Construction + Civil Engineering

In this article, Patrick Faulkner, lead engineer at Accu, looks at how to measure a screw effectively to ensure the right fastener is selected for an application.

“In engineering, precision is everything, and that principle extends right down to the smallest components, including screws. Whether an engineer is designing a robotic arm, constructing industrial machinery or assembling components for aerospace applications, selecting the correct fastener can determine whether a structure is successful or fails.

Why are precise screw measurements important? 

Precise screw measurements are essential for achieving the right mechanical fit and ensuring consistent and reliable performance across different applications. Even minor discrepancies in size can result in uneven load distribution, components loosening over time or accelerated wear. 

Incorrect dimensions of screws may cause misalignment, introduce unwanted vibrations or even lead to total mechanical failure in severe cases. Using fasteners that are too large or too small can also damage materials or compromise joint strengths. In both prototyping or large-scale manufacturing, standard screw dimensions support quality control.

Understanding the anatomy of a screw

Before taking measurements, it is important to understand the basic structure of a screw. A standard screw consists of four main parts; a head, a shank, a threaded portion and the tip. The head sits at the top and comes in various shapes, such as cap, pan, button, countersunk or hexagon. Just beneath the head is the shank, which may be fully or partially threaded based on the screw’s design. The threaded section is the spiralled section that allows the screw to grip and hold materials together. The tip is the end of the screw that goes into the material.

How to measure a screw

Length

The first step in measuring a screw is determining its length, a detail that often causes confusion due to variations in how different screw types are measured. Understanding where to start and stop the measurement largely depends on the design of the screw head and how the screw is intended to sit in the material. 

In general, screw length is measured from the tip of the screw to a specific reference point on the head. If the screw is designed to sit flush with or below the surface, like a countersunk or grub screw, the measurement should include the entire screw, from the top of the head to the tip. This is because the head is meant to “sink below” the material along with the threaded shaft.

On the other hand, if the screw head is intended to remain above the surface like a hex, pan, button or cap head screw, then the length should be measured from the underside of the head to the tip. In these cases, the head does not contribute to the length and therefore isn’t included in the measurement, something which often causes confusion when finding replacement screws.

Sometimes, gauging the length based on the depth of the material’s hole can help clarify which part of the screw will be embedded. But as a rule, always consider the screw’s head type and how it will sit to determine the correct point from which to measure.

Diameter

Accurate diameter measurements are essential for selecting the correct screw and ensuring a proper fit within a threaded hole or nut. Even a small error in measuring can lead to the wrong choice of thread standard, potentially causing misalignment or mechanical failure. When measuring a screw's width, the most commonly referenced dimension is the major diameter. This is the measurement taken across the crests of the threads on opposite sides of the screw, representing its widest point. It is typically what is meant when referring to the size of the screw.

To measure the major diameter correctly, the screw should be positioned perpendicularly between the flat jaws of a calliper. This prevents misreadings that can occur due to angular offset or uneven alignment. For added accuracy, these measurements can be compared against Accu’s thread size chart to help identify whether the screw follows a metric or imperial thread standard.

Thread pitch

Another essential aspect of screw measurement is the thread pitch. When combined with the thread diameter, it provides a complete profile of the screw’s threading. Understanding how to measure pitch correctly requires knowing the distinction between imperial and metric thread standards, as the two systems use different units and methods of measurement.

In the imperial system, thread pitch is expressed in threads per inch (TPI). To measure this, select a one-inch length along the threaded portion of the screw and count the number of thread crests along one side. The resulting number is the TPI. When combined with the screw’s major diameter, this value can be cross-referenced with an imperial thread chart to identify the correct thread size and standard.

For metric screws, thread pitch is measured in millimetres, representing the distance between two adjacent thread crests along the length of the screw. Rather than relying on a single measurement, greater accuracy can be achieved by measuring the distance across a series of threads and then dividing that distance by the number of threads counted. This provides an average pitch measurement, which minimises error caused by thread imperfections or inconsistencies.

Watch this video for a visual demonstration on how to measure screw thread diameter:

Screw head

The dimensions of a screw head are just as important as the thread and length, as they can impact clearance, safety and overall design. A head that's too large may interfere with nearby components, create snagging hazards or compromise the desired aesthetic.

Measuring the screw head is relatively straightforward. The head height is measured from the highest point of the head to the base where it meets the shank or shoulder. The head diameter is taken across the widest part of the head. For round or pan head screws, this means measuring the full circular width. 

Weight

In aerospace, robotics and electronics applications, the weight of a screw becomes a critical factor as even the slightest excess can impact performance, efficiency or balance. When dealing with very small screws, weighing them individually can be difficult due to the limitations of standard scales. To improve accuracy, it is often better to weigh multiple screws of the same type together, then divide the total weight by the quantity. This method increases the total measurable mass, reducing the margin of error and allowing for the use of less sensitive scales that may struggle to detect the weight of a single screw precisely.

Accurate screw measurements ensure the right fastener is used, maintains mechanical integrity and supports repeatability in manufacturing. From the major diameter to identifying the thread pitch and checking the screw's weight, each step plays a role in making sure the fastener does its job without compromise.”