Choosing the right screw for a job is essential to ensure you’re working with the strongest, most durable materials. Screws come in a variety of types and sizes, but they all work the same way: by drawing materials together. Get it wrong and a screw could split wood, damage metal, or even affect the structural integrity of a building. This is why understanding the three main screw measurements – gauge, length, and threads per inch – is so important for all tradespeople.
Gauge, or major diameter, measures the largest portion of a screw’s threads and is typically indicated by an imperial number ranging from #0 up to #12 – each one representing a fraction of an inch. To measure a screw’s diameter, line it up with a ruler and measure from the point where its head meets the surface of the material it’s going into. For screws with rounded heads, like oval countersunk screws, measure to the flat part where the oval top would rest on the material’s surface.
The second measurement you need to know is the thread pitch, which is a count of how many thread peaks there are in an inch of a screw’s length. This is an important indicator of how well a screw will hold its position when screwing into a specific hole, as screws with a higher thread pitch have coarser and thicker threads while those with lower thread pitches have finer and tighter threads. 5/16 lag bolt pilot hole