What is the bolt circle diameter of a flange?

The diameter of the circle the bolt holes are centred on — the dimension that decides whether two flanges will bolt together. It is not the flange outside diameter and not the raised face. Two flanges of different pressure classes can share a bolt circle and still differ in thickness, and at several sizes on this page they do.

How much bigger is a bolt hole than its bolt?

One eighth of an inch, on all 792 rows here that publish both figures, across all seven pressure classes and every size. There are no exceptions. Publishers round the pair to two decimals, so the subtraction shows as 0.12, 0.13 or 0.125 — all of them an eighth.

Does a higher pressure class always mean more bolts?

No — at the large sizes it means fewer. At NPS 24 the count falls from 20 bolts in Class 150 to 16 in Class 1500, while the flange itself grows from 32.00 to 46.00 inches across. The bolts get thicker faster than the bolt circle grows, so fewer of them fit.

Will a Class 600 flange bolt to a Class 300 flange?

At the small sizes, dimensionally yes — several adjacent classes share an outside diameter, bolt circle, bolt count and hole size, and differ only in thickness. That makes them bolt together and look almost alike. It does not make them interchangeable in service: the pressure rating belongs to the assembly, and the thinner flange sets it.

Is this data from ASME B16.5 itself?

No, and that is stated rather than glossed. ASME B16.5 is a paid publication and we do not hold a copy, so every figure here comes from manufacturers and stockists reproducing it. Nothing on this page is marked verified — the strongest claim made is that two or more independent publishers agree and the geometry checks out. That is weaker than the steel dimensions elsewhere on this site, which rest on the standard itself.

Pipe Flange Dimensions

ASME B16.5 raised face, seven pressure classes, 816 rows. Two things in here are worth knowing before you look anything up: the bolt hole is always an eighth of an inch bigger than its bolt, and at the largest sizes a higher class means fewer bolts, not more.

What This Data Rests On — Read First

Nothing on this page is marked verified, on purpose. ASME B16.5 is a paid publication and no copy of it is held here, so every table behind this page is a manufacturer or stockist reproducing a standard we have not read. That decision was taken before any of it was parsed, not after the numbers came out looking tidy. The strongest claim made anywhere here is that 8 independent publisher groups were compared, two or more agree on the figure, and every geometric check passes. Elsewhere on this site — steel pipe sizes, for instance — verified means the standard itself. How this data was verified.

There is a second reason the top tier is unreachable here, and it is worth stating because it is unusual. Every other dataset on this site has an arithmetic backstop: a mass formula that re-derives the published weight from the dimensions and catches a crossed column. A flange is not a prism and no source publishes a formula to check a flange weight against, so that backstop does not exist. In its place this dataset leans on geometry — bolt circles that must fall between the raised face and the rim, holes that must fit inside the ring, bolt counts that must be a multiple of four, and a weld-neck hub that must equal the pipe outside diameter it joins.

The Bolt Hole Is Always an Eighth Bigger

Of the 792 rows that publish both a bolt diameter and a hole diameter, the hole is one eighth of an inch larger on 792 of them. Not most. All of them, across seven pressure classes and every nominal size in the range.

It is worth saying why that is not obvious from a catalogue. Sources round both figures to two decimal places, so the subtraction comes out as 0.12 at some sizes, 0.13 at others and 0.125 where a publisher kept three decimals. Read down a column and it looks like the clearance wanders. It does not — every one of those is an eighth of an inch, and the wandering is the rounding.

At the Big Sizes, a Higher Class Means Fewer Bolts

The intuition is that a higher pressure class means more bolts. At the large sizes it is the opposite, and the flange gets bigger while the count comes down.

NPS 24 across the pressure classes that carry it. The flange grows, the bolts grow faster, and fewer of them fit.
Class	Flange OD (in)	Bolts	Bolt diameter (in)
150	32.00	20	1.250
300	36.00	24	1.500
400	36.00	24	1.750
600	37.00	24	1.875
900	41.00	20	2.500
1500	46.00	16	3.500

The bolt circle grows too, but not as fast as the bolts thicken, so the number that will fit around it falls. This is the one place on the page where reading down a column and expecting a trend gets you the wrong answer.

Where Two Classes Are the Same Flange

Pressure classes are not evenly spaced in metal. Over a run of the smaller sizes, adjacent classes share an outside diameter, a bolt circle, a bolt count and a hole size — so they bolt together and look nearly alike, differing only in thickness, if at all.

Class 300 and Class 400: Identical bolt pattern across 8 sizes, NPS ½ to 3.
Class 400 and Class 600: Identical bolt pattern across 9 sizes, NPS ½ to 3 ½.
Class 900 and Class 1500: Identical bolt pattern across 7 sizes, NPS ½ to 2 ½.

Two flanges bolting together is not two flanges being interchangeable. The rating belongs to the joint, and the thinner of the two sets it — which is exactly why the coincidence is worth knowing about rather than relying on.

Bolt Patterns by Pressure Class

The bolt pattern is a property of the class and the size, not of the flange type: a weld neck, a slip on and a blind of the same class and size carry the same outside diameter, bolt circle, count and hole. What changes between types is the hub, the bore and the length through the hub.

Class 150

ASME B16.5 Class 150, raised face — 21 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	3.50	2.38	4	0.62
¾	3.88	2.75	4	0.62
1	4.25	3.12	4	0.62
1 ¼	4.62	3.50	4	0.62
1 ½	5.00	3.88	4	0.62
2	6.00	4.75	4	0.75
2 ½	7.00	5.50	4	0.75
3	7.50	6.00	4	0.75
3 ½	8.50	7.00	8	0.75
4	9.00	7.50	8	0.75
5	10.00	8.50	8	0.88
6	11.00	9.50	8	0.88
8	13.50	11.75	8	0.88
10	16.00	14.25	12	1.00
12	19.00	17.00	12	1.00
14	21.00	18.75	12	1.12
16	23.50	21.25	16	1.12
18	25.00	22.75	16	1.25
20	27.50	25.00	20	1.25
22	29.50	27.25	20	1.38
24	32.00	29.50	20	1.38

Class 300

ASME B16.5 Class 300, raised face — 21 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	3.75	2.62	4	0.63
¾	4.62	3.25	4	0.75
1	4.88	3.50	4	0.75
1 ¼	5.25	3.88	4	0.75
1 ½	6.12	4.50	4	0.88
2	6.50	5.00	8	0.75
2 ½	7.50	5.88	8	0.88
3	8.25	6.62	8	0.88
3 ½	9.00	7.25	8	0.88
4	10.00	7.88	8	0.88
5	11.00	9.25	8	0.88
6	12.50	10.62	12	0.88
8	15.00	13.00	12	1.00
10	17.50	15.25	16	1.12
12	20.50	17.75	16	1.25
14	23.00	20.25	20	1.25
16	25.50	22.50	20	1.38
18	28.00	24.75	24	1.38
20	30.50	27.00	24	1.38
22	33.00	29.25	24	1.63
24	36.00	32.00	24	1.63

Class 400

ASME B16.5 Class 400, raised face — 21 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	3.75	2.62	4	0.63
¾	4.62	3.25	4	0.75
1	4.88	3.50	4	0.75
1 ¼	5.25	3.88	4	0.75
1 ½	6.12	4.50	4	0.88
2	6.50	5.00	8	0.75
2 ½	7.50	5.88	8	0.88
3	8.25	6.62	8	0.88
3 ½	9.00	7.25	8	1.00
4	10.00	7.88	8	1.00
5	11.00	9.25	8	1.00
6	12.50	10.62	12	1.00
8	15.00	13.00	12	1.12
10	17.50	15.25	16	1.25
12	20.50	17.75	16	1.38
14	23.00	20.25	20	1.38
16	25.50	22.50	20	1.50
18	28.00	24.75	24	1.50
20	30.50	27.00	24	1.62
22	33.00	29.25	24	1.75
24	36.00	32.00	24	1.88

Class 600

ASME B16.5 Class 600, raised face — 20 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	3.75	2.62	4	0.63
¾	4.62	3.25	4	0.75
1	4.88	3.50	4	0.75
1 ¼	5.25	3.88	4	0.75
1 ½	6.12	4.50	4	0.88
2	6.50	5.00	8	0.75
2 ½	7.50	5.88	8	0.88
3	8.25	6.62	8	0.88
3 ½	9.00	7.25	8	1.00
4	10.75	8.50	8	1.00
5	13.00	10.50	8	1.12
6	14.00	11.50	12	1.12
8	16.50	13.75	12	1.25
10	20.00	17.00	16	1.38
12	22.00	19.25	20	1.38
14	23.75	20.75	20	1.50
16	27.00	23.75	20	1.63
18	29.25	25.75	20	1.75
20	32.00	28.50	24	1.75
24	37.00	33.00	24	2.00

Class 900

ASME B16.5 Class 900, raised face — 19 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	4.75	3.25	4	0.88
¾	5.12	3.50	4	0.88
1	5.88	4.00	4	1.00
1 ¼	6.25	4.38	4	1.00
1 ½	7.00	4.88	4	1.12
2	8.50	6.50	8	1.00
2 ½	9.62	7.50	8	1.12
3	9.50	7.50	8	1.00
4	11.50	9.25	8	1.25
5	13.75	11.00	8	1.38
6	15.00	12.50	12	1.25
8	18.50	15.50	12	1.50
10	21.50	18.50	16	1.50
12	24.00	21.00	20	1.50
14	25.25	22.00	20	1.63
16	27.75	24.25	20	1.75
18	31.00	27.00	20	2.00
20	33.75	29.50	20	2.13
24	41.00	35.50	20	2.63

Class 1500

ASME B16.5 Class 1500, raised face — 19 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	4.75	3.25	4	0.88
¾	5.12	3.50	4	0.88
1	5.88	4.00	4	1.00
1 ¼	6.25	4.38	4	1.00
1 ½	7.00	4.88	4	1.12
2	8.50	6.50	8	1.00
2 ½	9.62	7.50	8	1.12
3	10.50	8.00	8	1.25
4	12.25	9.50	8	1.38
5	14.75	11.50	8	1.63
6	15.50	12.50	12	1.50
8	19.00	15.50	12	1.75
10	23.00	19.00	12	2.00
12	26.50	22.50	16	2.12
14	29.50	25.00	16	2.38
16	32.50	27.75	16	2.63
18	36.00	30.50	16	2.88
20	38.75	32.75	16	3.12
24	46.00	39.00	16	3.63

Class 2500

ASME B16.5 Class 2500, raised face — 14 sizes. The bolt pattern is a property of the class, not of the flange type: a weld neck and a blind of the same class and size bolt identically.
NPS	Flange OD (in)	Bolt circle (in)	Bolts	Hole (in)
½	5.25	3.50	4	0.88
¾	5.50	3.75	4	0.88
1	6.25	4.25	4	1.00
1 ¼	7.25	5.13	4	1.12
1 ½	8.00	5.75	4	1.25
2	9.25	6.75	8	1.12
2 ½	10.50	7.75	8	1.25
3	12.00	9.00	8	1.38
4	14.00	10.75	8	1.63
5	16.50	12.75	8	1.88
6	19.00	14.50	8	2.12
8	21.75	17.25	12	2.12
10	26.50	21.25	12	2.63
12	30.00	24.38	12	2.88

One Word, Two Thicknesses

“Flange thickness” is published two different ways under one name. Some sources measure it excluding the raised face, some including it, and at Classes 150 and 300 the two differ by a sixteenth of an inch — enough to look like a disagreement between publishers when it is a difference in what is being measured.

Almost nobody labels which one they mean. This dataset carries them as two separate fields and never averages them, because averaging a measurement with a different measurement of a different thing produces a number describing nothing. Above Class 400 the ambiguity disappears: no publisher includes the raised face there.

What We Could Not Verify

199 of 816 rows are published carrying a visible label rather than dropped, and the rest sit one tier below that.

ASME B16.5 ring-joint (RTJ) facing: ring numbers, groove dimensions and RTJ flange heights are not compiled. Sources carry stud lengths for RTJ but not the ring geometry.
flange weights: coastalflange and projectmaterials publish approximate weights. A flange is not a prism and there is no published formula here to check a weight against, so nothing was carried. The mass gate that guards every other dataset on this site does not exist for this one.
stud bolt lengths: published by metalspiping, texasflange and projectmaterials. They depend on the gasket, which is not part of B16.5, and the three do not use one basis.
ASME B16.47 (NPS 26 and above): Series A and B are a different standard with two incompatible bolt patterns at the same size. Not in scope and must not be blended into a B16.5 table.
EN 1092-1: acquired opportunistically only — valvias PN 6-40, roymech PN 16, pipingpipeline type 01, projectmaterials type 11, one globalsupplyline PDF. No type 05 or blind coverage worth the name. Needs its own pass.
octalflange and hu-steel: every dimension table is a raster scan or a JPG; /Font count is 0 in all four PDFs. OCR of a dimension table is the failure this project has logged three times.
flangeboltchart.com: a Next.js app whose server HTML contains only whole-number NPS rows. Every fractional size is absent without executing JavaScript.
htpipe.com: suspected machine-generated: hedging prose, an edition year outside B16.5's revision cadence, and an invented 'shell closure requirements' justification. Excluded so it cannot count towards agreement.
vishalsteel.com: demonstrated composite scrape of three other publishers. Not parsed at all.
wermac.org: 20 transposed per-size pages, metric-rounded, and its A/D/B/G/K/H/L symbols are defined nowhere on the page. Reading them would be the parser inventing a categorical.
globalsupplyline.com.au PDFs: six class charts with a real text layer, the best non-US-lineage cross-check available. Not parsed in this pass; the highest-value next addition.
simupipe, pipingx, apexflowsolutions, apiint, creativepiping, baleng: too small to test for copying, or no dimension table met the row threshold in server HTML.

Several publisher defects turned up on the way and are worth recording, because they are the reason this page does not simply reprint whichever chart came first: one source files an entire pressure class under the wrong heading, another drops a table cell on eighty-two rows so that every column after the bore slides one place left, and a third prints its own service-weight figures under a heavier heading. All of them produce tables that look internally consistent. Everything found that way is at errors found in published pipe data.

The other way of joining a pipe without a thread is to weld a fitting to it, and that one does rest on a standard we hold — see pipe fitting dimensions, where the evidence is a good deal stronger than it is here.

A weld-neck flange welds to the pipe, so its hub at the welding end is the pipe’s own outside diameter — which is how every weld-neck row here is checked against steel pipe sizes, a test no flange publisher can influence. For the diameters themselves see the full pipe size chart, and for why the nominal number is not a measurement, nominal pipe size explained.

Frequently Asked Questions

What is the bolt circle diameter of a flange?: The diameter of the circle the bolt holes are centred on — the dimension that decides whether two flanges will bolt together. It is not the flange outside diameter and not the raised face. Two flanges of different pressure classes can share a bolt circle and still differ in thickness, and at several sizes on this page they do.
How much bigger is a bolt hole than its bolt?: One eighth of an inch, on all 792 rows here that publish both figures, across all seven pressure classes and every size. There are no exceptions. Publishers round the pair to two decimals, so the subtraction shows as 0.12, 0.13 or 0.125 — all of them an eighth.
Does a higher pressure class always mean more bolts?: No — at the large sizes it means fewer. At NPS 24 the count falls from 20 bolts in Class 150 to 16 in Class 1500, while the flange itself grows from 32.00 to 46.00 inches across. The bolts get thicker faster than the bolt circle grows, so fewer of them fit.
Will a Class 600 flange bolt to a Class 300 flange?: At the small sizes, dimensionally yes — several adjacent classes share an outside diameter, bolt circle, bolt count and hole size, and differ only in thickness. That makes them bolt together and look almost alike. It does not make them interchangeable in service: the pressure rating belongs to the assembly, and the thinner flange sets it.
Is this data from ASME B16.5 itself?: No, and that is stated rather than glossed. ASME B16.5 is a paid publication and we do not hold a copy, so every figure here comes from manufacturers and stockists reproducing it. Nothing on this page is marked verified — the strongest claim made is that two or more independent publishers agree and the geometry checks out. That is weaker than the steel dimensions elsewhere on this site, which rest on the standard itself.