What Is a Coupling? Full vs Half Coupling

Quick Answer

A coupling is a short forged fitting used to connect two pipes in a straight line. A full coupling has internal connections (socket weld or threaded) on both ends, joining two pipe segments end to end. A half coupling has an internal connection on one end and a plain or butt weld end on the other, creating a branch outlet when welded to a header pipe. Both types are manufactured per ASME B16.11 in sizes NPS 1/8 to NPS 4.

Couplings are fundamental fittings in small-bore piping systems. They appear in instrument connections, vent and drain assemblies, utility lines, and anywhere socket weld or threaded joints are used.

Full Coupling vs Half Coupling

Feature	Full Coupling	Half Coupling
Connections	Both ends (SW or threaded)	One end (SW or threaded), one plain/BW end
Function	Joins two pipes in-line	Creates branch outlet from header
Bore	Through-bore	Blind bore (one end welded to header)
Typical use	Pipe extension, repair joints	Instrument taps, vent/drain outlets
Welded to header	No	Yes (fillet weld around base)

ASME B16.11 Specifications

Property	Detail
Standard	ASME B16.11
Size range	NPS 1/8 to NPS 4
Pressure classes	2000#, 3000#, 6000# (socket weld); 2000#, 3000#, 6000# (threaded)
End types	Socket weld (SW), threaded (NPT)
Materials	ASTM A105 (CS), A182 F304/F316 (SS), A182 F11/F22 (alloy), A350 LF2 (low-temp)
Manufacturing	Forged and machined

Dimensions (ASME B16.11)

NPS	OD (mm)	Length - Full Coupling (mm)	Length - Half Coupling (mm)	Socket Depth (mm)
1/2	27.7	38.1	22.4	9.7
3/4	34.0	38.1	25.4	9.7
1	42.2	47.8	28.4	11.2
1-1/2	55.6	57.2	34.5	12.7
2	68.3	63.5	38.1	14.2

Applications

Full coupling uses:

• Joining two threaded pipe lengths in utility and instrument piping
• Repair joints where a section of pipe is replaced
• Inline connections in small-bore socket weld piping

Half coupling uses:

• Instrument taps on headers and vessels (pressure, temperature connections)
• Vent connections at high points
• Drain connections at low points
• Sample points for process fluid sampling
• Thermowell boss connections

Half couplings welded to header pipes are an economical alternative to threadolets and sockolets for branch connections on smaller headers (NPS 2 to NPS 6). On larger headers, o-let fittings with contoured bases provide better stress distribution.

Reducing Couplings

A reducing coupling has different nominal sizes on each end (e.g., NPS 1 x NPS 3/4). This allows inline pipe size reduction in threaded or socket weld systems. Reducing couplings serve the same purpose as swage nipples but with both ends as female connections rather than male.

Configuration	Connection Type
Full coupling (equal)	Same NPS both ends
Reducing coupling	Different NPS each end
Half coupling	One end to header, one end receives pipe

Pressure Classes

ASME B16.11 pressure classes determine wall thickness:

Class	Socket Weld	Threaded
2000#	Not available for SW	Light duty
3000#	Standard	Standard
6000#	Heavy duty, high pressure	Heavy duty, high pressure

Class 3000# is the most common specification for general industrial and oil and gas applications. Class 6000# is reserved for high-pressure services and critical instrument connections.

Read the full guide to pipe fittings