As fasteners, screws come in a variety of types and sizes for an endless number of construction tasks—from woodworking to metal roof installations. But, choose the wrong length or width, and you can affect the load-bearing ability of a structure. As with staples, screw measurement is slightly more complicated than nails. Here are three essential measurements every tradesperson should know.
Screw Measurement, In Three Parts
To start off, there are three main screw measurement: gauge, length, and threads per inch (TPI). When shopping for collated screws at Nail Gun Depot, for instance, you’ll find screws described first by gauge, then usually length, as with the Duraspin #8 x 1-1/4" #08X125CBACTS Screw.
Screw Gauge
Gauge refers to width or the diameter measurement. Screw gauge is determined by the outside diameter of the thread, also called “major diameter.” For screws smaller than 1/4”, gauge is measured nominally, in sizes from #0 to #14. That number has a corresponding decimal number (in inches) to describes its size. Example: Size #0 = .013”.
That number increases by .013” as you go up each size. For the Duraspin screw (shown below), the measurement of the #8 gauge nominal size is 0.164”.
Beyond major diameter, a screw has several other width measurements. The measurement of the screw width beneath the threaded part is called root diameter (or “minor diameter”). The measurement of the unthreaded part of the screw (if it's not fully threaded) is called shank diameter.
Screw Length
The next important aspect of screw measurement is length. Not necessarily overall length, but shaft length. Essentially, this is the part of the fastener that drives into a surface. In this regard, countersinking screws and non-countersinking screws differ in how they’re measured.
Since flat-head screws can be countersunk, their length measurement includes the distance from the top of the head to the tip or point. The same goes for bugle-head, trim-head—and any other screw where the head can be driven beneath a surface.
With countersunk screws, such as a hex-head, length is measured from the bottom of the head to the tip. Other non-countersinking screws, like pan-, button-, round-, and truss-head screws, are measured from just under the head to the tip. An oval-head screw, which can be partially countersunk, is measured from the widest point of the head to the tip.
Below is an example of Timber screws from Simpson Strong-Tie, illustrating how length is measured for non-countersinking screws. The first screw has washer head with a low profile, the second screw has a more prominent head/drive.
Threads Per Inch
Threads per inch (or TPI) is a measurement of the number of threads in a one-inch section. You may have also heard the term "thread pitch" in regard to screw measurements. This refers to the number of threads per unit of measurement. FYI, the Brits use metric “thread pitch," which is the distance in millimeters between a single thread.
Wood screws, which generally have a coarse thread often have a low TPI, such as 7. A metal screw with a finer thread, might have a 22 TPI. The TPI measurement is listed after the screw diameter, separated by a hyphen. For example, a #10-12 screw is has a size 10 diameter, with 12 TPI.
Below, you can see detailed measurements for the Senco Duraspin 08X125CBACTS screw, mentioned above. The screw has 8 threads per inch, and an exact major diameter of 0.17, among other details.
If you're shopping for collated screws and need help, contact Customer Service for assistance.
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