thickness of the first piece of wood you will screw into. The shank of the screw you will be using, which is the portion without threads just below the head, should pass fully through the first piece of wood. This ensures that the screw will be held firmly in place without splitting the wood.
Measure the total amount of wood the screw will be entering. Compare it against the total length of the screw you will be selecting. There must be 1/4 inch or more of material beyond the end of the screw to ensure that it will not poke through or split the wood.
Check the width of the piece of wood you will be screwing into. Measure the root diameter of the screws you plan to use. The root diameter is the width of the threaded portion of the screw. Ensure that there is at least 1/8 inch of material on either side of the screw threads.
Double-check your calculations to compensate for differences in wood screw head types. There are three main head types on wood screws: flathead, round and oval. Countersink flathead screws if you use them. That means you have to factor the head length into your calculations. You also must countersink oval screws. Do not countersink round-head screws.
Root diameters on wood screws range from 1/4 inch to 3/64 inch.