What is a Tapped Hole?

A tapped hole is simplify a threaded hole. However not all threaded holes are tapped holes i.e. all thumbs are fingers but not all fingers are thumbs!

Check out our article on the differences between thread tapping and thread milling

Tapped holes are produced through a multi-step process:

1. First, a pilot hole is drilled using the appropriate tapping drill size
2. Next, a tap is inserted into the pilot hole, either by hand or machine. A tapping tool consists of a rod with sharp edges or teeth that cut into the sides of hole
3. The tap is rotated clockwise while applying downward pressure
4. As the tap turns, its cutting edges remove material and form the threads forming an internal spiral groove (thread)
5. Periodically, the tap is reversed to break and clear chips
6. This process continues until the desired thread depth is achieved

What is a Tapping Drill Size?

A tapping drill size is the specific diameter of hole that needs to be drilled before tapping threads into the material. This pre-drilled hole is slightly smaller than the major diameter of the thread, allowing the tap to cut the threads effectively.

The basic formula for calculating tapping drill size is:

Tap Drill Size = Major Diameter - Thread Pitch

The correct tapping drill size is crucial because:

• It determines the thread engagement and strength of the connection
• Too small a hole can lead to tap breakage
• Too large a hole will result in weak or stripped threads

Tapping Drill Sizes – Metric Threads

Fastener Size	Major Diameter	Minor Diameter	Thread Pitch	Tapping Drill Diameter
M3x0.50	3.000	2.459	0.50	2.500
M4x0.70	4.000	3.242	0.70	3.300
M5x0.80	5.000	4.134	0.80	4.200
M6x1.00	6.000	4.917	1.00	5.000
M8x1.25	8.000	6.647	1.25	6.800
M10x1.50	10.000	8.376	1.50	8.500
M12x1.75	12.000	10.106	1.75	10.200

Tapping Drill Sizes – Imperial Threads

Fastener Size	Major Diameter	Minot Diameter	Thread Pitch	Maj Dia – Pitch	Req. Drill Size	Tapping Drill Diameter
#6-32	3.505	2.532	0.794	2.711	#36	2.705
#8-32	4.166	3.193	0.794	3.372	#29	3.454
#10-32	4.826	3.853	0.794	4.032	#21	4.039
1/4-28	6.350	5.237	0.907	5.443	#3	5.410
5/16-24	7.938	6.640	1.058	6.879	I	6.909
3/8-24	9.525	8.227	1.058	8.467	Q	8.433
7/16-20	12.065	9.555	1.270	10.795	25/64	9.921
1/2-20	12.700	11.143	1.270	11.430	29/64	11.509