Tap Drill Chart – The Ultimate inch and metric table (mm and inches)

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Taps are cutting tools used by machinists to cut or form screw threads. The first recorded use of taps in metalworking is placed sometime in the 18th century, while the British engineer and industrialist Joseph Clement popularised the tool in the mid-19th century. We then saw standards emerge in threading with the tap drill chart. As a consequence the development of taps corresponded to this standardisation ensuring enhanced compatibility.

The Threading Process

As you would expect, taps come in all forms and sizes in order to accommodate standardised screw threads. Therefore the tap drill chart is extremely important. To create a thread properly, one would need a set of three taps that correspond to the same bit size; namely the bottoming, the intermediate, and the taper tap. The intermediate, or plug tap, is used first to start cutting out material from a non-threaded hole. That is why it features a tapered bit, allowing the machinist to align it correctly.

Unfortunately, due to the shape it can’t create a thread all the way to the end of the hole. So next comes the bottoming tap that has the capacity to do this. However, the bottoming tap cannot start the threading on an uncut hole, so it can only take part in the cutting process as the second or third tap. The taper tap is only used in certain situations such as when the material is very hard or the hole is too small. The taper tap features a smooth diameter transition that allows for a less aggressive cutting action. This minimises the risk of damaging the drilled hole. If the taper needs to go first, the intermediate tap follows second and the thread is finished with the bottoming tap.

Thread Geometry And Drilling The Hole

As is evident from the above, to create the thread we first need to drill a hole on the workpiece. Having determined the bolt that we want to use in the specific case, we already have a technical specification for the thread which will host it. Bolts are classified by specification codes such as:-

• M4-0.7 x 20

The “M” indicates the “metric diameter” of its thread (see tap drill chart below), which in our case is 4 mm. The second number (0.7) is the pitch which corresponds to the distance between two adjacent threads in millimeters. Finally, 20 is the length of the bolt in mm, measured from the tip of the thread to the bottom of the head (with the exception of oval headed bolts).

So, if we wanted to drill a hole for our example bolt, we would need to use a drill that is smaller than 4 mm. The material will have to be cut and pushed out in order to create a thread in that hole. If that is the case, can we simply use the next smallest available drill size. That would be a 3.7 mm drill bit, but using that drill would leave an inadequate margin for the creation of the appropriate thread depth. A rule of thumb is that the right tap drill is around 85% for coarse threads and 90% for fine threads which feature a higher number of threads per axial distance. In our case, the correct tap drill would be the closest to 3.4 mm from below, and that is the 3.3 mm or 0.13-inch tap drill. Lower percentages such as “75% of the thread depth” are also widely used and considered safe.

To help you get the grasp of the fine-coarse difference, the M4 classification comprises of the:-

• Coarse 0.7 mm thread
• Fine 0.5 mm thread
• Extra-fine 0.35 mm thread pitch

The smaller the pitch value, the less deep the threading needs to go. So for:-

• M4-07 we use the 3.3 mm tap drill
• M4-05 we use the 3.5 mm drill
• M4-035 we use the 3.6 mm drill bit

As per the ISO metric screw thread standard, the thread depth should be 0.614 x pitch, which is why a smaller pitch number also results in a lower thread depth value. Now engineers can sit down and manually calculate the right tap drill. Or they can consult a tap drill chart like those found in the next section.

Some prefer to use somewhat arbitrary but reliable formulas like: “Dtap = Nominal Diameter – Thread Pitch”. This formula works for both metric and imperial units, and for all 60-degree threads (all ISO, DIN, UTS, UNC, UNS, and UNF bolts). The first tap drill chart below is in mm and the next in inches. Both are available as a printable tap drill chart PDF as well.

Tap Drill Chart (mm and inches)

Drill sizes given are the ‘closest’ drill size. Download Tap Drill Chart PDF here.

TAP SIZE		CUTTING TAPS		TRU-FLO ™ FORMING TAPS
INCH	METRIC	DRILL SIZE	DECIMAL EQUIVALENT	DRILL SIZE	DECIMAL EQUIVALENT
0-80		3/64	.0469	54	.0550
	M1.6 X 0.35	1.25mm	.0492	1.45mm	.0571
	M1.8 X 0.35	1.45mm	.0571	1.65mm	.0650
1-64		53	.0595	51	.0650
1-72		53	.0595	51	.0650
	M2 X 0.40	1.60mm	.0630	1.80mm	.0709
2-56		50	.0700	5/64	.0781
2-64		50	.0700	47	.0785
	M2.2 X 0.45	1.75mm	.0689	2.00mm	.0787
	M2.5 X 0.45	2.05mm	.0807	2.30mm	.0906
3-48		47	.0785	43	.0890
3-56		46	.0810	2.30mm	.0905
4-40		43	.0890	38	.1015
4-48		42	.0935	2.60mm	.1024
	M3 X 0.50	2.50mm	.0984	7/64	.1094
5-40		38	.1015	33	.1130
5-44		37	.1040	2.90mm	.1142
	M3.5 X 0.60	2.90mm	.1142	3.20mm	.1260
6-32		36	.1065	1/8	.1250
6-40		33	.1130	3.25mm	.1280
	M4 X 0.70	3.30mm	.1299	3.70mm	.1457
8-32		29	.1360	25	.1495
8-36		29	.1360	24	.1520
	M4.5 X 0.75	3.70mm	.1476	4.10mm	.1614
10-24		26	.1470	11/64	.1719
10-32		21	.1590	16	.1770
	M5 X 0.80	4.20mm	.1654	14	.1820
12-24		16	.1770	5mm	.1969
12-28		15	.1800	7	.2010
	M6 X 1.00	5.00mm	.1969	7/32	.2188
1/4-20		7	.2010	1	.2280
1/4-28		3	.2130	15/64	.2340
	M7 X 1.00	6.00mm	.2362	F	.2570
5/16-18		F	.2570	L	.2900
5/16-24		I	.2720	M	.2950
	M8 X 1.25	6.70mm	.2638	7.40mm	.2913
	M8 X 1.0	7.00mm	.2756	19/64	.2969
3/8-16		5/16	.3125	S	.3480
3/8-24		Q	.3320	T	.3580
	M10 X 1.50	8.50mm	.3346	U	.3680
	M10 X 1.25	8.70mm	.3425	9.40mm	.3701
7/16-14		U	.3680	Y	.4040
7/16-20		25/64	.3906	Z	.4130
	M12 X 1.75	10.20mm	.4016	11.20mm	.4409
	M12 X 1.25	10.80mm	.4252	11.50mm	.4528
1/2-13		27/64	.4219	15/32	.4682
1/2-20		29/64	.4531	12.25mm	.4823
	M14 X 2.00	12.00mm	.4224	33/64	.5156
9/16-12		31/64	.4844	17/32	.5312
9/16-18		33/64	.5156	13.50mm	.5315
5/8-11		17/32	.5312	14.75mm	.5807
5/8-18		37/64	.5781	15.25mm	.6004
	M16 X 2.00	14.00mm	.5512	19/32	.5938
	M16 X 1.50	14.50mm	.5906	15.25mm	.6004
	M18 X 2.50	15.50mm	.6102	39/64	.6094
	M18 X 1.50	16.50mm	.6496	17.25mm	.6791
3/4-10		21/32	0.6562	45/64	0.7031
3/4-16		11/16	0.6875	23/32	0.7188
	M20 X 2.50	17.50mm	0.689
	M20 X 1.50	18.50mm	0.7283
	M22 X 2.50	19.50mm	0.7677
	M22 X 1.50	20.50mm	0.8071
7/8-9		49/64	0.7656
7/8-14		13/16	0.8125
	M24 X 3.00	21.00mm	0.8268
	M24 X 2.00	22.00mm	0.8661
1-8		7/8	0.875
1-12		59/64	0.9219
	M27 X 3.00	24.00mm	0.9449
	M27 X 2.00	25.00mm	0.9843
1-1/8-7		63/64	0.9844
1-1/8-12		1-3/64	1.0469
	M30 X 3.50	26.50mm	1.0433
	M30 X 2.00	28.00mm	1.1024
1-1/4-7		1-7/64	1.1094
1-1/4-12		1-11/64	1.1719
	M33 X 3.50	29.50mm	1.1614
	M33 X 2.00	31.00mm	1.2205
1-3/8-6		1-7/32	1.2188
1-3/8-12		1-19/64	1.2969
	M36 X 4.00	32.00mm	1.2598
	M36 X 3.00	33.00mm	1.2992
1-1/2-6		1-11/32	1.3438
1-1/2-12		1-27/64	1.4219
	M39 X 4.00	35.00mm	1.378
	M39 X 3.00	36.00mm	1.4173

Pipe Thread Drilled Hole Sizes

Drill sizes given are the ‘closest’ drill size. Download Pipe Thread Drilled Hole Sizes PDF here.

NOMINAL  SIZE	NPT	NPTF	NPSC	NPSM	NPSF	NPSI	BSPT	BSPP
1/16 – 27	D (0.246)	D (0.246)	0.25		0.25	0.25
1/8 – 27	Q (0.332)	Q (0.332)	0.3437	0.3593	0.3437	0.3437
1/8 – 28							0.3281	0.3437
1/4 – 18	0.4375	0.4375	0.4375	0.4687	0.4375	0.4375
1/4 – 19							0.4375	0.4531
3/8 – 18	0.5625	0.5781	0.5781	0.6093	0.5781	0.5781
3/8 – 19							0.5781	0.5937
1/2 – 14	0.7031	0.7031	0.7187	0.75	0.7187	0.7187	0.7187	0.7343
5/8 – 14								0.8125
3/4 – 14	0.9062	0.9218	0.9218	0.9646			0.938	0.965
7/8 – 14								1.1093
1 – 11							1.1718	1.1875
1 – 11-1/2	1.1406	1.1562	1.1562	1.2031
1-1/4 – 11							1.5	1.5468
1-1/4 – 11-1/2	1.4843	1.5	1.5	1.5468
1-1/2 – 11							1.75	1.7656
1-1/2 – 11-1/2	1.7343	1.7343	1.75	1.7913
1-3/4 – 11								2
2 – 11							2.2187	2.25
2 – 11-1/2	2.2031	2.2187	2.2187	2.2638

Also read:

Specifying tap drill diameters – good practice or micromanagement?