Flange Size Chart in MM

Selecting the correct flange size is essential for building a safe, efficient, and leak-free piping system. Flanges connect pipes, valves, pumps, and other equipment, making accurate sizing critical for maintaining pressure, preventing leaks, and simplifying maintenance.

This flange size chart in mm covers commonly used standards such as AS 4087, AS 2129, ANSI B16.5, and ISO 7005 (DIN). Whether you’re an engineer, fabricator, plumber, or DIY enthusiast, understanding flange dimensions helps ensure compatibility and long-term system reliability.

Flange Size Chart

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	95	67	4	M12	14
AS 2129	Table C	95	67	4	M12	14
	Table D	95	67	4	M12	14
	Table E	95	67	4	M12	14
	Table F	95	67	4	M12	14
	Table H	114	83	4	M16	17
	Table J	114	83	4	M16	17
ANSI B16.5	ANSI 150	89	60	4	13	16
	ANSI 300	95	67	4	13	16
	ANSI 600	95	67	4	13	16
	ANSI 900	121	83	4	19	22
	ANSI 1500	121	83	4	19	22
ISO 7005 (DIN)	PN6	80	55	4	M10	11
	PN10	95	65	4	M12	14
	PN16	95	65	4	M12	14
	PN20	90	60.5	4	M14	16
	PN25	95	65	4	M12	14
	PN40	95	65	4	M12	14

20mm (3/4″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	100	73	4	M12	14
AS 2129	Table C	100	73	4	M12	14
	Table D	100	73	4	M12	14
	Table E	100	73	4	M12	14
	Table F	100	73	4	M12	14
	Table H	115	83	4	M12	18
	Table J	115	83	4	M12	18
ANSI B16.5	ANSI 150	98	70	4	13	16
	ANSI 300	117	83	4	16	19
	ANSI 600	117	83	4	16	19
	ANSI 900	130	89	4	19	22
	ANSI 1500	130	89	4	19	22
ISO 7005 (DIN)	PN6	90	65	4	M10	11
	PN10	105	75	4	M12	14
	PN16	105	75	4	M12	14
	PN20	100	70	4	M14	16
	PN25	105	75	4	M12	14
	PN40	105	75	4	M12	14

25mm (1″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	115	83	4	M12	14
AS 2129	Table C	115	83	4	M12	14
	Table D	115	83	4	M12	14
	Table E	115	83	4	M12	14
	Table F	120	87	4	M16	18
	Table H	120	87	4	M16	18
	Table J	120	87	4	M16	18
ANSI B16.5	ANSI 150	110	79	4	13	16
	ANSI 300	125	89	4	16	19
	ANSI 600	125	89	4	16	19
	ANSI 900		102	4	22	25
	ANSI 1500		102	4	22	25
ISO 7005 (DIN)	PN6	100	78	4	M10	11
	PN10	115	85	4	M12	14
	PN16	115	85	4	M12	14
	PN20	110	79.5	4	M14	16
	PN25	115	85	4	M12	14
	PN40	115	85	4	M12	14

32mm (1 1/4″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	120	87	4	M12	14
AS 2129	Table C	120	87	4	M12	14
	Table D	120	87	4	M12	14
	Table E	120	87	4	M12	14
	Table F	135	98	4	M16	18
	Table H	135	98	4	M16	18
	Table J	135	98	4	M16	18
ANSI B16.5	ANSI 150	117	89	4	13	16
	ANSI 300	133	98	4	16	19
	ANSI 600	133	98	4	16	19
	ANSI 900	159	111	4	22	25
	ANSI 1500	159	111	4	22	25
ISO 7005 (DIN)	PN6	120	90	4	M12	14
	PN10	140	100	4	M16	18
	PN16	140	100	4	M16	18
	PN20	120	89	4	M14	16
	PN25	140	100	4	M16	18
	PN40	140	100	4	M16	18

40mm (1 1/2″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	135	98	4	M12	14
AS 2129	Table C	135	98	4	M12	14
	Table D	135	98	4	M12	14
	Table E	135	98	4	M12	14
	Table F	140	105	4	M16	18
	Table H	140	105	4	M16	18
	Table J	140	105	4	M16	18
ANSI B16.5	ANSI 150	127	98	4	13	16
	ANSI 300	156	114	4	19	22
	ANSI 600	156	114	4	19	22
	ANSI 900	178	124	4	25	29
	ANSI 1500	178	124	4	25	29
ISO 7005 (DIN)	PN6	130	100	4	M12	14
	PN10	150	110	4	M16	18
	PN16	150	110	4	M16	18
	PN20	130	98.5	4	M14	16
	PN25	150	110	4	M16	18
	PN40	150	110	4	M16	18

50mm (2″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	150	114	4	M16	18
	PN16	150	114	4	M16	18
	PN21	165	127	4	M16	18
	PN35	165	127	4	M16	18
AS 2129	Table C	150	114	4	M16	18
	Table D	150	114	4	M16	18
	Table E	150	114	4	M16	18
	Table F	165	127	4	M16	18
	Table H	165	127	4	M16	18
	Table J	165	127	4	M20	22
ANSI B16.5	ANSI 150	152	121	4	16	19
	ANSI 300	165	127	8	16	19
	ANSI 600	165	127	8	16	19
	ANSI 900	216	165	8	22	25
	ANSI 1500	216	165	8	22	25
ISO 7005 (DIN)	PN6	140	110	4	M12	14
	PN10	165	125	4	M16	18
	PN16	165	125	4	M16	18
	PN20	150*	120.5	4	M16	18
	PN25	165	125	4	M16	18
	PN40	165	125	4	M16	18
*When made from cast or ductile iron the OD is 155mm.

65mm (2 1/2″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	165	127	4	M16	18
	PN16	165	127	4	M16	18
	PN21	185	146	8	M16	18
	PN35	185	146	8	M16	18
AS 2129	Table C	165	127	4	M16	18
	Table D	165	127	4	M16	18
	Table E	165	127	4	M16	18
	Table F	185	146	8	M16	18
	Table H	185	146	8	M16	18
	Table J	185	146	8	M20	22
ANSI B16.5	ANSI 150	178	140	4	16	19
	ANSI 300	191	149	8	19	22
	ANSI 600	191	149	8	19	22
	ANSI 900	244	191	8	22	25
	ANSI 1500	244	191	8	25	29
ISO 7005 (DIN)	PN6	160	130	4	M12	14
	PN10	185	145	4	M16	18
	PN16	185	145	4	M16	18
	PN20	180	139.5	4	M16	18
	PN25	185	145	8	M16	18
	PN40	185	145	8	M16	18

80mm (3″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	185	146	4	M16	18
	PN16	185	146	4	M16	18
	PN21	205	165	8	M16	18
	PN35	205	165	8	M16	18
AS 2129	Table C	185	146	4	M16	18
	Table D	185	146	4	M16	18
	Table E	185	146	4	M16	18
	Table F	205	165	8	M16	18
	Table H	205	165	8	M16	18
	Table J	205	165	8	M20	22
ANSI B16.5	ANSI 150	191	152	4	16	19
	ANSI 300	210	168	8	19	22
	ANSI 600	210	168	8	19	22
	ANSI 900	241	192	8	22	25
	ANSI 1500	267	203	8	29	32
ISO 7005 (DIN)	PN6	190	150	4	M16	18
	PN10	200	160	8	M16	18
	PN16	200	160	8	M16	18
	PN20	190	152.5	4	M16	18
	PN25	200	160	8	M16	18
	PN40	185	160	8	M16	18

100mm (4″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	215	178	4	M16	18
	PN16	215	178	4	M16	18
	PN21	230	191	8	M16	18
	PN35	230	191	8	M16	18
AS 2129	Table C	215	178	4	M16	18
	Table D	215	178	4	M16	18
	Table E	215	178	8	M16	18
	Table F	230	191	8	M16	18
	Table H	230	191	8	M16	18
	Table J	230	191	8	M20	22
ANSI B16.5	ANSI 150	229	191	8	16	19
	ANSI 300	254	200	8	19	22
	ANSI 600	273	216	8	22	25
	ANSI 900	292	235	8	29	32
	ANSI 1500	311	241	8	32	35
ISO 7005 (DIN)	PN6	210	170	4	M16	18
	PN10	220	180	8	M16	24
	PN16	220	180	8	M16	18
	PN20	230	190.5	8	M16	18
	PN25	235	190	8	M20	22
	PN40	235	190	8	M20	22

125mm (5″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	255	210	8	M16	18
	PN16	255	210	8	M16	18
	PN21	280	235	8	M20	22
	PN35	280	235	8	M20	22
AS 2129	Table C	255	210	8	M16	18
	Table D	255	210	8	M16	18
	Table E	255	210	8	M16	18
	Table F	280	235	8	M20	22
	Table H	280	235	8	M20	22
	Table J	280	235	8	M24	26
ANSI B16.5	ANSI 150	254	216	8	19	22
	ANSI 300	279	235	8	19	22
	ANSI 600	330	267	8	25	29
	ANSI 900	349	279	8	32	35
	ANSI 1500	375	292	8	38	41
ISO 7005 (DIN)	PN6	240	200	8	M16	18
	PN10	250	210	8	M16	18
	PN16	250	210	8	M16	18
	PN20	255	216	8	M20	22
	PN25	270	220	8	M24	26
	PN40	270	220	8	M24	26

150mm (6″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	280	235	8	M16	18
	PN16	280	235	8	M16	18
	PN21	305	260	12	M20	22
	PN35	305	260	12	M20	22
AS 2129	Table C	280	235	8	M16	18
	Table D	280	235	8	M16	18
	Table E	280	235	8	M20	22
	Table F	305	260	12	M20	22
	Table H	305	260	12	M20	22
	Table J	305	260	12	M24	26
ANSI B16.5	ANSI 150	279	241	8	19	22
	ANSI 300	318	270	12	19	22
	ANSI 600	356	292	12	25	29
	ANSI 900	381	318	12	29	32
	ANSI 1500	394	318	12	35	38
ISO 7005 (DIN)	PN6	265	225	8	M16	18
	PN10	285	240	8	M20	22
	PN16	285	240	8	M20	22
	PN20	280	241.5	8	M20	22
	PN25	300	250	8	M24	26
	PN40	300	250	8	M24	26

200mm (8″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	335	292	8	M16	18
	PN16	335	292	8	M16	18
	PN21	370	324	12	M20	22
	PN35	370	324	12	M20	22
AS 2129	Table C	335	292	8	M16	18
	Table D	335	292	8	M16	18
	Table E	335	292	8	M20	22
	Table F	370	324	12	M20	22
	Table H	370	324	12	M20	22
	Table J	370	324	12	M24	26
ANSI B16.5	ANSI 150	343	298	8	19	22
	ANSI 300	381	330	12	19	22
	ANSI 600	419	349	12	29	32
	ANSI 900	470	394	12	35	38
	ANSI 1500	483	394	12	41	44
ISO 7005 (DIN)	PN6	320	280	8	M16	18
	PN10	340	295	8	M20	22
	PN16	340	295	12	M20	22
	PN20	345	298.5	8	M20	22
	PN25	360	310	12	M24	26
	PN40	375	320	12	M27	30

250mm (10″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	405	356	8	M20	22
	PN16	405	356	8	M20	22
	PN21	430	381	12	M24	26
	PN35	430	381	12	M24	26
AS 2129	Table C	405	356	8	M20	22
	Table D	405	356	8	M20	22
	Table E	405	356	12	M20	22
	Table F	430	381	12	M24	26
	Table H	430	381	12	M24	26
	Table J	430	381	12	M27	30
ANSI B16.5	ANSI 150	406	362	12	22	25
	ANSI 300	445	387	16	25	29
	ANSI 600	508	432	16	32	35
	ANSI 900	546	470	16	35	38
	ANSI 1500	584	483	12	41	51
ISO 7005 (DIN)	PN6	375	335	12	M16	18
	PN10	395	350	12	M20	22
	PN16	405	355	12	M24	26
	PN20	405	362	12	M24	26
	PN25	425	370	12	M27	30
	PN40	450	385	12	M30	33

300mm (12″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	455	406	12	M20	22
	PN16	455	406	12	M20	22
	PN21	490	438	16	M24	26
	PN35	490	438	16	M24	26
AS 2129	Table C	455	406	12	M20	22
	Table D	455	406	12	M20	22
	Table E	455	406	12	M24	26
	Table F	490	438	16	M24	26
	Table H	490	438	16	M24	26
	Table J	490	438	16	M27	30
ANSI B16.5	ANSI 150	483	432	12	22	25
	ANSI 300	521	451	16	29	32
	ANSI 600	559	489	20	32	35
	ANSI 900	610	533	20	35	38
	ANSI 1500	673	571	16	51	54
ISO 7005 (DIN)	PN6	440	395	12	M20	22
	PN10	445	400	12	M20	22
	PN16	460	410	12	M24	26
	PN20	485	432	12	M24	26
	PN25	485	430	16	M27	30
	PN40	515	450	16	M30	33

350mm (14″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	525	470	12	M24	26
	PN16	525	470	12	M24	26
	PN21	550	495	16	M27	30
	PN35	550	495	16	M27	30
AS 2129	Table C	525	470	12	M24	26
	Table D	525	470	12	M24	26
	Table E	525	470	12	M24	26
	Table F	550	495	16	M27	30
	Table H	550	495	16	M27	30
	Table J	550	495	16	M30	32
ANSI B16.5	ANSI 150	533	476	12	25	29
	ANSI 300	584	514	20	29	32
	ANSI 600	603	527	20	38	41
	ANSI 900	641	559	20	38	41
	ANSI 1500	749	635	16	57	60
ISO 7005 (DIN)	PN6	490	445	12	M20	22
	PN10	505	460	16	M20	22
	PN16	520	470	16	M24	26
	PN20	535	476	12	M27	35
	PN25	555	490	16	M30	33
	PN40	580	510	16	M33	36

400mm (16″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14	580	521	12	M24	26
	PN16	580	521	12	M24	26
	PN21	610	552	20	M27	30
	PN35	610	552	20	M27	30
AS 2129	Table C	580	521	12	M24	26
	Table D	580	521	12	M24	26
	Table E	580	521	12	M24	26
	Table F	610	552	20	M27	30
	Table H	610	552	20	M27	30
	Table J	610	552	20	M30	33
ANSI B16.5	ANSI 150	597	540	16	1″ (M24)	29
	ANSI 300	648	572	20	1-1/4″ (M30)	35
	ANSI 600	686	603	20	1-1/2″ (M36)	41
	ANSI 900	705	616	20	1-5/8″ (M42)	44
	ANSI 1500	826	705	16	2-1/2″ (M64)	67
ISO 7005 (DIN)	PN6	540	495	16	M20	22
	PN10	565	515	16	M24	26
	PN16	580	525	16	M27	30
	PN20	600	540	16	M27	29.5
	PN25	620	550	16	M33	36
	PN40	660	585	16	M36	39

450mm (18″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14 Flange	640	584	12	M24	26
	PN16 Flange	640	584	12	M24	26
	PN21 Flange	675	610	20	M30	33
	PN35 Flange	675	510	20	M30	33
AS 2129	Table C Flange	640	584	12	M24	26
	Table D Flange	640	584	12	M24	26
	Table E Flange	640	584	16	M24	26
	Table F Flange	675	610	20	M30	33
	Table H Flange	675	610	20	M30	33
	Table J Flange	675	610	20	M33	36
ANSI B16.5	ANSI 150FLANGEe	635	578	16	1-1/8″ (M27)	32
	ANSI 300 Flange	711	629	24	1-1/4″ (M30)	35
	ANSI 600 Flange	743	654	20	1-5/8″ (M42)	44
	ANSI 900 Flange	787	686	20	1-7/8″ (M48)	51
	ANSI 1500 Flange	914	775	16	2-3/4″ (M72)	73
ISO 7005 (DIN)	PN6 Flange	595	550	16	M20	22
	PN10 Flange	615	565	20	M24	26
	PN16 Flange	640	585	20	M27	30
	PN20 Flange	635	578	16	M30	32.5
	PN25 Flange	–	–	–	–	–
	PN40 Flange	–	–	–	–	–

500mm (20″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14 Flange	705	641	16	M24	26
	PN16 Flange	705	641	16	M24	26
	PN21 Flange	735	673	24	M30	33
	PN35 Flange	735	673	24	M30	33
AS 2129	Table C Flange	705	641	16	M24	26
	Table D Flange	705	641	16	M24	26
	Table E Flange	705	641	16	M24	26
	Table F Flange	735	673	24	M30	33
	Table H Flange	735	673	24	M30	33
	Table J Flange	735	673	24	M33	36
ANSI B16.5	ANSI 150 Flange	699	635	20	1-1/8″ (M27)	32
	ANSI 300 Flange	775	686	24	1-1/4″ (M30)	35
	ANSI 600 Flange	813	724	24	1-5/8″ (M42)	44
	ANSI 900 Flange	857	749	20	2″ (M48)	54
	ANSI 1500 Flange	984	832	16	3″ (M80)	79
ISO 7005 (DIN)	PN6 Flange	645	600	20	M20	22
	PN10 Flange	670	620	20	M24	26
	PN16 Flange	715	650	20	M30	33
	PN20 Flange	700	635	20	M30	32.5
	PN25 Flange	–	–	–	–	–
	PN40 Flange	–	–	–	–	–

600mm (24″)

Standard	Class	Diameter	Bolt Circle Diameter	Number of Bolts	Bolt Size	Diameter of Bolt Hole
AS 4087	PN14 Flanges	825	756	16	M27	30
	PN16 Flanges	825	756	16	M27	30
	PN21 Flanges	850	781	24	M33	36
	PN35 Flanges	750	781	24	M33	36
AS 2129	Table C Flanges	825	756	16	M27	30
	Table D Flanges	825	756	16	M27	30
	Table E Flanges	825	756	16	M30	33
	Table F Flanges	850	781	24	M33	36
	Table H Flanges	850	781	24	M33	36
	Table J Flanges	850	781	24	M36	39
ANSI B16.5	ANSI 150 Flanges	813	749	20	1-1/4″ (M30)	35
	ANSI 300 Flanges	914	813	24	1-1/2″ (M36)	41
	ANSI 600 Flanges	940	838	24	1-7/8″ (M48)	51
	ANSI 900 Flanges	1041	902	20	2-1/2″ (M64)	67
	ANSI 1500 Flanges	1168	991	16	3-1/2″ (M90)	92
ISO 7005 (DIN)	PN6 Flanges	755	705	20	M24	26
	PN10 Flanges	780	725	20	M27	30
	PN16 Flanges	840	770	20	M33	36
	PN20 Flanges	815	749.5	20	M33	36
	PN25 Flanges	–	–	–	–	–
	PN40 Flanges	–	–	–	–	–

What Are Flanges

A flange is a mechanical component used to connect pipes, valves, pumps, tanks, and other equipment within a piping system. Most flanges are circular with evenly spaced bolt holes that allow two flanges to be securely bolted together using a gasket to create a leak-resistant seal.

Flanges make installation, inspection, maintenance, and replacement much easier than permanently welded connections. Because they can be disassembled when needed, they’re widely used across industrial and commercial piping systems.

Why Choosing the Correct Flange Size Matters

Selecting the proper flange size affects much more than simply connecting two pipes. An incorrectly sized flange can lead to leakage, reduced system efficiency, excessive stress on piping components, and costly repairs.

Choosing the right flange ensures:

Proper alignment between connected components.
Reliable sealing with compatible gaskets.
Adequate pressure-handling capability.
Easier installation and maintenance.
Longer service life for the piping system.

Although larger flanges may provide greater strength, they also increase material costs and weight. Conversely, undersized flanges may fail under pressure. Balancing performance, safety, and cost is essential.

Understanding the Flange Size Chart

A flange size chart lists the critical dimensions required to match flanges with pipes and fittings. Depending on the standard, the chart typically includes:

Outside diameter (OD)
Bolt circle diameter (BCD or PCD)
Number of bolt holes
Bolt size
Bolt hole diameter
Pressure class or pressure rating

These dimensions vary depending on the flange standard, pressure class, and nominal pipe size. Always ensure that every specification matches before installation.

Common Flange Standards

Several international standards are used throughout the world. This chart includes some of the most widely adopted specifications.

AS 4087

An Australian standard commonly used for water, wastewater, and industrial piping systems.

AS 2129

An older Australian flange standard that remains widely used in existing installations and replacement projects.

ANSI/ASME B16.5

One of the world’s most recognized flange standards, commonly used throughout North America and many international industries. It includes pressure classes such as 150, 300, 600, 900, and 1500.

ISO 7005 (DIN)

This European standard uses pressure ratings such as PN6, PN10, PN16, PN25, and PN40 and is widely used throughout Europe and many international markets.

How to Read a Flange Size Chart

If you’re new to flange specifications, the amount of technical data can seem overwhelming. Fortunately, each column has a specific purpose.

Diameter – Overall outside diameter of the flange.
Bolt Circle Diameter (BCD/PCD) – Diameter of the imaginary circle passing through the centers of all bolt holes.
Number of Bolts – Total fastening bolts required.
Bolt Size – Recommended bolt diameter.
Bolt Hole Diameter – Diameter of each bolt hole.
Pressure Class – Maximum pressure rating according to the applicable standard.

Matching all of these dimensions ensures proper installation and reliable sealing.

Where Flanges Are Commonly Used

Flanges are found in countless industrial applications, including:

Oil and gas pipelines
Chemical processing plants
Water treatment facilities
HVAC systems
Fire protection systems
Food and beverage processing
Marine engineering
Power generation
Pharmaceutical manufacturing

Because these industries often operate under high pressure or extreme temperatures, selecting the correct flange size is essential for both safety and performance.

Types of Flanges

Flanges are available in several designs, each intended for specific pressure ratings, temperatures, and piping applications. Selecting the appropriate type improves sealing performance, simplifies maintenance, and extends the service life of the piping system.

Weld Neck Flange

A weld neck flange features a long tapered hub that gradually transfers stress from the flange to the pipe. It is commonly used in high-pressure and high-temperature systems where strength and reliability are essential.

Best for:

Oil and gas pipelines
Chemical plants
Steam systems
High-pressure applications

Slip-On Flange

Slip-on flanges slide over the pipe before being welded in place. They are easier to align and install than weld neck flanges, making them popular for low- and medium-pressure systems.

Best for:

Water distribution
Cooling systems
General industrial piping

Blind Flange

Blind flanges have no center opening and are used to seal the end of a pipe or pressure vessel. They also provide easy future access for inspection or expansion.

Best for:

Pipeline termination
Pressure testing
Future system modifications

Socket Weld Flange

Socket weld flanges are designed for small-diameter, high-pressure piping. The pipe fits into a recessed socket before welding, creating a smooth internal flow path.

Best for:

Hydraulic systems
Steam lines
High-pressure process piping

Threaded Flange

Threaded flanges connect to threaded pipes without welding. They are commonly used where welding is impractical or hazardous.

Best for:

Low-pressure systems
Gas distribution
Temporary installations

Lap Joint Flange

Lap joint flanges are used with stub ends, allowing the flange to rotate freely for easier bolt alignment. They are often chosen where piping requires frequent disassembly.

Best for:

Food processing
Pharmaceutical plants
Stainless steel piping systems

Understanding Flange Pressure Ratings

Every flange is designed to operate safely within a specified pressure range. Pressure ratings help determine which flange can safely withstand the operating conditions of a piping system.

Common pressure classes include:

ANSI / ASME Pressure Classes

Class 150
Class 300
Class 600
Class 900
Class 1500

Higher pressure classes generally feature:

Larger flange thickness
Stronger bolts
Greater sealing capability
Increased weight

DIN / ISO Pressure Ratings

European standards typically use:

PN6
PN10
PN16
PN20
PN25
PN40

The “PN” designation refers to the nominal pressure rating under standard operating conditions.

Selecting the wrong pressure class can lead to gasket failure, leakage, or equipment damage, making pressure compatibility just as important as physical dimensions.

Types of Flange Faces

The flange face determines how the gasket seals between two connected flanges. Different applications require different sealing surfaces.

Raised Face (RF)

Raised Face flanges are the most commonly used type. The slightly raised sealing surface concentrates gasket pressure, providing an excellent seal for most industrial applications.

Flat Face (FF)

Flat Face flanges have a completely flat sealing surface and are commonly paired with cast iron equipment to prevent cracking under bolt loads.

Ring Type Joint (RTJ)

RTJ flanges contain a machined groove that accommodates a metal ring gasket. They are designed for extremely high-pressure and high-temperature applications.

These flanges are commonly found in:

Refineries
Offshore platforms
Petrochemical facilities
High-pressure steam systems

Common Flange Materials

Material selection depends on pressure, temperature, corrosion resistance, and the fluid being transported.

Some of the most common flange materials include:

Carbon Steel
Stainless Steel
Alloy Steel
Cast Iron
Ductile Iron
PVC
CPVC
Brass
Bronze

Each material offers different advantages. Stainless steel provides excellent corrosion resistance, while carbon steel is often preferred for its strength and cost-effectiveness.

How to Select the Right Flange

Choosing the correct flange involves more than matching pipe diameter.

Consider the following factors:

Pipe size
Pressure rating
Operating temperature
Flange standard (ANSI, AS, DIN, ISO)
Pipe material
Flange material
Gasket compatibility
Bolt specifications
Fluid type
Installation environment

Balancing these factors ensures safe operation while avoiding unnecessary costs.

Tips for Choosing the Correct Flange

Before ordering a flange, verify every specification carefully.

Confirm the applicable flange standard.
Match the pressure rating with the system requirements.
Verify the nominal pipe size.
Check the bolt circle diameter.
Count the required bolt holes.
Select the correct flange face type.
Choose a compatible gasket.
Ensure the flange material suits the operating environment.
Review temperature and corrosion requirements.

Spending a few extra minutes checking specifications can prevent costly installation errors and downtime.

How to Measure Flange Size in MM

Accurate measurements are essential when selecting or replacing a flange. Even a small measurement error can result in poor alignment, gasket failure, or leaks. Fortunately, measuring a flange is straightforward if you know which dimensions to record.

1. Measure the Outside Diameter (OD)

Use a caliper or measuring tape to measure the flange’s widest point from one outer edge to the opposite edge.

2. Measure the Bolt Circle Diameter (BCD)

The Bolt Circle Diameter (also called the Pitch Circle Diameter or PCD) is the diameter of the imaginary circle passing through the center of all bolt holes.

This measurement is one of the most important when matching replacement flanges.

3. Count the Bolt Holes

Count the total number of bolt holes around the flange. Larger pressure classes generally require more bolts to distribute pressure evenly.

4. Measure the Bolt Hole Diameter

Measure the inside diameter of one bolt hole. This determines the correct bolt size needed during installation.

5. Identify the Nominal Pipe Size (NPS or DN)

Finally, identify the pipe size connected to the flange. The nominal pipe size helps determine the correct flange standard and dimensions.

Comparing these measurements with the flange size chart helps ensure an accurate replacement or new installation.

Flange Size Conversion

Flange sizes are commonly referenced using three measurement systems:

Inches
Millimeters (mm)
DN (Nominal Diameter)

Understanding these conversions makes it easier to compare international flange standards.

Common Flange Size Conversion

Nominal Size	Inches	DN (Approx.)
15 mm	1/2″	DN15
20 mm	3/4″	DN20
25 mm	1″	DN25
32 mm	1-1/4″	DN32
40 mm	1-1/2″	DN40
50 mm	2″	DN50
65 mm	2-1/2″	DN65
80 mm	3″	DN80
100 mm	4″	DN100
125 mm	5″	DN125
150 mm	6″	DN150
200 mm	8″	DN200
250 mm	10″	DN250
300 mm	12″	DN300
350 mm	14″	DN350
400 mm	16″	DN400
450 mm	18″	DN450
500 mm	20″	DN500
600 mm	24″	DN600

Converting Inches to Millimeters

Many flange standards use inch-based pipe sizes, while engineering drawings often specify dimensions in millimeters.

The basic conversion formula is:

1 inch = 25.4 mm

Examples:

Inches	Millimeters
1″	25.4 mm
2″	50.8 mm
4″	101.6 mm
6″	152.4 mm
8″	203.2 mm
10″	254 mm
12″	304.8 mm

Keep in mind that nominal pipe sizes are standardized designations and may not exactly match the pipe’s actual outside diameter.

Converting DN to MM

DN (Diameter Nominal) is the international metric designation used for pipes and flanges.

Unlike direct inch conversions, DN values represent nominal sizes rather than exact measurements.

Examples include:

DN15 ≈ 15 mm
DN20 ≈ 20 mm
DN50 ≈ 50 mm
DN100 ≈ 100 mm
DN150 ≈ 150 mm
DN300 ≈ 300 mm

Always verify flange dimensions using the applicable standard rather than relying solely on the DN designation.

Conclusion

Choosing the correct flange size is essential for creating safe, efficient, and reliable piping systems. By understanding flange dimensions, pressure ratings, face types, and international standards, you can confidently select components that fit your application. Whether you’re working with ANSI, AS, or DIN flanges, using an accurate flange size chart in millimeters and verifying every measurement before installation helps minimize leaks, reduce maintenance, and improve the long-term performance of your piping system.

FAQs

What is a flange size chart in mm used for?

A flange size chart in mm provides standardized dimensions for flanges, including outside diameter, bolt circle diameter, bolt size, and pressure class. It helps ensure compatible connections between pipes, valves, and equipment.

Are flange sizes standardized?

Yes. Most flange dimensions follow recognized standards such as ASME/ANSI B16.5, AS 2129, AS 4087, and ISO 7005 (DIN). Always confirm which standard your project requires.

Can I use ANSI and DIN flanges together?

Generally, no. ANSI and DIN flanges have different dimensions, bolt patterns, and pressure ratings. Mixing them can result in poor sealing and installation problems.

What is the difference between DN and NPS?

DN (Diameter Nominal) is the metric designation used internationally, while NPS (Nominal Pipe Size) is the North American inch-based designation. Although they correspond closely, they are not identical measurement systems.

How do I identify an unknown flange?

Measure the outside diameter, bolt circle diameter, bolt hole diameter, number of bolt holes, flange thickness, and pipe size. Then compare these measurements with a standard flange size chart.

Why is the pressure class important?

The pressure class determines the maximum pressure and temperature a flange can safely withstand. Using a flange with an insufficient pressure rating can lead to leaks or equipment failure.

Which flange face is most commonly used?

Raised Face (RF) flanges are the most widely used because they provide reliable sealing for a broad range of industrial piping systems.

What material is best for flanges?

The ideal material depends on the application. Carbon steel is commonly used for general industrial piping; stainless steel offers excellent corrosion resistance, while alloy steel, cast iron, and PVC are selected for specific operating conditions.