Ultimate Guide to US Screw Sizes

One of the popular items in the tool shed of any DIY enthusiast is screws. They come in different sizes and shapes.

When it comes to screws, there are two primary areas where people get confused. They are sheet metal screws and wood screws. The difference between the two is understanding metric screw vs. imperial sizes. The following guide will ensure you understand the difference.

Different Types of US Screws

Before you look at the different US screw sizes, you need to understand the types available. The following is a breakdown of three major types of US screws you can find:

Wood Screws

They are designed for connecting items to a wooden base (e.g. attaching metal coat hooks to a wooden rail).

Wood expands and shrinks depending on the surrounding temperature. This means a specialized screw is required for wood applications.

The best options are screws that bend before snapping. Wood screws have this capability and that is why they are not used on sheet metal. They come in a variety of sizes which are provided in the tables found later in this guide.

Sheet Metal Screws

They are typically sturdier and stronger than their wooden counterparts. They can be used in other materials apart from metal as they will stay strictly in place once screwed on.

Often they are self-tapping meaning that they may not always need a hole drilled into the source before they are inserted. Sheet metal screws come in several different sizes and styles.

Phillips Head Screw

A traditional (“flat”) screwdriver cannot be used on sheet metal cross head screws. It is a cross-shaped head kind of screw (see image above). Most sets of driver heads or screwdrivers have at least one of these types of screwdrivers.

screw sizes

Screw Acronyms

In addition to length of the screw, diameter, and gauge size, the packaging of many screws contains acronyms. These are given to show the added value of the item you purchase.

The next step is to understand popular screw acronyms. This information will help you further understand the screws and their sizes:

  • ST- Self-tapping: They don’t require a hole to be drilled and are time savers.
  • TT-Twin thread: It’s generally more secure if a screw can be installed and removed faster than a single thread equivalent that’s what it means by two threads.
  • TFT-Twin Fine Thread: They are a little less coarse than non-fine thread screws and thus may slot into a drilled hole a little easier.
  • ZP-Zinc Plating: It offers a layer of protection against corrosion.
  • ZYP-Zinc and Yellow Passivity: They are named for their yellow coating and have two layers of protection.

Understanding Screw Sizes

Unless you are conversant with the several screw sizes that are quoted on the packaging, sizing the right screw might be difficult in both metric and imperial. We normally deal with different sizes of screws and conversion from metric to imperial sizes in this project using our handy conversion table.

The table helps in converting metric screw sizes to imperial and vice versa easily. It, therefore, makes sure that you obtain the correct ones.

Imperial and Metric Screw Sizes

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

It is, therefore, crucial for you to know the difference between the two. It will ensure you minimize or avoid getting the wrong sizes.

Explanation on Imperial Screw Sizes

A screw for wood is sized by two distinct numbers, first is the gauge of the screw which is the diameter. This means, the larger the number the larger the diameter. Therefore, a number 12 screw is larger than a number 4 screw.

It is important to note that there is no direct connection between the gauge of an imperial screw and its head size. Although some sources may make you believe that this is how the gauge is calculated, it is by chance that from screw gauge 6 and above the gauge is almost twice the diameter’s head.

It is important again to note that the given length for a screw is the length that is buried in the wood or any other material but the head of a raised screw is not inclusive. The screw size is determined by length and gauge.

Explanation on Metric Screw Sizes

Though the metric system is a bit simple to understand for the unskilled, it can be a bit challenging if you are not conversant with it or still working in imperial.

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

It is by coincidence that the gauge is approximately the size of the screw head in millimeters. A 6 gauge screw will have a head almost equal to 6 mm wide.

Correlation between diameter (metric in mm), gauge(imperial) and head size is complicated. There is nowhere you will find information that can guarantee you can calculate this effectively. It may be the reason why professionals prefer to buy screws personally, to ensure they get the right screw head.

The imperial diameter (in 16th of an inch) of the screw head is twice the gauge (imperial). The formula below can be used to calculate an estimate of the screw head sizes and the 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 for imperial gauge, the diameter in mm is almost half the gauge. You should not be disappointed if you do not follow this since not many people know about the said relationship leave alone using them.

Since slotted screws are being outdated, the crosshead screws are now replacing them but it is important to note that a cross head screw can be either a superdrive/pozidrive or a Philips screw.

Conversion Chart For Metric Imperial Screws

screws comparison
Screws and their bolts

The table below can be used to match the imperial screw size to the metric size. You should note that the conversion is not exact and therefore margins of errors are allowed.

Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
6mm 150 12 x 6
130 12 x 5 1/8
110 12 x 4 3/8
100 12 x 4
90 12 x 3 1/2
80 12 x 3 1/4
75 12 x 3
70 12 x 2 3/4
60 12 x 2 3/8
Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
5 mm 100 10 x 4
90 10 x 3 1/2
80 10 x 3 1/4
75 10 x 3
70 10 x 2 3/4
60 10 x 2 3/8
50 10 x 2
45 10 x 1 3/4
40 10 x 1 1/2
35 10 x 1 3/8
30 10 x 1 ¼
25 10 x 1
Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
4.5 mm 75 9 x 3
70 9 x 2 3/4
60 9 x 2 3/8
50 9 x 2
45 9 x 1 3/4
40 9 x 1 1/2
35 9 x 1 3/8
30 9 x 1 1/4
25 9 x 1
Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
4 mm 70 8 x 2 3/4
60 8 x 2 3/8
50 8 x 2
45 8 x 1 3/4
40 8 x 1 1/2
30 8 x 1 1/4
25 8 x 1
20 8 x 3/4
16 8 x 5/8
12 8 x 1/2
Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
3.5 mm 40 6 x 1 ½
30 6 x 1 ¼
25 6 x 1
20 6 x ¾
16 6 x 5/8
16 6 x 5/8
Diameter (mm) Length (mm) Closest Imperial Size
Gauge x Length
3 mm 40 4 x 1 1/2
30 4 x 1 1/4
25 4 x 1
20 4 x ¾
16 4 x 5/8
12 4 x ½

screw tip

Image of a typical superdrive/prodive screw.

You can as well 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.

Gauge Metric diameter (mm) Pilot Hole size (mm) Masonry Rawl Plug Hole size for Rawl Plug (mm)
14 6.5 4 Blue 10
12 5.5 3.5 Brown 7
10 5 3 Brown 7
8 4 2.5 Red (or Brown) 6 (or 7)
6 3.5 2 Red 6
4 3 1.5 Yellow 5
3 2.5 1 Yellow 5

A chart for the above table:

us screw sizes

Figure 1.: Chart for imperial measurements. The X axis is the gauge size. The Y axis is measurement in millimeters.

Wrench Sizes Or Spanner

The table below shows common sizes of thread and their equivalent wrench or spanner size. For screws which have a hex head such as coach screws, the size is quoted as an ISO Metric which is the number starting with “M”. The numbers are used to describe the thread and relate it to the size of the spanner to be used on it.

Spanner size (mm)
M64 95
M56 85
M48 75
M42 65
M36 55
M30 46
M24 36
M20 30
M16 24
M12 19
M10 17
M8 13
M6 10
M5 8
M4 7
M3 5.5
M2.5 5
M2 4
M1.6 3.2

A chart for the above data

screw sizes chart

FAQs

How Do I Know The Best Screw Size for my Needs?

Trial and error usually! You will have to assess which size best serves your project by trying a variety of different sizes starting with the longest.

Is Size Important?

Yes! A screw that is too long or too small for your task is no use at all as it may be dangerous. Don’t cut off the end of your screw if it does not disappear fully into its source whilst leaving the ability to tighten. Use an alternative with a shorter length after you remove the screw in question.

Can Sheet Metal Screws be used in Wood?

Due to the flexibility of a wood screw, it may be stronger on wood. However, it may end up snapping if the surrounding temperatures change drastically.

Can Wood Screws be used in Sheet Metal?

No! Always use a sheet metal screw because the tensile strength of a wood screw cannot compare.

Can I Use the Same Type of Screws Outdoors and Indoors?

If you plan to use the screws outdoors then look for a screw labeled ‘exterior’ of the same. They will not corrode or rust as quickly in the rain because they have extra protection against the elements.

Do I Need a Screwdriver if I Have a Self-Tapping Screw?

It is advisable to have one on hand because it depends on what you are driving the screw into.

How Do I Loosen/Tighten a Screw?

Turn a screw anti-clockwise to loosen it and clockwise to tighten.

How Tight Does the Screw Need to Be?

Ensure that a screw is turned as tightly as you can manage because a loose screw could lead to a loose connection.

What Happens When My Screw Doesn’t Tighten?

Consider adding a little glue if you are screwing into the wood and you know that you have the correct style and size.

Is There a Formula for Matching Screw Sizes to Drill Bits?

It depends on whether you will be drilling into soft or hardwood, or if you’ll require the use of walls plugs in a foundational part of your home. The most effective drill bit you want to use means you need a variety of screw sizes at hand and work using the drill bit size as a guide.

Bottom Line

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

In many situations, you may find it challenging to know the head and the thread available and also the different parts that make up a screw. Also take note of the number of threads per inch. Use this guide to simplify the process and reduce the margin of error.