FittingKey

Fittings · BSPP (G)

British Standard Pipe Parallel, BSPP or G (ISO 228-1)

G 1/4 G 1/4 (19 TPI, 55 degree)

1/4 in trade size. Parallel thread, sealed on a face gasket, parallel thread, 55 degree flank angle. Basic major diameter 13.16 mm / 0.518 in.

Identify a fitting you are holdingAll BSPP (G) sizes
Measure across the crests
13 mm to 13.2 mm
Basic 13.16 mm / 0.518 in

The visual tell

What the end of the fitting looks like

Parallel thread, sealed on a face gasket, on a parallel threadWasher at the hex, plain noseParallel threadOpen end on the left
Schematic cross-section of the male half, upper side only, with the centre line dashed and the open end on the left. Cone angles are drawn at their true angle from the axis; nothing else is to scale and no dimension should be measured off it. Thread shown: G 1/4 (19 TPI, 55 degree).

A straight thread with a flat shoulder or a chamfered face behind it, carrying a bonded seal washer or an O-ring.

Where it seals: A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself.

Dimensions

Thread size, pitch and diameters for G 1/4.
DesignationG 1/4
ThreadG 1/4 (19 TPI, 55 degree)
Threads per inch / pitch19 TPI (1.337 mm pitch)
Basic major diameter (male)13.16 mm / 0.518 in
Measured across the male crests13 mm to 13.2 mm
Basic minor diameter (mating female)11.44 mm / 0.451 in - computed from the 55 degree thread profile, not measured
Thread form55 degree flank angle, parallel
SeatParallel thread, sealed on a face gasket
How the size is namedSized by trade size like NPT, and like NPT the number is not the thread diameter: G 1/4 measures about 13.16 mm across the threads.

Working pressure

No working pressure is published for this connection. Ratings for it vary by manufacturer, body material, port form and seal, and no two independent sources in the register give the same figure under conditions they both state. A generic number here would be a decoration, not a rating. Read the pressure from the catalogue of the fitting you are actually installing.

A fitting with two different ends is only ever as strong as the weaker end, and any rating assumes the fitting is undamaged, correctly assembled and within its temperature range. Pressure ratings are safety-relevant: confirm them against the manufacturer specification before the system is charged.

What it mates with

A female G 1/4 (19 TPI, 55 degree) parallel port. A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself. A parallel thread run into a parallel port with nothing at the face will leak no matter how hard it is tightened.

A parallel thread cannot seal on itself. If the bonded washer or O-ring is missing, the joint will leak however tight it is, and tightening it further only strips the port.

Do not mix these up

Fittings that measure like a G 1/4

Every size below sits within 0.9 mm of this one across the male thread crests, which is inside what a worn or plated fitting and a hurried measurement produce together. Those marked same thread will screw together with this fitting.

Fittings in other standards whose male thread diameter is close to G 1/4.
FittingThreadBasic major ØSeatWill it thread on?
R 1/4 · BSPT (R)R 1/4 (19 TPI, 55 degree, taper 1:16)13.16 mmTapered thread, sealed on the threadSame thread - it will screw together and the seal will be wrong
-04 JIS 30 (G 1/4) · JIS 30 degree flare (BSPP)G 1/4 (19 TPI, 55 degree)13.16 mm30 degree flare cone or seatSame thread - it will screw together and the seal will be wrong
-05 JIC · JIC 37 degree flare1/2-20 UNF12.7 mm37 degree flare coneNo - 0.46 mm apart on diameter, and a different pitch
-05 SAE ORB · SAE O-ring boss1/2-20 UNF12.7 mmO-ring boss (straight thread port)No - 0.46 mm apart on diameter, and a different pitch
5/16 in SAE 45 · SAE 45 degree flare1/2-20 UNF12.7 mm45 degree flare coneNo - 0.46 mm apart on diameter, and a different pitch
1/4 NPT · NPT / NPTF1/4-18 NPT13.72 mmTapered thread, sealed on the threadNo - 0.56 mm apart on diameter, and a different pitch

This list is computed from the published diameters in the database rather than kept as a hand-written list, so a correction to a table can never leave a stale warning behind.

Answers

Common questions about G 1/4

What thread is a G 1/4 fitting?

G 1/4 runs a G 1/4 (19 TPI, 55 degree) thread: 19 TPI (1.337 mm pitch), basic major diameter 13.16 mm / 0.518 in. Published identification charts read 13 mm to 13.2 mm across the male crests, which is the figure a caliper produces.

How do I tell a G 1/4 apart from a fitting that measures the same?

Look at the sealing surface. A straight thread with a flat shoulder or a chamfered face behind it, carrying a bonded seal washer or an O-ring. A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself. The thread is parallel (straight) and the flank angle is 55 degrees.

What does a G 1/4 screw into?

A female G 1/4 (19 TPI, 55 degree) parallel port. A bonded seal washer or O-ring at the face. The parallel thread cannot seal on itself. A parallel thread run into a parallel port with nothing at the face will leak no matter how hard it is tightened.

What pressure is a G 1/4 rated for?

No working pressure is published here for this connection. Ratings vary by manufacturer, body material and seal, and no two independent sources in the register give the same figure under stated conditions for it. Read the rating from the catalogue of the fitting you are installing.

BSPP (G)

Other sizes in this standard

G 1/161/16 in trade sizeG 1/16 (28 TPI, 55 degree)G 1/81/8 in trade sizeG 1/8 (28 TPI, 55 degree)G 3/83/8 in trade sizeG 3/8 (19 TPI, 55 degree)G 1/21/2 in trade sizeG 1/2 (14 TPI, 55 degree)G 5/85/8 in trade sizeG 5/8 (14 TPI, 55 degree)G 3/43/4 in trade sizeG 3/4 (14 TPI, 55 degree)G 7/87/8 in trade sizeG 7/8 (14 TPI, 55 degree)G 11 in trade sizeG 1 (11 TPI, 55 degree)G 1 1/41 1/4 in trade sizeG 1 1/4 (11 TPI, 55 degree)G 1 1/21 1/2 in trade sizeG 1 1/2 (11 TPI, 55 degree)G 22 in trade sizeG 2 (11 TPI, 55 degree)

Where these figures come from

Dimensions are as defined by ISO 228-1, BS 2779. FittingKey is an independent reference and is not affiliated with, endorsed by or approved by any standards body or fitting manufacturer.

Every value on this page was checked against 7 independent published sources:

Which published source supplied each column, and what it gave.
ColumnSourcesWhat they gave
Basic major diameteriso-228-1, amesweb-bspp, machiningdoctor-bspMajor, pitch and minor diameters read directly from ISO 228-1:2000 Table 1, and matched to three decimal places by two independent reproductions.
Measured male thread diametergerrard-id, adaptall-british, gates-idchart, apex-threadidFour identification charts. Three print identical figures at every size; the fourth rounds up by 0.1 mm. The span covers all four.
Working pressuregates-idchart, mcgill-idNeither identification source gives a working pressure for a BSPP connection, and none is published. It depends on the port form and the seal, not on the thread.