Wood screw sizes, screw gauge, and how to compare metric and imperial screws
As a constant DIY enthusiast, I am often putting furniture together with screws, repairing tools with screws or mounting shelves with screws.
Yes, I use a lot of screws and still find I need to check which sizes I need, which gauge is correct, is it metric or imperial? and how to compare them to what you can buy in your Hardware store.
Working out the right size is especially tricky when comparing Metric wood screws with Imperial wood screws unless you are constantly using them it is not easy, so I put together a simple guide to help compare imperial and metric wood screws, and here’s what I came up with;
Imperial wood screws use a gauge number for the diameter, the length is measured in inches, Metric wood screws use millimeters for both diameter and length, Example a 6×1” wood screw = 3.5mm diameter x 25mm long. Wood screws conform to standard – ANSI B18.6.1-1981(R1997) or EN 14592:2008
This quick comparison helps to remind me how to compare the different types of wood screws, however, as with most things, comparing imperial wood screws with metric wood screws is more complicated.
I added more detail on how to compare wood screws including how they are measured, how to read the label on packs of screws, and created a helpful and easy to use wood screw comparison chart in this post I hope it helps
What is the gauge of a screw?
The gauge of a wood screw refers to the diameter of the screw at the shank but does not really equal any measured unit, it is simply the position of a screw size in a numerical list, which is defined by the ANSI B18.6.1-1981 standard.
The ANSI B18.6.1-1981 gauge list ranges from 0 up to 24 for Flat, Pan, and Oval head wood screws, 0 being the smallest diameter and 24 the largest diameter.
There are a number of rules and calculations which offer to relate the gauge number of a wood screw to the diameter of the screw head such as:
The 16ths rule suggests that the wood screw head diameter is the screw gauge number divided 2 then add 1, the result is then converted into 16ths of an inch for example:
An 8 x 1 1/4” wood screw is 1 and a quarter inches long and the screw head diameter is 5/16”
8/2 = 4 +1 = 5 = 5/16
However, these are not completely accurate and are only a rule of thumb as the calculation does not work for gauge numbers smaller than 8 and odd numbers need to be converted into 32nds of an inch in the same way.
Generally, these calculations are not an issue as wood screws are often sized by eye and the issue becomes more relevant when you need to replace an old imperial wood screw with a metric version.
How do you measure a wood screw in mm?
I find that metric wood screws are simpler to fathom mainly because I use them more often and the metric system is less complicated than using imperial units of fractions of an inch.
A metric wood screw has two numbers the first is the diameter in millimeters, the second figure is the length. The diameter is measured over the outer diameter of the thread, often called the ‘major diameter’, for example, a 5.0 diameter screw will have a thread diameter of 5mm.
The length is also in millimeters so a 5.0 x 60 metric wood screw will be 5mm in diameter and 60 mm long.
The length of a screw is measured for the tip to the head, for countersunk screws this the overall length and includes the head depth, however for Pan head or dome head screws the length does not include the depth of the head, only the parts of the screw that will be below the surface when used.
What gauge is a 5mm screw and how to compare imperial to metric and metric to imperial screws?
Some rules of thumb compare the screw head size of a metric screw to the gauge of an imperial wood screw.
There are a couple of issues with this:
- Firstly it is not a reliable method as the smaller screws don’t fit the patterns suggested
- Secondly, manufacturers do not use head diameter on their packaging so you cannot compare them when buying screws.
For example, a 5mm diameter screw is equivalent to a 10 gauge imperial wood screw. However, a 4 gauge screw has a metric equivalent of 3.0mm diameter.
As the two different systems, imperial and metric do not directly relate to each other it is often easiest to use a comparison chart to compare the two versions and work out which imperial screw matches the closest metric version.
Metric to Imperial & Imperial to Metric wood screw comparison chart.
I have seen several charts but not all of them list the standard range of screws and compare the two systems, so I put this one together which includes metric and imperial units, and groups the metric and imperial equivalents together for both diameter and length. The information is based on the ANSI and BS EN standards for wood screws:
How read the label on a packet or box of wood screws
The labels on packs of wood screws have several sets of numbers used to identify different criteria, these include size and length of the screw, the quantity provided in the box, and the size of screwdriver bit needed.
The label will also tell you what type of screw threads have been manufactured and the purpose of the screw, the information will also include the material the screw is made from or if there has been a protective coating applied, normally a zinc plating is used which is used to help resist corrosion.
What do the letters mean on a box of wood screws?
There are also several abbreviations that are commonly used to define the type of thread and the material the wood screw has been coated in the most common ones are:
- TT – Twin thread
- TFT – Twin Fine Thread
- ST – Self Tapping; this type of screw allows them to be screwed into the material without a pilot hole being drilled.
- ZP – Zinc Plated; A thin layer of zinc plating has been added to help prevent corrosion
- ZYP – Zinc and Yellow Passivated; a thin layer of Zinc plating is added and an additional then a passivation treatment is used to provide further resistance to corrosion and gives the screws a yellow color
