Your guide to measuring screws

Measuring a screw may seem easy at first glance, but it isn’t as simple as getting a ruler and jotting down the length from the head to the tip. 

You need to know where to measure, what tools to use, and how long and wide each screw must be for the task at hand. If you get the measurements off and the screw is too big or too small, you could end up with insufficient grip and faulty installation.

So, do you want to learn how to measure your screws the right way? Stick with Efixings as we walk you through it. 

We stock a wide variety of professional screws and fixings for every project, perfect for those who work in construction or handy DIYers. 

Why should you measure your screws?

Whether you’re building home furniture, like a table or chairs, or you’re working on a structural project, stability and longevity are something you always want to aim for. 

Especially when you're building something that needs to handle a lot of constant pressure or weight, like floors, wooden decks, and load-bearing walls. Measuring your screws before every project will help you avoid screws that are too short to hold or long screws that protrude from the wall or surface, causing safety issues. 

Besides building structures that are safe for people to use, which is the main priority, knowing how to measure a screw properly will also help you avoid wasting time and money on incompatible screws. 

Screw size basics: metric vs. imperial

Screw sizes are typically given in metric in the UK (millimetres), but you might also encounter imperial sizing (inches and gauge numbers). 

Metric sizes are typically written as “diameter × length (mm)”. For example, a screw labelled 5 × 50 mm has a 5 mm thread diameter and is 50 mm long. 

While imperial screws are written as “gauge number × length (inches)”. For example, an 8 × 2" screw is approximately equivalent to a 4 mm diameter and 50 mm length. However, these are an older UK measuring system and aren’t much used anymore. 

The different parts of a screw

It’s best to know the four parts that make up a screw so you know which area you’re measuring. 

• The head: The top part of the screw that a screwdriver or drill fits into, which can be flat, rounded, or shaped (Phillips, Pozi, etc.).
• Shank (Shaft): The smooth or threaded body of the screw. The diameter of the shank (including threads) is what we measure for screw thickness.
• Thread: The helical ridge that wraps around the shank, which includes threads that are designed to bite into the material. The spacing between these threads is known as the thread pitch.
• Point: The sharp (or sometimes blunt) tip that allows the screw to pierce the surface of a material.

How to Measure Screw Length

When it comes to measuring the length of a screw, you’re best using a tape measure with metric markings or a ruler. We suggest going with the first if you can, for more accurate readings. 

If you need to measure a small screw, use a pair of digital callipers for a precise measurement. Once you’re ready, place your screw on a flat surface and align the ‘0’ mark on the ruler with the top of the screw’s head.

Then, do the following for each type of screw:

• Flat-headed screws (countersunk): measure from the very top of the head to the tip of the screw. The head will sit flush in the material, so the entire screw (including the head) counts toward the length.

• Round, pan, or hex-headed screws: measure from the flat underside of the head to the tip. For these, the head usually sits on top of the material, so the head’s height isn’t included in the length measurement.

• Oval head screws: While less common, these still crop up. Since they have a partially rounded head that sits partially into the material, measure from the widest point of the head to the tip.

How to Measure Screw Diameter (Gauge)

The diameter of a screw (or the gauge of a screw in imperial terms) tells you how thick the screw is. By measuring the diameter properly, you’ll be able to match the screw to the perfect-sized pilot hole or fitting (and avoid a messy installation).

When it comes to measuring different types of screws, the general guideline is the same; you’re measuring the major diameter, which is the distance across the outer edges of the threads.

We suggest using callipers or a screw gauge to provide a more accurate measurement in millimetres, but a standard ruler will do the job if you have nothing else. Place the screw on a flat surface and align the zero mark with the top of the head (unless it’s a round-head screw).

If the screw is countersunk, include the head in the length; if not, measure from underneath the head. Here’s a closer look at how to measure screws with different types of heads:

Self-drilling screws

1. Place the screw so the threaded section sits between your callipers or against a ruler.
2. Measure across the widest part of the threads, avoiding any fluted drilling section at the tip.

Flat head (countersunk) screws

1. Hold the screw just below the head and position the threaded shank between callipers or against a ruler.
2. Measure across the threads at a point where they are full depth, not near any taper at the tip.
3. Record the size in millimetres (e.g., 4 mm or 5 mm), and use it as your diameter.

Hex head screws

1. Grip the screw by the head and focus on the threaded shank (ignore the head).
2. Measure across the outer edges of the threads with callipers or a ruler.
3. Note the result in millimetres.

Oval head screws

1. Ignore the curved part of the head and look only at the threaded section.
2. Measure across the outer edges of the threads using callipers or a ruler, ideally at the middle of the shank where the threads are fully formed.
3. Write down the diameter in millimetres.

Don’t forget thread pitch!

When it comes to self-tapping or wood screws, you generally won’t need to measure the thread pitch, since these are chosen by the length and diameter, and wood screws generally have coarse pitches designed for biting into wood.

However, when it comes to bolts or machine screws, thread pitch is important. The thread pitch shows how close or far apart the threads on a screw are to each other and is measured in millimetres.

Coarse pitch threads tend to be more widely spaced, while fine pitch threads are closer together. To measure this, you can use a thread pitch gauge. Then, do the following:

1. Press the gauge’s teeth against the threads of your screw until you find the blade that meshes perfectly.
2. Look for the marking on the blade to find the pitch (TPI).
3. Alternatively, lay the screw alongside a ruler and count the number of threads over a given length (over 1 inch for imperial screws).

If you’re using a ruler, divide the number of threads by the length of the screw to calculate the thread pitch.

Shop screws for your next project today

Before you start your next project, take a moment to size up your screws using the tips above, so you aren’t left with money out of your pocket and useless screws in your toolbox. 

It only takes a few seconds and can save you a lot of hassle later. And when you’re ready to buy the perfect screws, remember that eFixings has you covered with a huge range of screw sizes and types, from classic wood screws to stainless steel screws.

Plus, they’re all clearly labelled in metric for your convenience!