Metric Thread Tap Drill


A threaded core hole is a hole into which an internal thread is cut. The core hole must always be slightly smaller than the nominal diameter of the thread to be cut.


As a rule of thumb for the core hole the following applies:


Thread Nominal diameter - Thread pitch = Core hole diameter


Example: Core hole M8

8mm - 1.25 mm = 6.8mm (core hole for M8)


A distinction is made between standard threads and fine threads for the core hole diameter. The table here shows the core hole diameter for metric standard threads. The thread core hole table for fine threads can be found here.


Metric Thread Tap Drill Chart

Metric thread


core hole



regular thread (mm)

M1 0,75 0,25
M1,1 0,85 0,25
M1,2 0,95 0,25
M1,4 1,1 0,30
M1,6 1,3 0,35
M1,8 1,5 0,35
M2 1,6 0,40
M2,2 1,8 0,45
M2,5 2,1 0,45
M3 2,5 0,50
M3,5 2,9 0,60
M4 3,3 0,70
M5 4,2 0,80
M6 5,0 1,00
M7 6,0 1,00
M8 6,8 1,25
M9 7,8 1,25
M10 8,5 1,50
M12 10,2 1,75
M14 12 2,00
M16 14 2,00
M18 15,5 2,50
M20 17,5 2,50
M22 19,5 2,50
M24 21 3,00
M27 24 3,00
M30 26,5 3,50
M36 32 4,00
M42 37,5 4,50
M48 43 5,00
M56 50,5 5,50
M64 58 6,00

* All information without guarantee.

Core Hole Depth

Core Hole M3, Core Hole M4, Core Hole M5, Core Hole M6, Core Hole M7, Core Hole M8, Core Hole M9, Core Hole M10, Core Hole M12
Core Hole and Core Hole Depth


The thread length should be at least twice and at most three times the nominal thread diameter in order to obtain a tight screw connection. The core hole depth must therefore be slightly more than the desired thread length so that the tap can cut the thread completely. A thread length of more than three times the nominal thread diameter should be avoided, as it does not lead to a better screw connection and has real disadvantages. One should know that 3 to 4 threads bear the full load when screwing! All other threads are so to speak without function.


For example: "core hole M3". An M3 thread has a nominal diameter of 3mm. This results in a thread length of 6mm to 9mm for (2 x d). As core hole depth you can choose 9mm here.