Metric wood screw dimensions explained

Metric wood screw sizes and why they beat imperial…

The old imperial method of measuring screws by gauge makes little sense to some of us today, (like most imperial measurements) and the logic of their origins is known only to Wikipedia (he says smiling!). So, now we are getting to grips with the common sense of metric wood screw measurements. (Arguments against on a postcard to the usual address please!). 

Right, so what do the numbers mean? Lets take a look at the packaging to start with…

What it said on the front of the packaging…

The basic info is the type of screwdriver (Torx in this case, a T20 drive bit), then it’s the length of the screw and the outer diameter (or gauge in ‘old money’). For this decking screw, the overall length is 5.5cm or 55mm and the threads are 4.8mm wide (which is your clearance hole size).

There is no measurement for the pilot hole size given on the packet. To be honest these particular decking screws have a special front thread, which in effect, drills the pilot hole as you drive them straight in. But for ordinary wood screws, you’ll need a pilot hole for best results (I wish I’d picked up a different box now, duh). 

The measurements from the back of the packaging

On the back of the metric wood screw box… The actual letters might be different in your country but they don’t matter as they are just identifiers really. Just look at the letter and match it up with the numbers in the legend…

Metric wood screw thread diameter and clearance holes

Measuring the outside diameter of the threads of a wood screw… (this is the clearance hole size).

Assuming yours is a regular metric wood screw and you’re making something on the bench, then in most cases you’ll need a clearance hole in the first piece of timber to allow the two pieces to pull up tightly.  As you can see from the above image, the outside thread diameter of this screw is 4.88mm wide. Now you could drill a clearance hole exactly this diameter (if you can find a drill bit that size!), but it is fine to go slightly under or over this and you’re unlikely to get into trouble. So 4mm to 5mm would be ideal.

Metric wood screw shank diameter and pilot holes

Measuring the shank diameter… (pilot holes, for this screw I’d go for 2.5mm to 3mm)

Again, assuming yours is a regular metric wood screw and you’re joining two pieces of wood together then in most cases you’ll need a pilot hole. This stops the threaded part of the screw breaking the second part of your workpiece once you’ve tightened it up.  As you can see from the above image, the shank diameter of this screw is 3.51mm wide. Now you could drill a pilot hole exactly this diameter. But to be honest, you’ll get a tighter and better fit if you go slightly under this. A 2.5mm to 3mm hole would be ideal, depending on how hard your workpiece is. Generally the harder the workpiece, the closer to 3.50mm the pilot hole needs to be. 

Learn more about screws here… wood screws explained and here… fixing stripped wood screw threads… 

Okay, I hope that helps you understand a little more when you look at a metric wood screw! 

Let me know if you get stuck,

Stay well

Ian

p.s. Oh, and why it beats imperial? Well, because you can physically measure stuff. E.g. a 4.8mm screw measures 4.8mm whereas the imperial gauge system requires a knowledge of the gauge sizes from experience or the need to consult a table.