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Screw gauge to mm sizing chart

Screw Gauge Chart

Screws are sized by gauge — a number that maps to a specific body diameter. Higher gauge = thicker screw. The chart below covers every standard gauge we see in trade, with the matching diameter in millimetres and inches.

Self tapping screw side profile showing the gauge is measured across the thread

Screw gauge is measured across the outside of the thread - not the shank.

Gauge	Diameter (mm)	Diameter (inch)
0g	1.5 mm	0.060"
1g	1.85 mm	0.073"
2g	2.2 mm	0.086"
3g	2.5 mm	0.099"
4g	2.85 mm	0.112"
5g	3.2 mm	0.125"
6g	3.5 mm	0.138"
7g	3.85 mm	0.151"
8g	4.2 mm	0.164"
9g	4.5 mm	0.177"
10g	4.8 mm	0.190"
12g	5.5 mm	0.216"
14g	6.3 mm	0.242"
16g	6.8 mm	0.268"
18g	7.45 mm	0.294"

Quick tips

Reading a screw label: the first number is the gauge (e.g. "8g x 50mm" means gauge 8, 50mm long).
Pilot hole: drill roughly 70–80% of the screw body diameter for hardwood, 60–70% for softwood.
Metric to imperial conversion: multiply mm by 0.0394 (or divide by 25.4) to get inches.

Browse our screws — Self Tapping Screws · Screws for Metal · Screws for Timber · Screws for Decking

Not sure which gauge to pick? Tell us what you're fixing and into what material — we'll match the right gauge, length and head style. Contact us or call 07 3192 9800.

Diameters rounded to common trade figures. Information sourced from various suppliers and provided as a guide only.