ISO 3601 O-Rings — International Metric Standard

ISO 3601 is the international standard governing O-Ring dimensions, tolerances and quality acceptance criteria. ISO 3601-1 defines dimensional tolerances for metric O-Rings; ISO 3601-2 covers quality acceptance criteria; ISO 3601-3 defines design guidance for glands. FITCO manufactures to ISO 3601 tolerance class A (tighter) and tolerance class B (general purpose) across the full metric CS/ID range.

Tolerance Class	Application
ISO 3601 Class A	Tight tolerance — aerospace and critical sealing applications
ISO 3601 Class B	General purpose — standard industrial and commercial sealing

ISO 3601 and AS568 are not directly interchangeable — they define different tolerances at similar nominal sizes. If your drawing references ISO 3601, specify the size as ID × CS in millimetres. FITCO will confirm the tolerance class on quotation.

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Material Technical Guides

NBR FKM EPDM FFKM HNBR Silicone

Standards Cross-Reference

AS568 Metric ISO 3601 JIS B2401 Custom

Available Compounds

NBR 70/90FKM 75/90FFKM EPDMHNBR 70/80/90Silicone FluorosiliconeAFLASNeoprene

Complete Reference

Full GB/T 3452.1 (China) size chart — every size

GB/T 3452.1 is China's national standard for O-Ring dimensions, closely aligned with ISO 3601. All 496 standard sizes across all 5 cross-section bands, with inside diameter and tolerance in millimetres. Confirm which standard your drawing references — GB/T 3452.1 and ISO 3601 use similar but not always identical dimensions at the same nominal size.

ID (mm)	ID Tol. (±)	CS (mm)
CS 1.8 mm
1.80	0.13	1.8
2.00	0.13	1.8
2.24	0.13	1.8
2.50	0.13	1.8
2.80	0.13	1.8
3.15	0.14	1.8
3.55	0.14	1.8
3.75	0.14	1.8
4.00	0.14	1.8
4.50	0.15	1.8
4.75	0.15	1.8
4.87	0.15	1.8
5.00	0.15	1.8
5.15	0.15	1.8
5.30	0.15	1.8
5.60	0.16	1.8
6.00	0.16	1.8
6.30	0.16	1.8
6.70	0.16	1.8
6.90	0.17	1.8
7.10	0.17	1.8
7.50	0.17	1.8
8.00	0.17	1.8
8.50	0.18	1.8
8.75	0.18	1.8
9.00	0.18	1.8
9.50	0.19	1.8
9.75	0.19	1.8
10.00	0.19	1.8
10.60	0.19	1.8
11.20	0.20	1.8
11.60	0.20	1.8
11.80	0.20	1.8
12.10	0.21	1.8
12.50	0.21	1.8
12.80	0.21	1.8
13.20	0.21	1.8
14.00	0.22	1.8
14.50	0.22	1.8
15.00	0.23	1.8
15.50	0.23	1.8
16.00	0.24	1.8
17.00	0.24	1.8
18.00	0.25	1.8
19.00	0.26	1.8
20.00	0.26	1.8
20.60	0.27	1.8
21.20	0.27	1.8
22.40	0.28	1.8
23.00	0.29	1.8
23.60	0.29	1.8
24.30	0.30	1.8
25.00	0.30	1.8
25.80	0.31	1.8
26.50	0.31	1.8
27.30	0.32	1.8
28.00	0.32	1.8
29.00	0.33	1.8
30.00	0.34	1.8
31.50	0.35	1.8
32.50	0.36	1.8
33.50	0.36	1.8
34.50	0.37	1.8
35.50	0.38	1.8
36.50	0.38	1.8
37.50	0.39	1.8
38.70	0.40	1.8
40.00	0.41	1.8
41.20	0.42	1.8
42.50	0.43	1.8
43.70	0.44	1.8
45.00	0.44	1.8
46.20	0.45	1.8
47.50	0.46	1.8
48.70	0.47	1.8
50.00	0.48	1.8
CS 2.65 mm
4.50	0.15	2.65
5.30	0.15	2.65
6.00	0.16	2.65
6.90	0.17	2.65
8.00	0.17	2.65
9.00	0.18	2.65
9.50	0.19	2.65
10.00	0.19	2.65
10.60	0.19	2.65
11.20	0.20	2.65
11.60	0.20	2.65
11.80	0.20	2.65
12.10	0.21	2.65
12.50	0.21	2.65
12.80	0.21	2.65
13.20	0.21	2.65
14.00	0.22	2.65
14.50	0.22	2.65
15.00	0.23	2.65
15.50	0.23	2.65
16.00	0.24	2.65
17.00	0.24	2.65
18.00	0.25	2.65
19.00	0.26	2.65
20.00	0.26	2.65
20.60	0.27	2.65
21.20	0.27	2.65
22.40	0.28	2.65
23.00	0.29	2.65
23.60	0.29	2.65
24.30	0.30	2.65
25.00	0.30	2.65
25.80	0.31	2.65
26.50	0.31	2.65
27.30	0.32	2.65
28.00	0.32	2.65
29.00	0.33	2.65
30.00	0.34	2.65
31.50	0.35	2.65
32.50	0.36	2.65
33.50	0.36	2.65
34.50	0.37	2.65
35.50	0.38	2.65
36.50	0.38	2.65
37.50	0.39	2.65
38.70	0.40	2.65
40.00	0.41	2.65
41.20	0.42	2.65
42.50	0.43	2.65
43.70	0.44	2.65
45.00	0.44	2.65
46.20	0.45	2.65
47.50	0.46	2.65
48.70	0.47	2.65
50.00	0.48	2.65
51.50	0.49	2.65
53.00	0.50	2.65
54.50	0.51	2.65
56.00	0.52	2.65
58.00	0.54	2.65
60.00	0.55	2.65
61.50	0.56	2.65
63.00	0.57	2.65
65.00	0.58	2.65
67.00	0.60	2.65
69.00	0.61	2.65
71.00	0.63	2.65
73.00	0.64	2.65
75.00	0.65	2.65
77.50	0.67	2.65
80.00	0.69	2.65
82.50	0.71	2.65
85.00	0.72	2.65
87.50	0.74	2.65
90.00	0.76	2.65
92.50	0.77	2.65
95.00	0.79	2.65
97.50	0.81	2.65
100.00	0.82	2.65
103.00	0.85	2.65
106.00	0.87	2.65
109.00	0.89	2.65
112.00	0.91	2.65
115.00	0.93	2.65
118.00	0.95	2.65
122.00	0.97	2.65
125.00	0.99	2.65
128.00	1.01	2.65
132.00	1.04	2.65
136.00	1.07	2.65
140.00	1.09	2.65
142.50	1.11	2.65
145.00	1.13	2.65
147.50	1.14	2.65
150.00	1.16	2.65
152.50	1.18	2.65
160.00	1.23	2.65
170.00	1.29	2.65
180.00	1.36	2.65
CS 3.55 mm
18.00	0.25	3.55
19.00	0.26	3.55
20.00	0.26	3.55
20.60	0.27	3.55
21.20	0.27	3.55
22.40	0.28	3.55
23.00	0.29	3.55
23.60	0.29	3.55
24.30	0.30	3.55
25.00	0.30	3.55
25.80	0.31	3.55
26.50	0.31	3.55
27.30	0.32	3.55
28.00	0.32	3.55
29.00	0.33	3.55
30.00	0.34	3.55
31.50	0.35	3.55
32.50	0.36	3.55
33.50	0.36	3.55
34.50	0.37	3.55
35.50	0.38	3.55
36.50	0.38	3.55
37.50	0.39	3.55
38.70	0.40	3.55
40.00	0.41	3.55
41.20	0.42	3.55
42.50	0.43	3.55
43.70	0.44	3.55
45.00	0.44	3.55
46.20	0.45	3.55
47.50	0.46	3.55
48.70	0.47	3.55
50.00	0.48	3.55
51.50	0.49	3.55
53.00	0.50	3.55
54.50	0.51	3.55
56.00	0.52	3.55
58.00	0.54	3.55
60.00	0.55	3.55
61.50	0.56	3.55
63.00	0.57	3.55
65.00	0.58	3.55
67.00	0.60	3.55
69.00	0.61	3.55
71.00	0.63	3.55
73.00	0.64	3.55
75.00	0.65	3.55
77.50	0.67	3.55
80.00	0.69	3.55
82.50	0.71	3.55
85.00	0.72	3.55
87.50	0.74	3.55
90.00	0.76	3.55
92.50	0.77	3.55
95.00	0.79	3.55
97.50	0.81	3.55
100.00	0.82	3.55
103.00	0.85	3.55
106.00	0.87	3.55
109.00	0.89	3.55
112.00	0.91	3.55
115.00	0.93	3.55
118.00	0.95	3.55
122.00	0.97	3.55
125.00	0.99	3.55
128.00	1.01	3.55
132.00	1.04	3.55
136.00	1.07	3.55
140.00	1.09	3.55
142.50	1.11	3.55
145.00	1.13	3.55
147.50	1.14	3.55
150.00	1.16	3.55
152.50	1.18	3.55
155.00	1.19	3.55
157.50	1.21	3.55
160.00	1.23	3.55
162.50	1.24	3.55
165.00	1.26	3.55
167.50	1.28	3.55
170.00	1.29	3.55
172.50	1.31	3.55
175.00	1.33	3.55
177.50	1.34	3.55
180.00	1.36	3.55
182.50	1.38	3.55
185.00	1.39	3.55
187.50	1.41	3.55
190.00	1.43	3.55
195.00	1.46	3.55
200.00	1.49	3.55
CS 5.3 mm
40.00	0.41	5.3
41.20	0.42	5.3
42.50	0.43	5.3
43.70	0.44	5.3
45.00	0.44	5.3
46.20	0.45	5.3
47.50	0.46	5.3
48.70	0.47	5.3
50.00	0.48	5.3
51.50	0.49	5.3
53.00	0.50	5.3
54.50	0.51	5.3
56.00	0.52	5.3
58.00	0.54	5.3
60.00	0.55	5.3
61.50	0.56	5.3
63.00	0.57	5.3
65.00	0.58	5.3
67.00	0.60	5.3
69.00	0.61	5.3
71.00	0.63	5.3
73.00	0.64	5.3
75.00	0.65	5.3
77.50	0.67	5.3
80.00	0.69	5.3
82.50	0.71	5.3
85.00	0.72	5.3
87.50	0.74	5.3
90.00	0.76	5.3
92.50	0.77	5.3
95.00	0.79	5.3
97.50	0.81	5.3
100.00	0.82	5.3
103.00	0.85	5.3
106.00	0.87	5.3
109.00	0.89	5.3
112.00	0.91	5.3
115.00	0.93	5.3
118.00	0.95	5.3
122.00	0.97	5.3
125.00	0.99	5.3
128.00	1.01	5.3
132.00	1.04	5.3
136.00	1.07	5.3
140.00	1.09	5.3
142.50	1.11	5.3
145.00	1.13	5.3
147.50	1.14	5.3
150.00	1.16	5.3
152.50	1.18	5.3
155.00	1.19	5.3
157.50	1.21	5.3
160.00	1.23	5.3
162.50	1.24	5.3
165.00	1.26	5.3
167.50	1.28	5.3
170.00	1.29	5.3
172.50	1.31	5.3
175.00	1.33	5.3
177.50	1.34	5.3
180.00	1.36	5.3
182.50	1.38	5.3
185.00	1.39	5.3
187.50	1.41	5.3
190.00	1.43	5.3
195.00	1.46	5.3
200.00	1.49	5.3
203.00	1.51	5.3
206.00	1.53	5.3
212.00	1.57	5.3
218.00	1.61	5.3
224.00	1.65	5.3
227.00	1.67	5.3
230.00	1.69	5.3
236.00	1.73	5.3
239.00	1.75	5.3
243.00	1.77	5.3
250.00	1.82	5.3
254.00	1.84	5.3
258.00	1.87	5.3
261.00	1.89	5.3
265.00	1.91	5.3
268.00	1.93	5.3
272.00	1.96	5.3
276.00	1.99	5.3
280.00	2.01	5.3
283.00	2.03	5.3
286.00	2.05	5.3
290.00	2.08	5.3
295.00	2.11	5.3
300.00	2.14	5.3
303.00	2.16	5.3
307.00	2.19	5.3
311.00	2.21	5.3
315.00	2.24	5.3
320.00	2.27	5.3
325.00	2.30	5.3
330.00	2.33	5.3
335.00	2.36	5.3
340.00	2.40	5.3
345.00	2.43	5.3
350.00	2.46	5.3
355.00	2.49	5.3
360.00	2.52	5.3
365.00	2.56	5.3
370.00	2.59	5.3
375.00	2.62	5.3
379.00	2.64	5.3
383.00	2.67	5.3
387.00	2.70	5.3
391.00	2.72	5.3
395.00	2.75	5.3
400.00	2.78	5.3
CS 7.0 mm
109.00	0.89	7.0
112.00	0.91	7.0
115.00	0.93	7.0
118.00	0.95	7.0
122.00	0.97	7.0
125.00	0.99	7.0
128.00	1.01	7.0
132.00	1.04	7.0
136.00	1.07	7.0
140.00	1.09	7.0
142.50	1.11	7.0
145.00	1.13	7.0
147.50	1.14	7.0
150.00	1.16	7.0
152.50	1.18	7.0
155.00	1.19	7.0
157.50	1.21	7.0
160.00	1.23	7.0
162.50	1.24	7.0
165.00	1.26	7.0
167.50	1.28	7.0
170.00	1.29	7.0
172.50	1.31	7.0
175.00	1.33	7.0
177.50	1.34	7.0
180.00	1.36	7.0
182.50	1.38	7.0
185.00	1.39	7.0
187.50	1.41	7.0
190.00	1.43	7.0
195.00	1.46	7.0
200.00	1.49	7.0
203.00	1.51	7.0
206.00	1.53	7.0
212.00	1.57	7.0
218.00	1.61	7.0
224.00	1.65	7.0
227.00	1.67	7.0
230.00	1.69	7.0
236.00	1.73	7.0
239.00	1.75	7.0
243.00	1.77	7.0
250.00	1.82	7.0
254.00	1.84	7.0
258.00	1.87	7.0
261.00	1.89	7.0
265.00	1.91	7.0
268.00	1.93	7.0
272.00	1.96	7.0
276.00	1.99	7.0
280.00	2.01	7.0
283.00	2.03	7.0
286.00	2.05	7.0
290.00	2.08	7.0
295.00	2.11	7.0
300.00	2.14	7.0
303.00	2.16	7.0
307.00	2.19	7.0
311.00	2.21	7.0
315.00	2.24	7.0
320.00	2.27	7.0
325.00	2.30	7.0
330.00	2.33	7.0
335.00	2.36	7.0
340.00	2.40	7.0
345.00	2.43	7.0
350.00	2.46	7.0
355.00	2.49	7.0
360.00	2.52	7.0
365.00	2.56	7.0
370.00	2.59	7.0
375.00	2.62	7.0
379.00	2.64	7.0
383.00	2.67	7.0
387.00	2.70	7.0
391.00	2.72	7.0
395.00	2.75	7.0
400.00	2.78	7.0
406.00	2.82	7.0
412.00	2.85	7.0
418.00	2.89	7.0
425.00	2.94	7.0
429.00	2.96	7.0
433.00	2.99	7.0
437.00	3.01	7.0
443.00	3.05	7.0
450.00	3.09	7.0
456.00	3.13	7.0
462.00	3.17	7.0
466.00	3.19	7.0
470.00	3.22	7.0
475.00	3.25	7.0
479.00	3.28	7.0
483.00	3.30	7.0
487.00	3.33	7.0
493.00	3.36	7.0
500.00	3.41	7.0
508.00	3.46	7.0
515.00	3.50	7.0
523.00	3.55	7.0
530.00	3.60	7.0
538.00	3.65	7.0
545.00	3.69	7.0
553.00	3.74	7.0
560.00	3.78	7.0
570.00	3.85	7.0
580.00	3.91	7.0
590.00	3.97	7.0
600.00	4.03	7.0
608.00	4.08	7.0
615.00	4.12	7.0
623.00	4.17	7.0
630.00	4.22	7.0
640.00	4.28	7.0
650.00	4.34	7.0
660.00	4.40	7.0
670.00	4.47	7.0

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