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Wood screws are sized using two numbers: the first is the gauge of the screw, which refers to its diameter, and the second is its length. The larger the number, the larger the diameter or length. For example, a number 12 screw has a larger diameter than a number 4 screw. The length given for a screw is the length that is buried in the wood or other material, and does not include the head of a raised or domed screw. While the gauge of a screw is related to its head size, there is no direct link between the two.
Characteristics of Imperial Wood Screw Sizes
| Characteristics | Values |
|---|---|
| Numbering | #6, #8, etc. |
| Diameter | The larger the number, the larger the diameter. |
| Length | The length given is the length buried in the wood or other material, excluding the head of a raised screw. |
| Gauge | The gauge refers to the diameter of the screw. |
| Head Size | There is no direct link between the head size and the gauge. |
| Conversion | The imperial system can be converted to metric using conversion charts. |
| Measurement | The imperial system measures diameter and length in inches. |
| Acronyms | Acronyms on packaging include ST, TT, TFT, ZP, and ZYP. |
| Thread | The number of threads per inch is an important consideration when choosing a screw. |
| Pilot Hole | Matching screw sizes with appropriate pilot holes is crucial to prevent damage to the material. |
| Rawl Plug | The size of the hole for a rawl plug is also an important consideration. |
| Wrench Size | For screws with a hex head, the size is quoted as an ISO Metric starting with "M". |
Imperial vs metric
The choice between using imperial or metric screw sizes depends on a variety of factors, including personal preference, the target market, and the specific application. Here is a detailed comparison between the two systems in the context of screw sizes, particularly for wood screws:
Metric System
The metric system is based on measurements in millimetres and centimetres, making it easier to scale sizes up or down. The diameter and length of screws are both measured in millimetres. For example, M8 indicates a metric screw with an outer diameter of 8 millimetres. The metric system is used globally, making it more convenient for international collaboration and communication. It also has a straightforward subdivision of units, with each unit of measurement differing by a factor of ten, making it simpler to understand and reducing errors.
Imperial System
The imperial system, also known as the American Standard, uses inches or fractions of inches for measurements. The gauge of the screw, which refers to its diameter, is represented by a number, with larger numbers indicating larger diameters. The length of the screw is also measured in inches. For example, a 1/4"-20×3" screw has a diameter of 1/4", 20 threads per inch, and a length of 3". The imperial system is predominantly used in the United States, although there is a growing trend towards adopting the metric system in certain parts of the country.
Application Considerations
When deciding between imperial and metric screw sizes, it is important to consider the target market and application. If selling products in markets that predominantly use the metric system, such as Europe, it would be advantageous to use metric fasteners. This ensures consistency and avoids potential regional variations in measurements. On the other hand, if primarily targeting the US market, imperial fasteners may be more suitable, although the increasing use of metric measurements in certain US industries should be taken into account.
Additionally, the metric system offers a wider range of sizes within a small range, making it a preferable choice when working with applications that require a variety of closely scaled sizes. The metric system's ease of scaling also makes it a more convenient option for manufacturers and engineers.
In summary, while the choice between imperial and metric screw sizes can depend on personal preference, the metric system offers advantages in terms of global standardization, ease of subdivision, and scalability, making it the preferred choice for many manufacturers, especially those with international operations. However, the imperial system remains prevalent in certain industries and regions, particularly in the United States.
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Screw gauge
The gauge of a wood screw refers to its diameter. The higher the number, the larger the diameter. For example, a #12 screw has a larger diameter than a #4 screw. It's worth noting that there is no direct relationship between the head size of an imperial screw and its gauge. From gauge 6 and above, the gauge is roughly twice the head diameter, although this is coincidental and not a method for calculating the gauge.
The imperial gauge range was developed in part by the world-renowned screw manufacturer 'Guest, Keen & Nettlefolds' of Birmingham, England. The largest wood screw they produced was a size 32, with a head diameter of one inch and a shank diameter of half an inch. Their smallest screw was a size 0000, with a head diameter of 108/1000 of an inch and a shank diameter of 54/1000 of an inch.
The metric system, on the other hand, uses the diameter in millimetres instead of a gauge table. So, a 6 gauge screw will have a head that is approximately 6mm wide.
The formula for calculating the gauge of an imperial screw is:
> (Head diameter in sixteenths of an inch x 2) - 2 = Gauge
For example, a screw with a 5/16 head has:
> (5/16 x 2) - 2 = 8 gauge
The above formula means that for an imperial gauge, the diameter in mm is approximately half the gauge.
When buying screws, it's important to know the difference between imperial and metric sizes. Many companies provide both measurements on the packaging, but when buying online, you may only see one or the other.
| Metric Diameter (mm) | Imperial Gauge |
|---|---|
| 1.6 | 1 |
| 2.0 | 2 |
| 2.5 | 3 |
| 3.0 | 4 |
| 3.5 | 6 |
| 4.0 | 8 |
| 4.5 | 9 |
| 5.0 | 10 |
| 5.5 | 12 |
| 6.0 | 14 |
| 8.0 | 16 |
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Screw length
When determining the appropriate screw length, several factors come into play. Firstly, the thickness of the material being fastened plays a role. As a general rule, the screw should penetrate at least half of the thickness of the attached material. For thicker materials, longer screws may be required to achieve a firm hold. Conversely, shorter screws may be adequate for thinner materials.
The type of screw head also impacts the measurement of screw length. Most screw heads are flat on the bottom (e.g., pan head, round head, truss head), and the length is measured from the bottom of the head to the tip. Flat head, countersunk screws, on the other hand, are measured from the top of the head to the tip since they sit flush with the surface of the material. Oval head screws are an exception, with their length measured from the widest point of the head to the tip.
It is worth noting that the stated length of a wood screw typically refers to the portion of the screw that is buried in the wood or other material, excluding the head of a raised or domed head screw. Therefore, when selecting a wood screw, consider the thickness of the material and choose a screw with a length that will provide sufficient grip without penetrating all the way through.
Additionally, the McFeelys 2/3 Rule provides a helpful guideline for screw length selection. According to this rule, it is generally recommended that about 2/3 of the screw be threaded into the "secondary" piece or thinner material. As the thickness of the lumber increases, this ratio can be adjusted so that only half of the shank is threaded into the secondary piece.
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Screw head size
The head size of a screw is not the same as its gauge, despite what some sources may suggest. The gauge of an imperial screw refers to its diameter, so the larger the number, the larger the diameter. For example, a number 12 screw has a larger diameter than a number 4 screw.
The length of a screw is also provided in its sizing, but this does not include the head of a raised or domed screw. This length refers to the part of the screw that is buried in the wood or other material.
When measuring the length of a screw, it is important to start from wherever the screw head would rest when it is fully embedded. For a flat-headed screw, this will be the top of the head, whereas for a screw with a rounded head, you should measure from where the oval top and the countersunk half meet. For round-headed screws that aren't countersunk, measure from the flat underside of the head.
The largest wood screw produced by Nettlefolds had a head diameter of one inch, with the smallest being 108/10000 of an inch. The head diameters decreased by 28/10000 of an inch for each size, until they reached screw size 1, where the decrease was only 6/1000 of an inch.
While there is no direct relationship between the head size and gauge of an imperial screw, it is worth noting that for screw gauges 6 and above, the gauge is close to twice the head diameter.
To calculate the head size of an imperial screw, you can use the following formula:
> (Head diameter in sixteenths of an inch x 2) - 2 = Gauge
For example, if the head diameter is 5/16 of an inch, multiplying this by two gives 10. Subtracting two then gives us a gauge of 8.
As the gauge is approximately equal to the head size in millimetres, this formula can also be used to estimate the head size in metric units.
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Screw diameter
The diameter of a wood screw is the first of two numbers used to size it, known as the gauge. The larger the number, the larger the diameter. For example, a #12 screw has a larger diameter than a #4 screw. The gauge refers to the diameter of the screw shank, not the head.
The gauge (imperial) is half the imperial diameter (in 16ths of an inch) of the screw head, roughly. To calculate the gauge, you can use the following formula:
Gauge = (Head diameter in 16ths of an inch x 2) - 2
For example, a screw head that is 5/16 of an inch wide would be calculated as follows:
5/16 head x 2 = 10, minus 2 = 8. The gauge is 8.
It is worth noting that there is no direct link between the head size of an imperial screw and its gauge. It is a coincidence that, from screw gauges 6 and above, the gauge is close to twice the head diameter.
When it comes to metric screws, the diameter is measured in millimetres, and the same is true for imperial screws when using a digital caliper to measure.
The following are some examples of metric to imperial conversions for wood screws:
- Metric 1.6 mm = 1 Gauge (British Imperial)
- Metric 2.0 mm = 2 Gauge (British Imperial)
- Metric 2.5 mm = 3 Gauge (British Imperial)
- Metric 3.0 mm = 4 Gauge (British Imperial)
- Metric 3.5 mm = 6 Gauge (British Imperial)
- Metric 4.0 mm = 8 Gauge (British Imperial)
- Metric 5.0 mm = 10 Gauge (British Imperial)
- Metric 6.0 mm = 14 Gauge (British Imperial)
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Frequently asked questions
A wood screw is sized using two numbers. The first is the gauge of the screw, which refers to the diameter. The larger the number, the larger the diameter. The second number is the length of the screw, which refers to the length buried in the wood or other material, excluding the head of a raised screw.
Metric wood screws have a slightly different shank, thread, and head diameter compared to imperial wood screws. The difference is often subtle, especially with smaller gauge screws, but can be more noticeable with larger sizes, particularly around the head and shank.
It depends on your application. As a general rule of thumb, older slotted wood screws are typically manufactured to an imperial gauge, while newer stainless steel slotted wood screws are usually made to a metric gauge.
The imperial wood screw sizes range from a size 32, the largest, to a size 0000, the smallest.
