How Do I Measure The Length of a Screw?
Knowing how to identify the nominal length of a screw is vital for choosing the correct component for your application. Measuring the length of a screw is relatively straightforward, but the process is not as obvious as it may seem. Identifying screw length starts with knowing the difference between the overall 'length' and the 'nominal length' of a screw.
For more information on how to measure the other dimensions of a screw, see our article, How do I Measure a Screw?
What is Nominal Length?
The nominal length of a screw is the length or size which is used to define the length of the fastener, it is the name of the screw size, and not necessarily the overall length of the component. Nominal lengths are usually found within product names or catalogue listings - they are simplified terms used to refer to a screw when literal sizes or micron-accurate measurements would be inconvenient. Nominal length differs from length in that 'length' is a vague term which could refer to several different dimensions, whereas 'nominal length' is more specific.
Nominal sizes are most easily represented by the diameter sizing of nuts and washers, both of which are oversized for the simple reason that if they were exactly the same size as their accompanying screw or bolt, they would not fit together. Threaded fasteners, in comparison, are undersized for the same reason, and yet both types of fasteners are referred to as being the same diameter - this is an example of nominal sizing.
In the same way, nominal length is not always the literal length of the screw, and usually relates to a screw’s functional length, or a length rounded to the nearest millimetre to avoid unwieldy decimals.
Does Screw Length Include the Head?
The main consideration when measuring the nominal length of a screw is usually whether or not to include the length of the head. An easy way to test is to remember that the nominal length of a screw relates to the depth of the required hole.
For instance, the length of a cap head screw is defined by the length of its shaft, because the head sits above the required hole. The length of a countersunk screw, on the other hand, is defined by its total length (including its head) because the head sits below the surface, thus making up part of the required hole. The nominal length of a screw is usually, though not always, measured from below the head to the tip of the screw. If no head exists, such as on a grub screw or threaded bar, the screw would be measured from end-to-end.
For more information on measuring the length of screws, see our article How to Measure the Length of a Screw.
Distinctions in the measurement of length can be complicated by atypical screw types, such as shoulder screws. As shoulder screws often have little variety in thread length, and are used more for the attributes of their precisely machined shoulder section than their thread, their nominal length is usually defined by the size of their shoulder.
Some care should be taken when measuring the length of a screw with a raised head, particularly if the screw is countersunk. The raised section of a raised countersunk screw is designed to sit above the required hole, so is not counted as part of the nominal length - this means that raised head screws cannot simply be measured end-to-end.
There is also some confusion around screws with specialised tips, such as nylon tip grub screws, where the tip length is either counted or not counted towards the total length of the screw, dependent on manufacturer.
Female screws, though arguably nuts by many definitions and often referred to as 'barrel nuts', are also not measured by conventional methods. Female screws are measured based on their internal thread length, which is not necessarily directly consistent with their shaft length.
Of course, all of these rules are subject to misinterpretation by manufacturers, which can mean that choosing a screw based on its defined length always has to potential to be a minefield of misinformation. To avoid possible confusion, always make sure to check the engineering drawings of a fastener, as these will clearly identify the important dimensions of a design.