Metric Thread Tap Drill Size


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 = drill size diameter (Core hole diameter or hole size)


Example: M8 drill size

8mm - 1.25 mm = 6.8mm (M8 drill size)


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


Metric Thread Tap Drill Size Chart

Metric thread


hole size or drill size



regular thread (mm)

M1 drill size

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