Metric & Imperial Wood Screws Explained

Is there a difference between a Metric or Imperial Wood Screws?

Yes, fundamentally a metric wood screw, will have a slightly different shank, thread and head diameter (size) to that of its imperial equivalent wood screw. Sometimes this is only very subtle and un-noticeable, especially with the smaller gauge wood screws however with larger sizes there can be a considerable difference, especially surrounding the head and shank of the screw. 

Which screws are metric and which are imperial?

As a general rule of thumb, typically all new & old slotted wood screws are offered on our website in brass, steel, chrome, nickel, sherardized, black japanned finishes and are still manufactured to an imperial gauge, unless otherwise stated.  

Our Stainless Steel Slotted, Countersunk, Round and Raised Head Slotted Wood Screws are all made to a DIN Metric standard, unless otherwise stated. 

How do I know which gauge/size of screw to buy?

It all depends on what the application and what you are using the screws for.

The following metric to imperial conversions are widely excepted as the industry norm here in the UK, however there are a few exceptions to the rule, which we will happily advise on if required.

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)

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When buying Stainless Slotted wood screws what should I be aware of?

Product Fact: Typically, all stainless steel slotted wood screws widely available in the UK market are manufactured to a metric gauge (size) and are then advertised & sold with an imperial equivalent (old British size).

What this means is sometimes, more notably with the larger gauges (12’s & 14’s), the head, shank and thread diameter can differ from that of an old imperial (British) wood screw, as the two gauging methods, (metric & imperial gauge), don’t always allow for an exact match, hence the use of the term ‘ Imperial Equivalent’.

It is widely accepted that this is less of a consideration when buying stainless slotted wood screws for use in new joinery work. however, if you are replacing old screws or going into existing ironmongery then you might wish to contact us for additional information & guidance. 

backGround Surrounding imperial and metric wood screws....

Historically, for over 200 years, slotted wood screws were manufactured here in United Kingdom and exported all around the world.

Prior to the 1960’s, the majority of traditional slotted wood screws were manufactured to an imperial gauge range (inches), which was developed in part, by world renowned screw manufacturer ‘Guest, Keen & Nettlefolds’ of Birmingham, England.

The imperial gauge range can be traced back to the industrial revolution and was the adopted standard for slotted wood screws sold and used throughout the British empire for many years.

This was formalised in the early 1960’s with the introduction of an official 'British Manufacturing Standard' for wood screws developed by the British Standards Institute in conjunction with a wide range of contributors from industry and manufacturing alike.

Unfortunately, the screw manufacturing industry in the UK witnessed a steady decline in the late 1980’s and early 1990’s as British manufacturers found it increasingly difficult to compete with European, Chinese and Indian competitors and also new advances in manufacturing techniques.

Around the late 90’s, we started seeing screws coming into the UK, influenced by the European market, which were produced to a metric standard (millimetres).

Similar to that of the British Standard for wood screws, the Europeans often produced wood screws to a metric standard called ‘DIN’ Standards (DIN = is the German Institute for Standards), which specifies acceptable  tolerance  such as the head,  shank and thread diameter to name but a few, however these tolerances differed from those set out in the British wood screw standard of the 1960's, all be it that there is an overlap and similarities between the two standards. 

Today in the UK market place, there is a small range of screws produced to the old British Imperial Standard, however most wood screws (excluding brass) are imported into the UK and sold as metric screws (and not always to a DIN Standard). They are often given an imperial equivalent and thus you will often see screws advertised in both metric and imperial, which can be confusing at times. i.e. (5.0 x 25 mm) 10 x 1”.

I’m still unsure, can you help me?

Yes, of course we can help. Please get in touch either via email, contact form or phone if you are still unsure and we will answer any questions you might have as we have a wide range of practical and technical information to hand.


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