ISO Metric Thread

The ISO metric screw threads are the worldwide most commonly used type of general-purpose screw thread. They were one of the first international standards agreed upon when the International Organization for Standardization (ISO) was set up in 1947.

The design principles of ISO general-purpose metric screw threads ("M" series threads) are defined in international standard ISO 68-1. Each thread is characterized by its major diameter and its pitch. ISO metric threads consist of symmetric V-shaped thread. In the plane of the thread axis, the flanks of the V have an angle of 60° to each other.

ISO Metric Thread Prefered Size

ISO 261 specifies a detailed list of preferred combinations of outer diameter and pitch for ISO metric screw threads. The 1st choice column follows the R10 series from ISO 3, while the 2nd choice column is rounded off values from the R20 series from ISO 3.

ISO 262 specifies a shorter list of thread dimensions - a subset of ISO 261.

Superfine pitch metric threads are occasionally used in automotive components, such as suspension struts, and are commonly used in the aviation manufacturing industry. This is because extra fine threads are more resistant to coming loose from vibrations.

Thread Count (TPI)

TPI stands for Threads Per Inch, which means the total number of threads per inch measured along the length of a fastener. TPI is used only with American fasteners. Metric Fasteners use a thread Pitch. In general, smaller fasteners have finer threads, so the thread count is higher.

Thread Pitch

Metric fasteners are specified with a thread pitch, instead of a thread count. The thread pitch is the distance between threads expressed in millimeters. In general, smaller fasteners have finer threads, so they have a lower thread pitch.

ISO Metric Thread Charts

ISO Metric Thread Chart | Metric Thread Dimension

ISO Metric profile	External Thread						Internal Thread
Thread	Major Diameter		Pitch Diameter		Minor Diameter		Minor Diameter		Pitch Diameter		Major Diameter		Tap
Size x Pitch	max	min	max	min	max	min	min	max	min	max	min	max	Drill
M 2 x 0.4	1.981	1.886	1.721	1.654	1.548	1.408	1.567	1.679	1.74	1.83	2	2.148	1.6
M 2.2 x 0.45	2.18	2.08	1.888	1.817	1.693	1.54	1.713	1.838	1.908	2.003	2.2	2.36	1.75
M 2.3 x 0.45	2.28	2.18	1.988	1.917	1.793	1.64	1.813	1.938	2.008	2.103	2.3	2.46	1.85
M 2.5 x 0.45	2.48	2.38	2.188	2.117	1.993	1.84	2.013	2.138	2.208	2.303	2.5	2.66	2.05
M 2.6 x 0.45	2.58	2.48	2.288	2.217	2.093	1.94	2.113	2.238	2.308	2.393	2.6	2.75	2.15
M 3 x 0.5	2.98	2.874	2.655	2.58	2.439	2.272	2.459	2.599	2.675	2.775	3	3.172	2.5
M 3.5 x 0.6	3.479	3.354	3.089	3.004	2.829	2.635	2.85	3.01	3.11	3.222	3.5	3.699	2.9
M 4 x 0.7	3.978	3.838	3.523	3.433	3.22	3.002	3.242	3.422	3.545	3.663	4	4.219	3.3
M 4.5 x 0.75	4.478	4.338	3.991	3.901	3.666	3.439	3.688	3.878	4.013	4.131	4.5	4.726	3.75
M 5 x 0.8	4.976	4.826	4.456	4.361	4.11	3.869	4.134	4.334	4.48	4.605	5	5.24	4.2
M 5.5 x 0.5	5.48	5.374	5.155	5.065	4.939	4.757	4.959	5.099	5.175	5.295	5.5	5.692	5
M 6 x 1	5.974	5.794	5.324	5.212	4.891	4.596	4.917	5.153	5.35	5.5	6	6.294	5
M 7 x 1	6.974	6.794	6.324	6.212	5.891	5.596	5.917	6.153	6.35	6.5	7	7.294	6
M 8 x 1.25	7.972	7.76	7.16	7.042	6.619	6.272	6.647	6.912	7.188	7.348	8	8.34	6.75
M 9 x 1.25	8.972	8.76	8.16	8.042	7.619	7.272	7.647	7.912	8.188	8.348	9	9.34	7.75
M 10 x 1.5	9.968	9.732	8.994	8.862	8.344	7.938	8.376	8.676	9.026	9.206	10	10.396	8.5
M 11 x 1.5	10.97	10.73	9.994	9.862	9.344	8.938	9.376	9.676	10.026	10.196	11	11.387	9.5
M 12 x 1.75	11.97	11.7	10.83	10.68	10.072	9.601	10.106	10.441	10.863	11.063	12	12.453	10.25
M 14 x 2	13.96	13.68	12.66	12.5	11.797	11.271	11.835	12.21	12.701	12.913	14	14.501	12
M 15 x 1.5	14.97	14.73	13.99	13.85	13.344	12.93	13.376	13.676	14.026	14.216	15	15.407	13.5
M 16 x 2	15.96	15.68	14.66	14.5	13.797	13.271	13.835	14.21	14.701	14.913	16	16.501	14
M 17 x 1.5	16.97	16.73	15.99	15.85	15.344	14.93	15.376	15.676	16.026	16.216	17	17.407	15.5
M 18 x 2.5	17.96	17.62	16.33	16.16	15.252	14.624	15.294	15.744	16.376	16.601	18	18.585	15.5
M 20 x 2.5	19.96	19.62	18.33	18.16	17.252	16.624	17.294	17.744	18.376	18.6	20	20.585	17.5
M 22 x 3	21.95	21.58	20	19.82	18.704	17.97	18.752	19.252	20.051	20.296	22	22.677	19
M 24 x 3	23.95	23.58	22	21.8	20.704	19.955	20.752	21.252	22.051	22.316	24	24.698	21
M 25 x 2	24.96	24.68	23.66	23.49	22.797	22.261	22.835	23.21	23.701	23.925	25	25.513	23
M 26 x 1.5	25.97	25.73	24.99	24.84	24.344	23.92	24.376	24.676	25.026	25.226	26	26.417	24.5
M 27 x 3	26.95	26.58	25	24.8	23.704	22.955	23.752	24.252	25.051	25.316	27	27.698	24
M 28 x 2	27.96	27.68	26.66	26.49	25.797	25.261	25.835	26.21	26.701	26.925	28	28.513	26
M 30 x 3.5	29.95	29.52	27.67	27.46	26.158	25.306	26.211	26.771	27.727	28.007	30	30.785	26.5
M 32 x 2	31.96	31.68	30.66	30.49	29.797	29.261	29.835	30.21	30.701	30.925	32	32.513	30
M 33 x 3.5	32.97	32.54	30.7	30.48	29.179	28.327	29.211	29.771	30.727	31.007	33	33.785	29.5
M 35 x 1.5	34.97	34.73	33.99	33.84	33.344	32.92	33.376	33.676	34.026	34.226	35	35.416	33.5
M 36 x 4	35.94	35.47	33.34	33.12	31.61	30.654	31.67	32.27	33.402	33.702	36	36.877	32
M 38 x 1.5	37.97	37.73	36.99	36.84	36.344	35.92	36.376	36.676	37.026	37.226	38	38.417	36.5
M 39 x 4	38.94	38.47	36.34	36.12	34.61	33.654	34.67	35.27	36.402	36.702	39	39.877	35
M 40 x 3	39.95	39.58	38	37.8	36.704	35.955	36.752	37.252	38.051	38.316	40	40.698	37
M 42 x 4.5	41.94	41.44	39.01	38.78	37.066	36.006	37.129	37.799	39.077	39.392	42	42.965	37.5
M 45 x 4.5	44.94	44.44	42.01	41.78	40.066	39.006	40.129	40.799	42.077	42.392	45	45.965	40.5
M 48 x 5	47.93	47.4	44.68	44.43	42.516	41.351	42.587	43.297	44.752	45.087	48	49.057	43
M 50 x 4	49.94	49.47	47.34	47.11	45.61	44.642	45.67	46.27	47.402	47.717	50	50.892	46
M 52 x 5	51.93	51.4	48.68	48.45	46.516	45.365	46.587	47.297	48.752	49.067	52	53.037	47
M 55 x 4	54.94	54.47	52.34	52.11	50.61	49.642	50.67	51.27	52.402	52.717	55	55.892	51
M 56 x 5.5	55.93	55.37	52.35	52.09	49.971	48.7	50.046	50.796	52.428	52.783	56	57.149	50.5
M 58 x 4	57.94	57.47	55.34	55.11	53.61	52.642	53.67	54.27	55.402	55.717	58	58.892	54
M 60 x 5.5	59.93	59.37	56.35	56.09	53.971	52.7	54.046	54.796	56.428	56.783	60	61.149	54.5
M 62 x 4	61.94	61.47	59.34	59.11	57.61	56.642	57.67	58.27	59.402	59.717	62	62.892	58
M 63 x 1.5	62.97	62.73	61.99	61.83	61.344	60.91	61.376	61.676	62.026	62.238	63	63.429	61.5
M 64 x 6	63.92	63.32	60.02	59.74	57.425	56.047	57.505	58.305	60.103	60.478	64	65.241	58
M 65 x 4	64.94	64.47	62.34	62.11	60.61	59.642	60.67	61.27	62.402	62.717	65	65.892	61
M 68 x 6	67.92	67.32	64.02	63.74	61.425	60.047	61.505	62.305	64.103	64.478	68	69.241	62
M 70 x 6	69.92	69.32	66.02	65.74	63.425	62.047	63.505	64.305	66.103	66.478	70	71.241	64
M 72 x 6	71.92	71.32	68.02	67.74	65.425	64.047	65.505	66.305	68.103	68.478	72	73.241	66
M 75 x 6	74.92	74.32	71.02	70.74	68.425	67.047	68.505	69.305	71.103	71.478	75	76.241	69
M 76 x 6	75.92	75.32	72.02	71.74	69.425	68.047	69.505	70.305	72.103	72.478	76	77.241	70
M 78 x 2	77.96	77.68	76.66	76.48	75.797	75.251	75.835	76.21	76.701	76.937	78	78.525	76
M 80 x 6	79.92	79.32	76.02	75.74	73.425	72.047	73.505	74.305	76.103	76.478	80	81.241	74
M 82 x 2	81.96	81.68	80.66	80.48	79.797	79.251	79.835	80.21	80.701	80.937	82	82.525	80
M 85 x 6	84.92	84.32	81.02	80.74	78.425	77.047	78.505	79.305	81.103	81.478	85	86.241	79
M 90 x 6	89.92	89.32	86.02	85.74	83.425	82.047	83.505	84.305	86.103	86.478	90	91.241	84
M 95 x 6	94.92	94.32	91.02	90.72	88.425	87.027	88.505	89.305	91.103	91.503	95	96.266	89
M 100 x 6	99.92	99.32	96.02	95.72	93.425	92.027	93.505	94.305	96.103	96.503	100	101.27	94

ISO Metric Thread Chart | ISO 262 selected sizes for screws, bolts and nuts

1st choice	2nd choice	Coarse	Fine
1	-	0.25	0.2
1.2	-	0.25	0.2
-	1.4	0.3	0.2
1.6	-	0.35	0.2
-	1.8	0.35	0.2
2	-	0.4	0.25
2.5	-	0.45	0.35
3	-	0.5	0.35
-	3.5	0.6	0.35
4	-	0.7	0.5
5	-	0.8	0.5
6	-	1	0.75
-	7	1	0.75
8	-	1.25	1 \| 0.75
10	-	1.5	1.25 \| 1
12	-	1.75	1.5 \| 1.25
-	14	2	1.5
16	-	2	1.5
-	18	2.5	2 \| 1.5
20	-	2.5	2 \| 1.5
-	22	2.5	2 \| 1.5
24	-	3	2
-	27	3	2
30	-	3.5	2
-	33	3.5	2
36	-	4	3
-	39	4	3
42	-	4.5	3
-	45	4.5	3
48	-	5	3
-	52	5	4
56	-	5.5	4
-	60	5.5	4
64	-	6	4