Are you a bit confused about the different screw sizes you see listed in plans, instructions or specs? If so, you’re not alone. This is a common question among woodworkers.

The confusion stems from a number of factors. Incompatibility between different thread forms and their respective screw holes is one reason. However, a screw’s size is determined by two other measurements: its gauge and its thread per inch (TPI).

There are two different measurement systems used to identify screw sizes: the unified screw thread standard (UTS) and the metric system. The UTS is a set of inch-based measurements that includes coarse and fine threads series. The metric system uses millimeters to identify screw thread sizes.

A screw’s size is identified by its major diameter, or its outer thread diameter. The number of thread peaks in one inch, or its thread pitch, is the second element that determines its screw size. For example, a screw labeled “#6 x 2” has a major diameter of 6mm and a thread pitch of 1mm.

Most screw packaging lists the major diameter first, followed by the thread count and shaft length in inches. Occasionally, manufacturers will include the threads per inch in between, such as “10 x 36.” When this occurs, the numbers are used interchangeably to indicate the screw’s corresponding UTS size. In these cases, the screw is considered to be of a normal size for that type and class. A good rule of thumb is to select a screw that will enter the material at least half its thickness. metric to standard

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