1-3/8 in Drill Size

fractional drill

1-3/8″ 1.375 in 34.93 mm

A 1-3/8 in drill measures 1.375 in. It taps 2 threads in this chart.

What a 1-3/8 in drill taps

Every thread this drill cuts between 50% and 85% of thread, deepest first. Outside that band it is the wrong drill for the job, so nothing below that line is listed.

Threads a 1-3/8 in drill taps, with the percent of thread each one gets and the published chart drill for comparison.
Thread	% thread	Chart drill	Chart %
M39 x 4	78.4%	35 mm	77.0%
1-1/2-6	57.7%	1-11/32″	72.1%

Other jobs for a 1-3/8 in

Hard-material tap drill: The size to use in stainless or tool steel for 1-1/2-6, where it leaves a shallower thread than the standard drill and cuts tapping torque.

Drills either side

1-23/64″ 1.3594 in · 34.53 mm Next size down
35 mm 1.378 in · 35 mm Next size up

All drill sizes → · Open this drill in the calculator →

1-3/8 in drill: common questions

What size is a 1-3/8 in drill?

1.375 inches, which is 34.93 mm. It is a fractional drill.

What can I tap with a 1-3/8 in drill?

2 threads: M39 x 4 at 78.4%, 1-1/2-6 at 57.7%.

What drill is next to a 1-3/8 in?

Down: 1-23/64 in at 1.3594 in. Up: 35 mm at 1.378 in.