How to Read Screw Thread Designation?

Have you ever wondered how to read the screw thread designation in engineering drawings? Sometimes it’s quite confusing to read the Hole thread designation on the drawings. Let us discuss How to read Screw Thread Designation from the hole or shaft screw thread callout on the engineering drawings.

Designation meaning: The act of indicating or identifying the purpose

According to the Indian standard IS: 4218 (Part IV) 1976 (Reaffirmed 1996), The screw thread designation callout must include two parts. One is the size designation and the other one is tolerance designation.

Size Designation

The size of the screw thread is designated by the letter “M” followed by the diameter and pitch, the two being separated by the sign ×. When there is no indication of the pitch, it shall mean that a coarse pitch is implied.

Tolerance designation

This Tolerance designation includes the following information on the screw thread callout leader note on the engineering drawing.

1. A figure designating tolerance grade as indicated below:‘7’ for fine grade, ‘8’ for normal (medium) grade, and ‘9’ for coarse grade.
2. A letter designating the tolerance position as indicated below :‘H’ for unit thread, ‘d’ for bolt thread with allowance, and ‘h’ for bolt thread withoutallowance.

For example, A bolt thread of 6 mm size of the coarse pitch and with allowance on the threads and normal (medium) tolerance grade is designated as M6-8d.

Screw Thread Designation for a tap hole with Example drawing

Let us have a closer look at an engineering drawing callout for a tap hole on the flange to understand the screw thread designation in a more detailed manner. Following is the screenshot of the tap hole callout.

In the above tap hole callout, we have given two callouts to understand it in both metric and inch systems.

There are 3 parts for each callout.

1. The first part represents the quantity of the hole. Which is 3 in the case of inch thread and 2 in the case of metric thread from the above tap hole callouts.
2. The second part is the tap hole screw thread designation. (Discussed seperately for each unit system below)
3. The third part is the Bolt Circle Diameter where the hole need to be placed.

Metric Screw Thread Designation

From the Above Metric Screw thread designation for the tap hole callout, the M8x1.7 represents the screw thread size and 8d represents the tolerance details of the thread.

M8x1.7 = 8mm of the diameter and 1.75mm of pitch distance of each thread.

8d = 8 means a normal tolerance grade and d represents that there should be an allowance on the threads.

M8x1.75-8d = Thread diameter of 8mm with 1.75mm pitch of each thread with an allowance on the threads of normal tolerance grade.

Inch Screw thread designation

From the above callout inch screw thread designation, 5/8-11 represents the Size of the screw thread and UNC/UNF represents the tolerance details.

5/8-11 = 0.625 inches diameter with 11 threads per inch

UNC/UNF = Unified National Coarse Thread / Unified National Fine Thread

The Unified National Coarse threads are comparable with the ISO metric threads.

5/8-11 UNC = a screw thread of 0.625inch diameter with 11 threads per inch of coarse threads.

UNC – Unified National Coarse Thread according ANSI B1.1

Major Diameter	Threads per inch	Major Diameter	Major Diameter	Tap Drill Size	Pitch
(in)	(tpi)	(inch)	(mm)	(mm)	(mm)
#1 – 64	64	0.073	1.854	1.5	0.397
#2 – 56	56	0.086	2.184	1.8	0.453
#3 – 48	48	0.099	2.515	2.1	0.529
#4 – 40	40	0.112	2.845	2.35	0.635
#5 – 40	40	0.125	3.175	2.65	0.635
#6 – 32	32	0.138	3.505	2.85	0.794
#8 – 32	32	0.164	4.166	3.5	0.794
#10 – 24	24	0.19	4.826	4	1.058
#12 – 24	24	0.216	5.486	4.65	1.058
1/4″ – 20	20	0.25	6.35	5.35	1.27
5/16″ – 18	18	0.313	7.938	6.8	1.411
3/8″ – 16	16	0.375	9.525	8.25	1.587
7/16″ – 14	14	0.438	11.112	9.65	1.814
1/2″ – 13	13	0.5	12.7	11.15	1.954
9/16″ – 12	12	0.563	14.288	12.6	2.117
5/8″ – 11	11	0.625	15.875	14.05	2.309
3/4″ – 10	10	0.75	19.05	17	2.54
7/8″ – 9	9	0.875	22.225	20	2.822
1″ – 8	8	1	25.4	22.85	3.175
1 1/8″ – 7	7	1.125	28.575	25.65	3.628
1 1/4″ – 7	7	1.25	31.75	28.85	3.628
1 3/8″ – 6	6	1.375	34.925	31.55	4.233
1 1/2″ – 6	6	1.5	38.1	34.7	4.233
1 3/4″ – 5	5	1.75	44.45	40.4	5.08
2″ – 4 1/2	4 1/2	2	50.8	46.3	5.644
2 1/4″ – 4 1/2	4 1/2	2.25	57.15	52.65	5.644
2 1/2″ – 4	4	2.5	63.5	58.5	6.35
2 3/4″ – 4	4	2.75	69.85	64.75	6.35
3″ – 4	4	3	76.2	71.1	6.35
3 1/4″ – 4	4	3.25	82.55	77.45	6.35
3 1/2″ – 4	4	3.5	88.9	83.8	6.35
3 3/4″ – 4	4	3.75	95.25	90.15	6.35
4″ – 4	4	4	101.6	96.5	6.35

UNF – Unified National Fine Thread according ANSI B1.1

Major Diameter	Threads per inch	Major Diameter	Major Diameter	Tap Drill Size	Pitch
(in)	(tpi)	(inch)	(mm)	(mm)	(mm)
#0 – 80	80	0.06	1.524	1.25	0.317
#1 – 72	72	0.073	1.854	1.55	0.353
#2 – 64	64	0.086	2.184	1.9	0.397
#3 – 56	56	0.099	2.515	2.15	0.453
#4 – 48	48	0.112	2.845	2.4	0.529
#5 – 44	44	0.125	3.175	2.7	0.577
#6 – 40	40	0.138	3.505	2.95	0.635
#8 – 36	36	0.164	4.166	3.5	0.705
#10 – 32	32	0.19	4.826	4.1	0.794
#12 – 28	28	0.216	5.486	4.7	0.907
1/4″ – 28	28	0.25	6.35	5.5	0.907
5/16″ – 24	24	0.313	7.938	6.9	1.058
3/8″ – 24	24	0.375	9.525	8.5	1.058
7/16″ – 20	20	0.438	11.112	9.9	1.27
1/2″ – 20	20	0.5	12.7	11.5	1.27
9/16″ – 18	18	0.563	14.288	12.9	1.411
5/8″ – 18	18	0.625	15.875	14.5	1.411
3/4″ – 16	16	0.75	19.05	17.5	1.587
7/8″ – 14	14	0.875	22.225	20.4	1.814
1″ – 12	12	1	25.4	23.25	2.117
1 1/8″ – 12	12	1.125	28.575	26.5	2.117
1 1/4″ – 12	12	1.25	31.75	29.5	2.117
1 3/8″ – 12	12	1.375	34.925	32.75	2.117
1 1/2″ – 12	12	1.5	38.1	36	2.117

Standard Dimensions of Screw Threads for Bolt and Nuts

According to IS : 4218 (Part III) 1976 (Reaffirmed 1996)

Metric Threads Coarse Threads Series

Designation	Pitch mm	Major or nominal diameter  for Nut and Bolt (d = D) mm	Effective or pitch diameter  for Nut and Bolt (dp) mm	Minor or core diameter (dc)  for Bolt in mm	Minor or core diameter (dc)  for Nut in mm	Depth of thread  for bolt -mm	Stress area mm2
M 0.4	0.1	0.400	0.335	0.277	0.292	0.061	0.074
M 0.6	0.15	0.600	0.503	0.416	0.438	0.092	0.166
M 0.8	0.2	0.800	0.670	0.555	0.584	0.123	0.295
M 1	0.25	1.000	0.38	0.693	0.729	0.153	0.460
M 12	0.25	1.200	1.038	0.893	0.929	0.158	0.732
M 1.4	0.3	1.400	1.205	1.032	1.075	0.184	0.983
M 1.6	0.35	1.600	1.373	1.171	1.221	0.215	1.27
M 1.8	0.35	1.800	1873	1.371	1.421	0.215	1.70
M 2	0.4	2.000	1.740	1.509	1.567	0.245	2.07
M 22	0.45	2.200	1.908	1.648	1.713	0.276	2.48
M 25	0.45	2.500	2.208	1.948	2.013	0.276	3.39
M 3	0.5	3.000	2.675	2.387	2.459	0.307	5.03
M 35	0.6	3.500	3.110	2.764	2.850	0.368	6.78
M 4	0.7	4.000	3.545	3.141	3.242	0.429	8.78
M 4.5	0.75	4.500	4.013	3580	3.688	0.460	11.3
M 5	0.8	5.000	4.480	4.019	4.134	0.491	14.2
M 6	1	6.000	5350	4.773	4.918	0.613	20.1
M7	1	7.000	6.350	5.773	5.918	0.613	28.9
M8	1.25	8.000	7.188	6.466	6.647	0.767	36.6
M 10	1.5	10.000	9.026	8.160	8.876	0.920	58.3
M 12	1.75	12.000	10.863	9.858	10.106	1.074	84.0
M 14	2	14.000	12.701	11.546	11.835	1.227	115
M 16	2	16.000	14.701	13.546	13.835	1.227	157
M 18	2.5	18.000	16.376	14.933	15.294	1.534	192
M 20	2.5	20.000	18.376	16.933	17.294	1.534	245
M 22	2.5	22.000	20.376	18.933	19.294	1.534	303
M 24	3	24.000	22.051	20.320	20.752	1.840	353
M 27	3	27.000	25.051	23.320	23.752	1.840	459
M 30	3.5	30.000	27.727	25.706	26.211	2.147	561
M 33	3.5	33.000	30.727	28.706	29.211	2.147	694
M 36	4	36.000	33.402	31.093	31.670	2.454	817
M 39	4	39.000	36.402	34.093	34.670	2.454	976
M 42	4.5	42.000	39.077	36.416	37.129	2.760	1104
M 45	4.5	45.000	42.077	39.416	40.129	2.760	1300
M 48	5	48.000	44.752	41.795	42.587	3.067	1465
M 52	5	52.000	48.752	45.795	46.587	3.067	1755
M 56	5.5	56.000	52.428	49.177	50.046	3.067	2022
M 60	5.5	60.000	56.428	53.177	54.046	3.374	2360

Metric Threads Fine Thread Series

Designation	Pitch mm	Major or nominal diameter  for Nut and Bolt (d = D) mm	Effective or pitch diameter  for Nut and Bolt (dp) mm	Minor or core diameter (dc)  for Bolt in mm	Minor or core diameter (dc)  for Nut in mm	Depth of thread  for bolt -mm	Stress area mm2
M 0.4	0.1	0.400	0.335	0.277	0.292	0.061	0.074
M 0.6	0.15	0.600	0.503	0.416	0.438	0.092	0.166
M 0.8	0.2	0.800	0.670	0.555	0.584	0.123	0.295
M1	0.25	1.000	0.838	0.693	0.729	0.153	0.460
M 1.2	0.25	1.200	1.038	0.893	0.929	0.158	0.732
M 1.4	0.3	1.400	1.205	1.032	1.075	0.184	0.983
M 1.6	0.35	1.600	1.373	1.171	1.221	0.215	1.27
M 1.8	0.35	1.800	1.573	1.371	1.421	0.215	1.70
M2	0.4	2.000	1.740	1.509	1.567	0.245	2.07
M 2.2	0.45	2.200	1.908	1.648	1.713	0.276	2.48
M 2.5	0.45	2.500	2.208	1.948	2.013	0.276	3.39
M3	0.5	3.000	2.675	2.387	2.459	0.307	5.03
M 3.5	0.6	3.500	3.110	2.764	2.850	0.368	6.78
M4	0.7	4.000	3.545	3.141	3.242	0.429	8.78
M 4.5	0.75	4.500	4.013	3.580	3.688	0.460	11.3
MS	0.8	5.000	4.480	4.019	4.134	0.491	14.2
M6	1	6.000	5.350	4.773	4.918	0.613	20.1

Metric Screw Threads – Coarse Threads according ISO 724

Size – Nominal Diameter	Pitch	Clearance Drill	Tap Drill	Tensile Stress Area
(mm)	(mm)	(mm)	(mm)	(mm2)
M 1.6	0.35	1.8	1.25
M 2	0.4	2.4	1.6
M 2.5	0.45	2.9	2
M 3	0.5	3.4	2.5
M 3.5	0.6	3.9	2.9
M 4	0.7	4.5	3.3	8.78
M 5	0.8	5.5	4.2	14.2
M 6	1	6.6	5	20.1
M 8	1.25	9	6.8	36.6
M 10	1.5	12	8.5	58
M 12	1.75	14	10.2	84.3
M 14	2	16	12
M 16	2	18	14	157
M 20	2.5	22	17.5	245
M 22	2.5	25	19.5
M 24	3	27	21	353
M 27	3	30	24
M 30	3.5	33	26.5	561
M 36	4	40	32	817
M 42	4.5	46	37.5	1120
M 48	5	53	43	1470
M 56	5.5	62	50.5	2030
M 64	6	70	58	2680
M 68	6	74	62

Metric Screw Threads – Fine Threads according ISO 724

Size – Nominal Diameter	Pitch	Tap Drill	Tensile Stress Area
(mm)	(mm)	(mm)	(mm2)
M 1.0 x 0.2	0.2	0.8
M 1.1 x 0.2	0.2	0.9
M 1.2 x 0.2	0.2	1
M 1.4 x 0.2	0.2	1.2
M 1.6 x 0.2	0.2	1.4
M 1.8 x 0.2	0.2	1.6
M 2 x 0.25	0.25	1.75
M 2.2 x 0.25	0.25	1.95
M 2.5 x 0.35	0.35	2.1
M 3 x 0.35	0.35	2.6
M 3.5 x 0.35	0.35	3.1
M 4 x 0.5	0.5	3.5
M 4.5 x 0.5	0.5	4
M 5 x 0.5	0.5	4.5
M 5.5 x 0.5	0.5	5
M 6 x 0.75	0.75	5.2	20.1
M 7 x 0.75	0.75	6.2
M 8 x 0.75	0.75	7.2
M 8 x 1.0	1	7	39.2
M 9 x 0.75	0.75	8.2
M 9 x 1	1	8
M 10 x 0.75	0.75	9.2
M 10 x 1	1	9	64.5
M 10 x 1.25	1.25	8.8	61.2
M 11 x 0.75	0.75	10.2
M 11 x 1	1	10
M 12 x 1	1	11
M 12 x 1.25	1.25	10.8	92.1
M 12 x 1.5	1.5	10.5	88.1
M 14 x 1.0	1	13
M 14 x 1.25	1.25	12.8
M 14 x 1.5	1.5	12.5
M 15 x 1	1	14
M 15 x 1.5	1.5	13.5
M 16 x 1	1	15	178
M 16 x 1.5	1.5	14.5	167
M 17 x 1.0	1	16
M 17 x 1.5	1.5	15.5
M 18 x 1.0	1	17
M 18 x 1.5	1.5	16.5
M 18 x 2.0	2	16
M 20 x 1.0	1	19
M 20 x 1.5	1.5	18.5	272
M 20 x 2.0	2	18	258
M 22 x 1.0	1	21
M 22 x 1.5	1.5	20.5
M 22 x 2.0	2	20
M 24 x 1.0	1	23
M 24 x 1.5	1.5	22.5	401
M 24 x 2.0	2	22	384
M 25 x 1.0	1	24
M 25 x 1.5	1.5	23.5
M 25 x 2.0	2	23
M 27 x 1.0	1	26
M 27 x 1.5	1.5	25.5
M 27 x 2.0	2	25
M 28 x 1.0	1	27
M 28 x 1.5	1.5	26.5
M 28 x 2.0	2	26
M 30 x 1.0	1	29
M 30 x 1.5	1.5	28.5
M 30 x 2.0	2	28	621
M 30 x 3.0	3	27	581
M 32 x 1.5	1.5	30.5
M 32 x 2.0	2	30
M 33 x 1.5	1.5	31.5
M 33 x 2.0	2	31
M 33 x 3.0	3	30
M 35 x 1.5	1.5	33.5
M 35 x 2.0	2	33
M 36 x 1.5	1.5	34.5
M 36 x 2.0	2	34	915
M 36 x 3.0	3	33	865
M 39 x 1.5	1.5	37.5
M 39 x 2.0	2	37
M 39 x 3.0	3	36
M 40 x 1.5	1.5	38.5
M 40 x 2.0	2	38
M 40 x 3.0	3	37
M 42 x 1.5	1.5	40.5
M 42 x 2.0	2	40
M 42 x 3.0	3	39	1210
M 42 x 4.0	4	38	1150
M 45 x 1.5	1.5	43.5
M 45 x 2.0	2	43
M 45 x 3.0	3	42
M 45 x 4.0	4	41
M 48 x 1.5	1.5	46.5
M 48 x 2.0	2	46
M 48 x 3.0	3	45	1600
M 48 x 4.0	4	44	1540
M 50 x 1.5	1.5	48.5
M 50 x 2.0	2	48
M 50 x 3.0	3	47
M 52 x 1.5	1.5	50.5
M 52 x 2.0	2	50
M 52 x 3.0	3	49
M 52 x 4.0	4	48
M 55 x 1.5	1.5	53.5
M 55 x 2.0	2	53
M 55 x 3.0	3	52
M 55 x 4.0	4	51
M 56 x 1.5	1.5	54.5
M 56 x 2.0	2	54
M 56 x 3.0	3	53
M 56 x 4.0	4	52
M 58 x 1.5	1.5	56.5
M 58 x 2.0	2	56
M 58 x 3.0	3	55
M 58 x 4.0	4	54
M 60 x 1.5	1.5	58.5
M 60 x 2.0	2	58
M 60 x 3.0	3	57
M 60 x 4.0	4	56
M 62 x 1.5	1.5	60.5
M 62 x 2.0	2	60
M 62 x 3.0	3	59
M 62 x 4.0	4	58
M 64 x 1.5	1.5	62.5
M 64 x 2.0	2	62
M 64 x 3.0	3	61
M 64 x 4.0	4	60
M 65 x 1.5	1.5	63.5
M 65 x 2.0	2	63
M 65 x 3.0	3	62
M 65 x 4.0	4	61
M 68 x 1.5	1.5	66.5
M 68 x 2.0	2	66
M 68 x 3.0	3	65
M 68 x 4.0	4	64
M 70 x 1.5	1.5	68.5
M 70 x 2.0	2	68
M 70 x 3.0	3	67
M 70 x 4.0	4	66
M 70 x 6.0	6	64
M 72 x 1.5	1.5	70.5
M 72 x 2.0	2	70
M 72 x 3.0	3	69
M 72 x 4.0	4	68
M 72 x 6.0	6	66
M 75 x 1.5	1.5	73.5
M 75 x 2.0	2	73
M 75 x 3.0	3	72
M 75 x 4.0	4	71
M 75 x 6.0	6	69
M 76 x 1.5	1.5	74.5
M 76 x 2.0	2	74
M 76 x 3.0	3	73
M 76 x 4.0	4	72
M 76 x 6.0	6	70
M 80 x 1.5	1.5	78.5
M 80 x 2.0	2	78
M 80 x 3.0	3	77
M 80 x 4.0	4	76
M 80 x 6.0	6	74
M 85 x 2.0	2	83
M 85 x 3.0	3	82
M 85 x 4.0	4	81
M 85 x 6.0	6	79
M 90 x 2.0	2	88
M 90 x 3.0	3	87
M 90 x 4.0	4	86
M 90 x 6.0	6	84
M 95 x 2.0	2	93
M 95 x 3.0	3	92
M 95 x 4.0	4	91
M 95 x 6.0	6	89
M 100 x 2.0	2	98
M 100 x 3.0	3	97
M 100 x 4.0	4	96
M 100 x 6.0	6	94

Conclusion: The above-given list of the threads is not the complete list. ISO 724 provides thread sizes up to 300. let us know in the comment section below if you understood how to read your screws threads on the drawings correctly.