Screw Gauge Comparison

 

Screw Size Identification

The description of the size of screws is commonly listed as three figures – firstly the gauge, second the threads per inch (TPI) and the third figure is the length either by inches or millimeters (mm).

For example:

  • 14-10 x 25mm indicates  a 14 gauge screw with 10 TPI and 25mm in length
  • It could also be written as 14g-10 x 25mm; or
  • Often in the case of timber screws it is not common to show the second figure e.g; 14 x 25mm or 14 x 1"

Gauge

The gauge of the screw is determined by the basic size of the outer thread diameter (major diameter). As it is quite common to purchase screws using gauge as a measure in Australia we have referenced the metric equivalent here as a guide.

Metric screws have also made there way into the market however these are normally denoted with a M prefix and represent the same outside diameter i.e; M8 = 8mm, M10 = 10mm, etc,.

Gauge    
  Metric Ref. Inch Ref. Approx
0g 1.5mm 0.060
1g 1.9mm 0.073
2g 2.25mm 3/32"
3g 2.5mm 0.099
4g 2.85mm 7/64"
5g 3.3mm 1/8"
6g 3.5mm 9/64"
7g 3.9mm 5/32"
8g 4.2mm 11/64"
9g 4.5mm 0.177
10g 4.8mm 3/16"
12g 5.5mm 7/32"
13g 6.1mm 15/64"
14g 6.3mm 1/4"
15g 6.5mm 1/4"
16g 6.8mm 0.268
18g 7.5mm 0.294
20g 8.1mm 0.320
24g 9.4mm 0.372

Screw Length

The length of a screw is determined from the tip of point to the underside of the head, this means for any screw head type that protrudes the surface, the head height is not counted. So for all screw heads that are to be flush with the surface such as a countersunk head the length stated is the full length from the tip of the screw to the top of the head.

Self Drillers (Metal Teks)

Self drillers (SD) or self drilling screws for metal a.k.a metal teks are made with a drill point similar to a tip of your common jobber drill bit. Without the need to pre-drill using these screws the gauge, TPI and screw tip will determine what thickness of metal can be fixed.

A coarse thread screw is considered as up to 16 TPI and should be used with thin steel up to 2.4mm, alternatively a fine thread screw is anything greater than 16 TPI and can be found on screws for steel up-to 12mm thick.

Gauge TPI Max. Drill Capacity
6g 20 2.3mm
8g 18 2.5mm
10g 16 3.5mm
24 4.5mm
12g 14 4.5mm
24 6.0mm
12g (SD500) 24 12.5mm
14g 10 3.0mm
20 6.5mm

Common shape for a drill point is a shorter No.3 drill point and for larger screws to drill 6mm and above a longer No.5 drill point is used such as SD500® self drillers. Other types of drill points include winged drill points which as described features wings between the point and threads. These wings aid in boring through timber before hitting metal allowing the head of the screw to pull in flush i.e; in decking or cladding applications.

Type 17 Timber Screws (T17)

A self drilling milled point screw tip for timber – the Type 17 tip is found mostly on screws designed for heavy timber construction. The sharp point is also capable of cutting through thin sheet metal if required.

Gauge TPI Min. Embedment
6g 18 20mm
8g 15 20mm
10g 12 25mm
12g 11 30mm
14g 10 35mm

Self-Tapping Screws (Type AB)

A self tapper features a die produced point and a rolled thread which leave a thread start on the taper. Not to be confused with a self-driller you will need to pre-drill prior to fixing in most applications. The Type AB gets its name from the tapping action it creates in the undersized pre-drilled hole.

Gauge Metric Ref TPI Pilot Holes Pre-Drill or Punched Holes* Drill Capacity
      Softwood Metal [min-max] Metal Thickness [min-max]
2g 2.2mm 32 1.5mm 1.6 – 1.95mm 0.45–1.52mm
4g 2.9mm 24 2.0mm 2.05 – 2.60mm 0.45–2.03mm
6g 3.5mm 20 7/64" (2.8mm) 2.35 – 3.20mm 0.45–2.64mm
8g 4.2mm 18 1/8" (3.2mm) 2.90 – 3.80mm 0.71–3.18mm
10g 4.8mm 16 9/64" (3.6mm) 3.40 – 4.50mm 0.71–4.75mm
12g 5.5mm 14 4.2mm 4.10 – 5.10mm 0.71– 4.75mm
14g 6.3mm 14 4.9mm 4.80 – 6.00mm 1.22 – 6.35mm

*Due to the variety of the hardness of metals and different operating conditions, it may be required to vary these dimensions which should be used only as a guide