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Screws: How They’re Classified And How To Know What Kind To Use

image of screws how they are classified and how to know what kind to use
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Do you have a disorganized drawer that holds loose or leftover screws and bolts? Maybe you have 10 screws of a certain kind and need 20 more, but want to make sure you get the right kind. If you’ve ever encountered a household collection of random screws and want to know how they can be used, or just want to know how to select the right screw for your project, this article can help.

Jaycon Systems engineers often use small screws that hold small plastic parts or printed circuit boards together. As they are part of the plastic design process, we need to know how to sort them into groups and choose the right screw for each client’s project.

We also know that using the wrong size or type can really “screw” things up, so we’ve shared some of our knowledge about these crucial parts to help you identify screws and choose one for your own project — whether it’s a DIY creation or something you’re repairing.

Below you’ll find an overview of how screws are classified according to shape and size.

Screw head types

The screw head type is the first piece of information you’ll need to consider when figuring out what kind of screws you have or choosing the right kind of screws for your project.

You might choose a certain screw head type because of personal preference or for functional purposes. If you want a completely flush, flat appearance on the outside of the product you’re building, for example, then you’ll probably want to use a flat countersunk head. If you have a product with thick or strong outer walls, on the other hand, you’ll probably need a thicker screw head that rests on the outer surface of the product. In addition to utility, screw heads can help create high-quality interaction design and aesthetically-pleasing products.

Some of the most commonly used head types are pictured below in Figure 1. From left to right, you can see an oval countersunk head, a flat countersunk head, a round head, and a pan head. The dashed line in Figure 1 indicates where the screw will rest on a surface when installed. The first two are counter sunk heads. When installing that type of screw, the head ends up below the surface.

image of screws head types
Figure 1: oval countersunk head, a flat countersunk head, a round head and a pan head. The red line indicates where the screw will rest on a surface when installed.

Screw Values

A screw’s values encompass diameter, thread spacing and length. This is the second set of information you’ll need to consider when determining what kind of screws you have or choosing the right kind of screws for your project.

Values can appear in either metric or imperial (standard) measurement.

Imperial values usually have a three-value identifier, such as #3–35 x ½. Metric values sometimes have only two values, or they may appear in a way that is similar to this example: M3–0.5 x 5. The first value of the sets indicate the diameter, the second value indicates the thread spacing, and the last is the length.


The diameter listed in a value set is the major diameter — the widest part of the thread — as seen in Figure 2. In the metric system, the value generally begins with an M and may appear in a format following the examples M2 or M2.5, which indicate 2 mm and 2.5 mm respectively.

image of screw diameter
Figure 2: The major diameter is the widest part of the thread. It’s included as the diameter value in a screw’s value set.

In the imperial system, the first number is listed as #0 through #10 or as a fraction, such as ⅝. Use one the following equations to calculate the values marked with #:

D= (# * 0.013) + 0.06


# = (D — 0.06) / 0.013


In the imperial system, the thread spacing value is determined by threads per inch. Thread counts in this system are commonly listed as 35 or 40 — meaning there are 35 or 40 threads per inch on that screw. It’s also common for more than one thread count to be available for any diameter size, so it’s important to note the thread count when selecting corresponding nuts.

image of thread spacing
Figure 3: In the metric system, the spacing value represents the distance per thread, measured in millimeters.

In the metric system, the spacing value represents the distance per thread, measured in millimeters. The distance from one thread to the next is known as the pitch. It may appear as 0.45 or 0.45 mm. Sometimes the pitch is not even mentioned in a value set. That’s because one pitch is typically used per diameter size. M2 diameters, for example, generally use 0.4 as the pitch.


Screw length may seem self-explanatory, but it doesn’t simply refer to the length of the screw from one end to the other. The length actually encompasses the distance from the end opposite the screw head to the area where the screw would rest on a surface.

image of screw length
Figure 4: The dashed line indicates the starting measure point for a screw. All the screws shown here have the same length.

The dashed line in Figure 4 indicates the starting measure point for a screw. Some include all or most of the head if it’s a countersunk screw; others do not include the head at all. All the screws in Figure 4 have the same length, according to screw value measurement standards.

When clearance is an issue, think ahead to make sure the end of the screw where the surface rests will fit into the available space on the product you’re working with.


It can be easy to identify a piece once you’ve taken into account the head type, diameter, thread spacing, and length. Further, these values will help you choose the right screw for your project according to cosmetics, structure, or cost.

To really drive the point home, let’s look at a real-life situation that illustrates how important a screw can be and why it’s helpful to know how screws are classified. When we complete projects at Jaycon, we select screws based on the specific design or function of each particular project. The process usually goes something like this: we make an overall design, get an idea of where the screws will be placed and how they will function, look for the most fitting or appropriate piece, and then make any necessary modifications to the design in order to fit the screw for final manufacturing and assembly. This is part of the design for manufacturing.

But that’s not always the case. A client might request that we build a product using the same hardware — including screws — as another one of the client’s products so they can purchase the same set of pieces in large batches. This is such a common occurrence, we’ve made shared inventories a standard feature of Product Hubb. This practical approach to project planning can help companies save money, and when it’s used, almost the whole project revolves around the screw!

Learning the piece characteristics discussed in this article may also help you save time and money.

Now, instead of wandering around the hardware store looking for the right fastener — or being unsure about which screws to choose from a drawer full of miscellaneous screws — you’ll be able to evaluate a based on its head type and values.

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