Screws are essential tools for a wide range of projects, from building to carpentry, but using the wrong screw size can damage your project or worse still, compromise its structural integrity. Knowing how to accurately measure a screw’s diameter, length and thread pitch is vital for accurate selection and ensuring optimal performance.
The correct screw size for a given job is determined by a number of factors, including the type of material being fastened and its thickness, as well as the intended load capacity and environmental conditions. It is also crucial to consider the screw type itself, as some are designed for multipurpose use while others are specialised for specific applications.
Measuring the diameter of a screw is done by measuring from the crest of one side to the crest of the other, referred to as its major diameter. Alternatively, the smallest dimension of the thread (known as its minor diameter) can be measured from the base or ‘root’ of each flank of the thread to the opposite flank. For imperial screws smaller than 1/4″, a number system is commonly used to designate the size, with each digit added as the screw gets thinner and shorter. For example, a #12 screw is larger than a #8 screw.
The length of a screw is the distance from its tip to the underside of its head, and can be measured by either a ruler or tape measure, though using a caliper is recommended for greater accuracy. It is also essential to ensure that the screw is long enough to penetrate the material fully without risk of splitting or damaging it, and this can be achieved by selecting a length three times as long as the thickness of the material to prevent tearing. #6 screw size