What Is ASME B16.21?

Quick Answer

ASME B16.21 is the American standard for non-metallic flat gaskets used with ASME B16.5 and B16.47 pipe flanges. It covers soft gasket dimensions (ID, OD, thickness, bolt hole patterns) for raised face and flat face flanges in Classes 150 through 2500. The standard does not specify gasket materials; that choice is left to the engineer.

ASME B16.21 works in tandem with ASME B16.20 (metallic gaskets). Together, the two standards cover every gasket type used on ASME flanges. While B16.20 governs metallic and semi-metallic gaskets (SWG, RTJ, Kammprofile), B16.21 addresses flat gaskets made entirely from non-metallic materials.

Scope and Coverage

Attribute	Detail
Full title	Nonmetallic Flat Gaskets for Pipe Flanges
Gasket types	Full-face and ring-type flat gaskets
Materials	Standard does not mandate specific materials; common choices include PTFE, flexible graphite, compressed fiber, rubber
Flange size range	NPS 1/2 through NPS 24 (ASME B16.5)
Pressure classes	150, 300, 600, 900, 1500, 2500
Flange faces	Raised face (RF), flat face (FF)
Companion standard	ASME B16.20 (metallic gaskets)

Gasket Dimensions per ASME B16.21

The standard tabulates ID, OD, and bolt hole locations for two gasket configurations:

Configuration	Where Used	OD Limit	Bolt Holes
Ring gasket	RF flanges	Extends to inner edge of bolt holes	No holes in gasket
Full-face gasket	FF flanges	Extends to flange OD	Holes match flange bolt pattern

For ring gaskets on RF flanges, the gasket OD is sized to sit within the bolt circle so that the bolts pass outside the gasket. For full-face gaskets on FF flanges, the gasket has punched holes aligned with the flange bolt holes and covers the entire face.

Standard Thicknesses

ASME B16.21 recognizes three nominal gasket thicknesses:

Nominal Thickness	Metric Equivalent	Typical Use
1/16 inch	1.5 mm	Standard; preferred for most applications
1/8 inch	3.0 mm	Rough or warped flange faces
1/4 inch	6.0 mm	Heavily damaged or misaligned flanges (not recommended for new construction)

Thinner gaskets are always preferred. A thinner gasket has less material to creep, less volume to blow out, and generates higher seating stress for the same bolt load. Specifying 1/4” gaskets as standard practice is a common but avoidable mistake.

What ASME B16.21 Does Not Cover

The standard defines dimensions only. It does not:

• Specify gasket material or quality
• Define pressure-temperature ratings for gaskets (these come from the material manufacturer)
• Cover semi-metallic gaskets (spiral wound gaskets fall under B16.20)
• Cover RTJ gaskets (also B16.20)
• Provide installation torque values (refer to ASME PCC-1)

Material selection is governed by the process design conditions (fluid, pressure, temperature) and the project’s piping material specification.

Relationship to Other Standards

Standard	Role
ASME B16.5	Defines flange dimensions that B16.21 gaskets fit
ASME B16.20	Companion standard for metallic/semi-metallic gaskets
ASME B31.3	Process piping code referencing B16.21
ASME PCC-1	Bolt assembly guidelines including gasket seating stress
EN 1514-1	European equivalent for non-metallic gaskets on EN 1092-1 flanges

Pro Tip

On purchase orders, always specify the gasket material alongside the ASME B16.21 dimensional reference. Writing “gasket per ASME B16.21, NPS 4, Class 150, ring type, 1/16” thick, flexible graphite” eliminates ambiguity. Stating only “gasket per B16.21” leaves the material to the vendor’s discretion.

Properly dimensioned soft gaskets paired with correctly torqued stud bolts and hex nuts ensure a leak-free joint in low-to-moderate pressure service.

Read the full guide to gasket selection