Whether you’re working in wood, drywall or machinery, selecting the correct screw size is critical for a successful project. The diameter and length you choose must fit the material and be strong enough to support the load without deforming or tearing. Measuring the diameter and thread count are a good place to start, but understanding how different systems measure these characteristics is key to getting it right.
Generally speaking, screws are measured in either imperial or metric units. In the imperial system, a screw’s size is determined by its diameter and thread count (the number of threads per inch). The thread pitch is the distance between each thread. For wood screws, this is typically coarse (with a pitch of 35 to 40 threads per inch), while metal screws are often fine-pitched.
Measuring a screw’s diameter can be done by using a tape measure or ruler. In most cases, you’ll want to get the major diameter measurement, which is the largest diameter of the threads from one end to the other. The minor diameter is the smallest measurement of the threads, usually taken from the root (bottom) of one side to the crest of the other. A simplified measurement called a nominal diameter is sometimes used to omit the need for precise measurements.
Measuring a screw’s length is much easier, as it simply involves lining up the screw with a bolt scale and finding the number on the scale that matches the screw’s length. If you don’t have a bolt scale, try using a stick or straw to see how long the screw sticks out of the hole. pilot hole 5/16 lag screw