Understanding screw sizes and gauges

When attempting to select the right screws for a particular application it can be very confusing, and they may all look the same. In some cases it is possible to use the wrong type or size of screw for a particular job, but you won’t get away with this for long. It is important to choose the right size screw because what you are attempting to fasten, secure or connect will work loose and can become damaged or cause injury over time, if not immediately, and there will be the cost of repairing damage and replacing the item to secure it properly, if you make the wrong selection.

So it is important to understand how different screws can be identified and why these differences are important, so in this guide we will describe the basic elements of all screws, and explain how and why they differ in size.

What are the common elements of all screws?

Screws are essentially made up of four main features:

• A head – round, square or hexagonal in shape and with a different notch in the top to indicate how the screw should be driven into the material (cross, slotted etc)
• A thread – to grip the material and help you drive the screw into it and maintain a hold
• Shank – the main body of the screw, part of this can be unthreaded
• Tip – the end point of the screw which is usually pointed to assist in grabbing hold of the base material, but can be flat.

How do we measure the size of a screw?

When determining the size of a screw we are effectively measuring three separate features, the head, the gauge and the length. Traditionally, an imperial measurement was used, ie. inches, but now we use a metric system measuring in millimetres (mm). In the US, imperial measurements are still used, but in the UK and Europe we mainly only use the metric system.

• Head size – With most screws this isn’t an important measurement, but if you have a screw with a hexagonal head, such as a stainless steel wood screw, this uses a different measurement system known as M numbers. These are defined using the standard DIN 571 and use an M number which ranges from M2 to M80. These numbers refer to the head size and thread diameter of the screw, and so you need to refer to a table such as this:

• Gauge – This effectively refers to the thickness or diameter of the shank or thread. It will include the extent to which the threaded teeth extend away from the shank. Gauge is usually a system starting below 1 and extending to 40, and again uses a reference table, such as this:

• Length – This is important because it is the depth by which the screw will penetrate the material, so it needs to match what you are driving into. If the length is too long it will protrude from the underside of the material, and will look bad and could cause an injury, if it is too short it may not provide enough hold and stability. Measuring the length is different to the other measurements because we are looking at how much of the screw will be in the end material, and therefore we need to also look at the head type. There are typically three different head types in this respect and the length is measured slightly differently with each, because in two cases the head is meant to protrude above a surface and hence is not in the base material and doesn’t need to be counted as ‘length’.
• Countersunk – This head type is flat and sits flush to the surface of the material, and hence the full extent of the screw should be measured as ‘length’.
• Raised head – Part of the head is above the material surface and part is below, so to calculate length you should measure from the tip to the centre line or lip of the head, ie. you don’t include the domed part which sits above the surface.
• Round head – Here, the whole of the screw head sits above the surface and so you can measure from the tip to the flat underside of the head.

Check out our full range of screws and fixings and see how head size, gauge and length is indicated as necessary. This should help you make the right selection when you order online through MB Direct.