Understanding Wood Screw Sizes: Metric Vs Imperial Standards

wood screw sizes metric imperial

Wood screw sizes differ between the metric and imperial systems. In the metric system, both the diameter and length of screws are measured in millimetres, while in the imperial system, the diameter is indicated by a gauge number and the length is measured in inches. While the UK historically used the imperial system, the metric system has become more prevalent due to European influence and the decline of British screw manufacturers. Today, most wood screws in the UK are imported and sold as metric screws, although they may also be advertised with imperial equivalents, which can be confusing.

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Screw size conversion charts

Screw sizes can be quoted in imperial or metric units, and many manufacturers will include both on their packaging. However, when purchasing online, many retailers will only include one unit format in the product title.

In the imperial system, the first number is the gauge of the screw, which refers to the diameter. The larger the number, the larger the diameter. The length is also represented in the imperial system, but it is measured in inches rather than millimetres.

The metric system is considered simpler as it measures the diameter and length of screws in millimetres. The M rating, which stands for Metric, indicates the diameter of the screw in millimetres. For example, M6 is a 6mm wide screw.

| Metric Diameter (mm) | Approx. Imperial Equivalent |

| --- | --- |

| 4 | #8 |

| 5 | #10 |

| 6 | #12 |

| 8 | 1/4 x 40 |

| 10 | 5/16 x 32 |

And here is a chart for imperial to metric:

| Imperial Size (No x Inch) | Metric Size (mm approx.) |

| --- | --- |

| #8 x 1 | M4 |

| #10 x 1 1/4 | M5 |

| #12 x 1 1/2 | M6 |

| 1/4 x 2 | M6 |

| 5/16 x 2 | M8 |

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Screw sizes on packaging

Sizing a screw can be a complicated process unless you are familiar with the various screw sizes quoted on the packaging, which is often in both imperial and metric units. The sizing of screws is determined by the length and gauge of the screw.

Imperial Screw Sizes

An imperial wood screw is sized using two different numbers. The first number is the gauge of the screw, which refers to the diameter. The larger the number, the larger the diameter. For example, a number 12 screw has a larger diameter than a number 4 screw. It is worth noting that there is no direct link between the head size of an imperial screw and the gauge. The length of the screw is represented by the second number and refers to the length of the screw that is buried in the wood or other material. It does not include the head of a raised or domed screw.

Metric Screw Sizes

The metric system is considered simpler to understand as it measures the diameter of the screw in millimetres, rather than using a gauge. The length of the screw is also measured in millimetres and is the same as in the imperial system. The first number on the packaging of screws measured in metric refers to the diameter, and the second number refers to the length. For example, a package of screws that says 5.0 x 60 means that the screws have a 5 mm diameter and are 60 mm long.

Converting Between Imperial and Metric

Although many manufacturers put both the imperial and metric sizes on packaging, when purchasing online, many retailers do not. This is because the title of the product becomes too long and cumbersome. There are conversion charts available to help you convert between the two systems and ensure you are ordering the correct size.

Additional Information on Packaging

In addition to the length and diameter/gauge of the screw, the packaging of many screws contains acronyms. These are included to show the added value of the item. Common acronyms include:

  • ST – Self-tapping: These screws can be screwed into wood without a pilot hole being drilled, saving time.
  • TFT – Twin Fine Thread: These screws are less coarse than non-fine thread screws and thus may slot into a drilled hole more easily.
  • ZP – Zinc Plated: These screws have been plated with zinc to prevent corrosion.
  • ZYP – Zinc and Yellow Passivated: These screws have been zinc plated and then a passivation layer is applied to provide further protection against corrosion, resulting in a yellow colour.

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Screw head sizes

When it comes to screw head sizes, there are several factors to consider. Firstly, it's important to distinguish between the head diameter and the shank diameter of a screw. The head diameter refers to the width of the screw head, while the shank diameter refers to the width of the unthreaded portion of the screw body, just below the head.

In the case of flat head and oval head screws, the head diameter is explicitly designed for these head shapes. Round head screws, on the other hand, can be about 15% smaller in head size, with the shank size remaining the same. It's worth noting that as the screw size decreases, the head diameter reduction becomes more significant.

The largest wood screw produced by Nettlefolds, a size 32 screw, had a head diameter of one inch and a shank diameter of half an inch. Conversely, their smallest screw, a size 0000, featured a head diameter of 108/1000 of an inch and a shank diameter of 54/1000 of an inch.

When selecting a screw, it's crucial to consider the application and the specific requirements of the project. For instance, the #4 screw size is designed for small crafts, jewellery boxes, and attaching hinges, while the #10 size is suitable for general construction, heavy-duty furniture, and outdoor projects.

Additionally, the choice between traditional wood screws and modern production screws can impact the sizing considerations. Traditional wood screws have a shank diameter that exactly matches the thread's outer dimensions, resulting in maximum holding power. In contrast, modern production screws have a reduced shank diameter, creating a deep thread profile for improved holding power, and eliminating the need for critical depth control during drilling.

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Pilot holes

When joining two boards, it is essential to pre-drill a hole only through the first board, leaving the mating piece untouched. This allows the screw's threads to grip the second board securely. The size of the pilot hole should be slightly smaller than the thickness of the screw's threads, ensuring enough material for the threads to bite into and hold the screw firmly in place.

The thickness of the screw will dictate the required size of the drill bit for the pilot hole. The screw's packaging will typically indicate its size, with a higher number representing a thicker screw shaft. Most drill bit sets include standard sizes suitable for creating pilot holes. It is crucial to select the correct size to avoid issues during the woodworking process.

The type of wood being used is another factor to consider when determining the size of the pilot hole. For hardwood, opt for a slightly larger pilot hole, while a smaller pilot hole is sufficient for softwood. Additionally, when drilling pilot holes, always wear safety glasses to protect your eyes from potential debris or shattered drill bits.

Drilling pilot holes is a straightforward task, but it requires careful execution. It is essential to clamp the board securely and maintain a steady hand to keep the drill straight. Start drilling slowly, and gradually increase the speed once the hole is established. Be cautious not to drill too fast, as it may result in an angled hole or accidental penetration through the other side of the board.

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Shank diameter

The shank of a screw is the unthreaded portion of its body, between the threaded portion and the head. The diameter of the shank is an important factor in screw selection, as it determines the amount of force that can be exerted on the material being joined.

In the metric system, the diameter of the shank is measured in millimetres. The larger the number, the larger the diameter. For example, a metric screw with a diameter of 5.0 mm would be larger than one with a diameter of 4.0 mm.

The imperial system, on the other hand, uses a gauge number to indicate the diameter of the shank. The gauge number refers to the number of wires of that size that can fit into an inch. So, a higher gauge number means a smaller diameter. For example, a #10 screw has a smaller diameter than a #8 screw.

When converting between metric and imperial screw sizes, it's important to consider the shank diameter. For example, a metric screw with a diameter of 4.0 mm is equivalent to a #8 screw in imperial. Here are some more examples of metric-imperial conversions for screw shank diameters:

  • 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 4.5 mm = 9 Gauge (British Imperial)
  • Metric 5.5 mm = 12 Gauge (British Imperial)

Frequently asked questions

Metric wood screws have sizes that are measured in millimetres, whereas imperial wood screws are measured in inches.

Metric to imperial conversions are widely accepted as the industry norm in the UK. Here is a conversion chart: 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 4.5 mm = 9 Gauge (British Imperial); Metric 5.0 mm = 10 Gauge (British Imperial); Metric 5.5 mm = 12 Gauge (British Imperial); Metric 6.0 mm = 14 Gauge (British Imperial); Metric 8.0 mm = 16 Gauge (British Imperial).

Yes, a metric wood screw will have a slightly different shank, thread, and head diameter compared to its imperial equivalent.

In the UK, most wood screws are imported and sold as metric screws. However, they are often given an imperial equivalent, which can be confusing.

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