Screw Sizes and Thread Sizes

Screws are used in a wide variety of applications, and the wrong length or diameter can cause the screw to break or fail. When shopping for screws, tradespeople need to know three essential measurements: screw gauge, thread size and length. This knowledge can save time and money by avoiding the frustration of having to return a box of incorrect fasteners.

Screw gauge refers to the size of the external thread in inches. Anything less than a quarter inch is labeled, from #0 to #14. A screw with a smaller gauge will have smaller outside diameter and vice versa. Each screw gauge has a decimal equivalent, and you can use a handy screw chart to convert between imperial and metric sizes.

The next measurement you need to consider when choosing a screw is its threads per inch. A screw’s thread is the spiral shape that runs around its cylinder. Threads can be tapered or parallel, and they can be full or partial. The number of threads per inch will tell you how much pressure the screw can withstand, and you can match it to the material it will be fastening to.

The unified thread system supports several different thread counts for each screw diameter, and the higher or coarser threads are described as Unified Coarse Thread (UNC). The lower or finer threads are called Unified Fine Thread (UNF). When selecting a screw, you will need to take into account the head type, such as square, slotted or Phillips head, and its head depth. This will determine whether the screw is a good fit for your tool, or if you need to drill a pilot hole before applying the screw. 1/8 in to mm



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