Flange Dimensions & Bolt Chart

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Four numbers define a flange's bolting geometry: the outside diameter, the bolt circle diameter (the circle through the centres of the bolt holes), the number of holes, and the hole diameter. Add the flange thickness and you have everything a fitter needs to check a mating pair. Two flanges of the same NPS and class from different makers bolt together because ASME B16.5 fixes all five numbers; two flanges of different classes almost never do — compare the two tables below and the bolt circles diverge at every size.

Use the tables to check a specification, a drawing or a delivered flange. When ordering from the flange range, the governing standard and class on your enquiry are what the product is confirmed against — the wall thickness that pairs with a weld neck bore is a separate question, covered by the pipe schedule chart, and facings are covered by the flange face guide.

Class 150 Dimensions (ASME B16.5-2020, Table 8C)

Class 300 Dimensions (ASME B16.5-2020, Table 11C)

Reading the Bolt Chart

The bolts per joint equal the number of holes, and the bolt diameter is the hole diameter minus the standard 1/8 inch drilling clearance — a 0.75 inch hole takes a 5/8 inch bolt. Bolt length is not in the tables because it belongs to the joint, not the flange: the stud must pass through two flange thicknesses, the gasket, two nut heights, and still show thread beyond the nuts. Worked example, NPS 2 Class 150 raised face: each flange's minimum thickness tf is 0.69 inch and its raised face adds a further 1/16 inch, so two flanges give 1.50 inch of grip, a 1/8 inch gasket adds 0.13, two 5/8 inch nuts add about 1.10, and thread protrusion about 0.25 — a total of roughly 3.0 inch, so 3-1/4 inch studs are the practical selection. Treat any calculated length as approximate: round up to the next standard length and confirm against the project bolting specification.

Thickness and Raised Face Notes

The first thickness column is tf, the minimum flange thickness of ASME B16.5-2020 for the class (Table 8C for Class 150, Table 11C for Class 300) — the figure the class's flanges are dimensioned from, with one exception: lap joint flanges carry their own thickness column, covered below. Under §6.4.1 of the standard, Class 150 and Class 300 flanges are regularly furnished with a 1/16 inch raised face, and that raised face is in addition to tf — the tabulated thickness is measured without it, so a raised-face flange measures tf plus 1/16 inch across the face. Section 6.4.2.1 states the same rule from the other side: the minimum flange thickness tf shall be provided first, and the raised face then added to it. Some published tables print a second thickness figure beside tf: on the Texas Flange tables that second figure (their TJ column) is the lap joint thickness covered below, not a version of tf with the raised face included — tf itself never includes the raised face. Lap joint flanges are not dimensioned from tf at all: Tables 8C and 11C print a separate minimum thickness column for them, greater than tf at every size in these tables, and that column is printed here beside tf, so a lap joint flange is read from its own column. Lap joint flanges are also furnished with flat faces under §6.4.3, so the raised-face note above does not apply to them. On Class 400 and above the raised face stands 1/4 inch and is likewise in addition to tf — which is one reason high-class joints are assembled with more care about gasket compression. B16.5 covers NPS 1/2 to 24 in classes up to 2500; large diameters continue in ASME B16.47, whose bolt circles differ again. Metric PN flanges follow EN 1092-1 and share neither the bolt circles nor the thicknesses of the class system — a PN16 flange does not bolt to a Class 150 flange of the same nominal size.

Dimensions Are Not an Order

These tables describe the standard, not a warehouse. A specific flange product — its type, class, facing, bore and grade — is confirmed against your enquiry before order, as each page in the flange range states. Table sources: ASME B16.5-2020, Table 8C (Class 150) and Table 11C (Class 300); the values are cross-checked against the B16.5 flange tables in the Texas Flange & Fitting Product Catalog, Ver 5.00, pp. 3–4, whose thickness column T is the same tf used here and whose TJ column is the same lap joint thickness printed beside it. Verify tabulated values against ASME B16.5-2020 before machining mating parts or cutting gaskets.

Frequently Asked Questions

Will a Class 150 flange bolt up to a Class 300 flange of the same size?

No. The bolt circle, hole count and flange diameter all differ — at NPS 2, for example, Class 150 has 4 holes on a 4.75 inch circle and Class 300 has 8 holes on a 5.00 inch circle. The two classes are different joints, not different strengths of one joint.

What bolt size fits an NPS 2 Class 150 flange?

Four bolts of 5/8 inch diameter per joint: the holes are drilled 0.75 inch and bolts run 1/8 inch under the hole diameter. Length follows the joint make-up — see the worked example above.

Are the dimensions in these tables the sizes you stock?

The tables are ASME B16.5 reference dimensions for checking and specifying. They are not a stock list: the size, class and execution of a specific product are confirmed against your enquiry before order.

Where can I find dimensions for Class 600 and above, or PN flanges?

Class 600 to 2500 follow the same B16.5 tables in the standard itself, and large diameters are in B16.47; PN flanges are dimensioned by EN 1092-1. If your specification uses those systems, name the standard and class or PN on the enquiry and the execution will be confirmed for the specific product.

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