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How to measure screw sizes: expert guide

how to measure screw sizes

Our guide on how to measure screw sizes also outlines the difference between imperial and metric sizing, how to convert screw sizes, and the differences between wood and metal screws.

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Just about the most used materials in any project are screws. They come in an astonishly wide range of different sizes and shapes, and it can be easy to be left wondering about the differences and which screw you need for the job in hand, which is why you need to know accurately how to measure screw sizes.

How to measure screw sizes

What are the different parts of a screw called?

how to measure screw sizes (1)

Before you look at how to measure screw sizes, you should first understand the terminology of the parts that make up a screw, so you understand which areasyou are measuring. But in simple terms, a screw essentially consists of 4 main parts, each of which has sub-areas:

  • Head
  • Shaft
  • Thread
  • Top

If it is a fully threaded screw, i.e. the thread pulls through to the screw head, the shaft is not required. Otherwise, each screw is built on the same principle.

How to measure a screw

how to measure screw sizes length and diameter

If you want to measure a screw, you should pay attention to the following. Screw sizes are always specified according to the same principle: diameter x length. Here is a small example: 5 x 120 mm would mean the screw is 5 mm thick and 120 mm long.

If you now want to measure a screw yourself, proceed as follows: First of all, you should use a caliper to measure. The length of the screws is measured in this case, including the head, as shown on the sketch and in the photo below. When determining the diameter, you have to make sure that the slide is placed on the thread and not accidentally slip between the cutting edges. You insert the screw in the caliper as shown in the photos above.

Often the diameter of the screw is determined incorrectly because many people apply the caliper as shown in Image C. Thus the diameter of course deviates from the information. Even with a partially threaded screw, it is important to ensure that the screw is correctly inserted into the caliper and not to determine the screw shaft as the diameter.

However, if you use a screw with a countersunk head (like the screw in our diagram above), the head will be included in the length when measuring. The reason for this is very simple. The countersunk head of a screw disappears completely into the wood and is therefore included in the length of the screw.

However, if you measure a screw whose head is not countersunk in the material, for example a pan-head screw (sketch on the right), then only the part that actually disappears in the material will be counted as length.

Since the head lies on the material in this case, it is not included in the calculation. How to measure screw sizesnow sounds easy, doesn’t it?

How to measure screw sizes: Complete guide to screw sizes

When it comes to how to measure screw sizes for different applications, there are two main areas where people get confused. They are metal screws and wood screws. The difference between the two is understanding metric screws versus imperial sizes. The following guide will make sure you understand the difference.

Table of Contents

Different types of US screws

Before looking at the different US screw sizes, you need to know the types available. The following is a breakdown of three main types of US screws that you can find:

Wood screws

They are used to connect objects with a wooden base (e.g. attaching metal coat hooks to a wooden rail). 

Wood expands and shrinks depending on the ambient temperature. This means that a special screw is required for wood applications.

The best options are screws that bend before they click into place. Wood screws have this ability and are therefore not used for sheet metal. They are available in different sizes, which are listed in the tables later in this manual.

Sheet metal screws

They are usually sturdier and stronger than their wooden counterparts. They can also be used in other materials besides metal as they stay strictly in place after screwing on.

Often times they are self tapping, which means they may not always have to drill a hole in the source before inserting them. Self-tapping screws are available in different sizes and designs.

Phillips screw

A conventional (“flat”) screwdriver cannot be used for sheet metal crosshead screws. It’s a cruciform head screw (see picture above). Most sets of driver heads or screwdrivers have at least one of these screwdrivers.

Screw acronyms

In addition to the length, diameter, and size of the gauge, many screws come with acronyms. These are provided to show the added value of the item you have purchased.

The next step is to understand the common acronyms for screws. This information will help you better understand the screws and their sizes:

  • ST- Self-cutting: You don’t need a hole to be drilled and you save time.
  • TT twin threads: It is generally safer if a screw can be installed and removed more quickly than a single thread, which is what is meant by two threads.
  • TFT-Twin Fine Thread: They are a little less coarse than non-fine thread screws and can therefore be inserted into a drill hole a little easier.
  • ZP galvanizing: Provides an anti-corrosion layer.
  • ZYP zinc and yellow passivity: they are named for their yellow coating and have two protective layers.

Understanding screw sizes

If you are not familiar with the various screw sizes listed on the package, sizing the correct screw can be difficult, both metric and imperial. Normally in this project we deal with different sizes of screws and the conversion from metric to imperial sizes using our practical conversion table.

The table helps to easily convert metric screw sizes to imperial and vice versa. So it makes sure you get the right ones.

Imperial and metric screw sizes

Many companies put both imperial and metric sizes on the same box of screws, which is very helpful. However, when buying online in the US, most retailers do not. The main reason is that the title of the product becomes too long and awkward, which means something has to go.

It is therefore important that you know the difference between the two. This will ensure that you minimize or avoid the wrong sizes.

Declaration on imperial screw sizes

A screw for wood has two different numbers. First, the strength of the screw is the diameter. This means the larger the number, the larger the diameter. Hence, a number 12 screw is larger than a number 4 screw.

It is important factor in how to measure screw sizes to note that there is no direct link between the strength of an Imperial screw and its head size. Although some sources suggest that the gauge is calculated this way, it just so happens that from gauge 6 and above the gauge is almost twice the diameter of the diameter.

Again, it is important to note that the length listed for a screw is the length that is buried in the wood or other material, but does not include the head of a raised screw. The screw size is determined by its length and strength.

Explanation of metric screw sizes

While the metric system is a little easy to understand for the unskilled, it can be a challenge if you are not familiar with it or are still working at the Imperial.

The metric system uses the diameter in millimeters instead of a “gauge” table. As in the imperial system, the length is measured in millimeters.

It is a coincidence that the measuring device is roughly the size of the screw head in millimeters. A 6-gauge screw has a head that is almost 6 mm wide.

The correlation between diameter (metric in mm), thickness (imperial) and head size is complicated. Nowhere can you find information that guarantees you can calculate this effectively. This may be why professionals prefer to buy screws in person to make sure they are getting the correct screw head.

The imperial diameter (in 16 inches) of the screw head is twice as large as the imperial diameter (imperial). The following formula can be used to calculate an estimate of the bolt head sizes and gauge.

Gauge = (head diameter in sixteenths of an inch x 2) – 2. E.g. 5/16 head times two equals 10, minus two equals 8. The gauge is 8.

The above formula means that the diameter in mm for the imperial gauge is almost half that of the gauge. You shouldn’t be disappointed if you don’t follow this as not many people know about the relationship in question.

Since slotted head screws are obsolete, now replace the Phillips head screws. However, it is important to note that a Phillips screw can either be a Superdrive / Pozidrive or a Phillips screw.

Conversion table for metric imperial screws

The table below can be used to adjust the imperial screw size to the metric size. You should note that the conversion is not accurate and therefore limits of error are allowed.

Diameter (mm)Length (mm)Next imperial  size indicator x length
6mm15012 x 6
13012 x 5 1/8
11012 x 4 3/8
10012 x 4
9012 x 3 1/2
8012 x 3 1/4
7512 x 3
7012 x 2 3/4
6012 x 2 3/8
Diameter (mm)Length (mm)Next imperial  size indicator x length
5 mm10010 x 4
9010 x 3 1/2
8010 x 3 1/4
7510 x 3
7010 x 2 3/4
6010 x 2 3/8
5010 x 2
4510 x 1 3/4
4010 x 1 1/2
3510 x 1 3/8
3010 x 1 ¼
2510 x 1
Diameter (mm)Length (mm)Next imperial  size indicator x length
4,5 mm759 x 3
709 x 2 3/4
609 x 2 3/8
509 x 2
459 x 1 3/4
409 x 1 1/2
359 x 1 3/8
309 x 1 1/4
259 x 1
Diameter (mm)Length (mm)Next imperial  size indicator x length
4 mm708 x 2 3/4
608 x 2 3/8
508 x 2
458 x 1 3/4
408 x 1 1/2
308 x 1 1/4
258 x 1
208 x 3/4
168 x 5/8
128 x 1/2
Diameter (mm)Length (mm)Next imperial  size indicator x length
3,5 mm406 x 1 ½
306 x 1 ¼
256 x 1
206 x ¾
166 x 5/8
166 x 5/8
Diameter (mm)Length (mm)Next imperial  size indicator x length
3 mm404 x 1 1/2
304 x 1 1/4
254 x 1
204 x ¾
164 x 5/8
124 x ½
Screw point

Picture of a typical Superdrive / Prodive screw.

You can also use the table below to find the metric screw size from an imperial measurement. The sizes for the Rawl plugs and pilot holes have been added.

SpurMetric diameter (mm)Pilotlochgröße (mm)Mauerwerk Rawl PlugHole size for Rawl Plug (mm)
842.5Red (or brown)6 (or 7)

A table for the above table:

Figure 1 .: Diagram for imperial measurements. The X-axis is the measurand. The Y-axis is measured in millimeters.

Wrench sizes or wrenches

The following table shows the common thread sizes and the corresponding wrench or wrench sizes. For screws with a hexagon head, such as B. Cylinder screws, the size is specified as an ISO metric, where the number begins with “M”. The numbers are used to describe the thread and to relate it to the size of the wrench that will be used on it.

Wrench size (mm)

A chart for the above data


How do I know the best screw size for my needs?

Trial and error usually! You need to judge which size is best for your project by trying different sizes, starting with the longest.

Does size matter?

Yes! A screw that is too long or too small for your job is of no use as it can be dangerous. Don’t cut the end of your screw if it doesn’t completely disappear into the source while you can tighten it. Use an alternative with a shorter length after removing the screw in question.

Can self-tapping screws be used in wood?

Because of the flexibility of a wood screw, it can be stronger on wood. However, crackling can occur when ambient temperatures change drastically.

Can wood screws be used in sheet metal?

No! Always use a self-tapping screw as the tensile strength cannot be compared to a wood screw.

Can I use the same type of screw outdoors and indoors?

If you plan to use the screws outdoors, look for a screw labeled “outside”. They don’t corrode or rust as quickly in the rain because they provide additional protection against the elements.

Do I need a screwdriver if I have a self-tapping screw?

It is advisable to have one on hand as it will depend on what you are driving the screw into.

How do I loosen / tighten a screw?

Turn a screw counterclockwise to loosen it and clockwise to tighten it.

How tight does the screw have to be?

Make sure to turn a screw as tight as possible, as a loose screw can lead to a loose connection.

What if my screw doesn’t tighten?

Consider adding a little glue when you screw into the wood knowing you are the right style and size.

Is there a formula for adjusting screw sizes to suit drill bits?

This depends on whether you are drilling in softwood or hardwood, or whether you need to use wall plugs in a basic part of your home. The most effective drill bit you want to use means you’ll need a variety of screw sizes and use the drill bit size as a guide.

Bottom line

The size of a screw is a very challenging thing, but there are different types of screws that you can use for a variety of different tasks.

In many situations it can be difficult to know the head and thread and the different parts of a screw. Use this guide on how to measure screw sizes to simplify the process and reduce the chance of errors.

gerhardt-richter Gerhardt Richter is a writer and a trainer at trade technical colleges, specializing in carpentry, plumbing, mechanics and construction.