Wood screw diameter chart

A gauge is a diameter, not a length. This is what each gauge number actually measures across the outside of the threads.

The major thread diameter is measured on the outside of the threads, not the shank.
Gauge Major thread diameter Nearest fraction In this catalogue
#0 0.060 in 1/16
#1 0.073 in 5/64
#2 0.086 in 3/32
#3 0.099 in 7/64
#4 0.112 in 7/64
#5 0.125 in 1/8 We stock it
#6 0.138 in 9/64 We stock it
#7 0.151 in 5/32 We stock it
#8 0.164 in 5/32 We stock it
#9 0.177 in 11/64 We stock it
#10 0.190 in 3/16 We stock it
#11 0.203 in 13/64
#12 0.216 in 7/32 We stock it
#14 0.242 in 1/4 We stock it
#16 0.268 in 17/64
#18 0.294 in 19/64
#20 0.320 in 5/16

Reading the number

The gauge number is a diameter. It says nothing about how long the screw is - a #8 is a #8 whether it is half an inch or three inches long, and the two numbers on a label are independent of each other.

The numbers are not millimetres and they are not a fraction. They are an old sizing series, which is why the fraction column exists: it is the nearest drill or gauge you are likely to have in your hand.

Not every designation here is a gauge

A few products in this catalogue carry a machine-thread or metric designation instead - something like 8-32, 5/16-18 or M6.3. Those are not wood-screw gauges and they do not appear on this chart. A machine designation gives a diameter and a thread pitch; a metric one gives millimetres across the threads.

Picking a pilot

A pilot hole should clear the shank and let the threads bite, so it is sized against the screw’s root diameter rather than the major diameter on this chart - and the right size changes with the material. Softwood wants a smaller pilot than hardwood, and many of the screws here have a point designed not to need one at all.

If you are unsure, call and ask. It is a thirty-second question.

Length is measured differently again — see how lengths are measured — and every other value on a product page is defined in the fastener glossary.