Lap Joint Flange Dimensions

A loose backing flange used with a stub end. The flange never contacts the fluid — it only carries bolt load — so it can be carbon steel behind a stainless or alloy stub end, and it swivels freely for bolt-hole alignment.

Standard: ASME B16.5
Abbreviation: LJ
Pressure classes: 7
Size range: NPS 1/2 – NPS 24
Typical service: Systems dismantled often, or alloy lines where flange cost matters.

Lap Joint Flange in section, and a Class 150 flange from the front. The section shows what each column of the dimension tables measures, with welds in orange; it is not to scale. The front view is an NPS 6 Class 150 flange drawn to scale, with its bolt holes straddling the centrelines.

Dimensions by pressure class

Class 150NPS 1/2 – 2420 sizes Class 300NPS 1/2 – 2420 sizes Class 400NPS 1/2 – 2420 sizes Class 600NPS 1/2 – 2420 sizes Class 900NPS 1/2 – 2419 sizes Class 1500NPS 1/2 – 2419 sizes Class 2500NPS 1/2 – 1214 sizes

Strengths

Spins freely, so bolt holes always line up — ideal for large or awkward spools
Cheap carbon steel flange behind an expensive alloy stub end
Easy to dismantle for cleaning or inspection

Limitations

Weakest of the six in fatigue
Requires a matching stub end, adding a component

Class 150 dimensions

Dimensions are given in inches, with the millimetre equivalent beneath in grey.

ASME B16.5 Class 150 lap joint flange dimensions — the most commonly specified class.
Size	Flange OD	Thickness	Bolt circle	Bolts	Bolt dia.	Raised face OD
NPS 1/2	3.5088.90 mm	0.389.65 mm	2.3860.45 mm	4	1/2 in	1.3835.05 mm
NPS 3/4	3.8898.55 mm	0.4411.18 mm	2.7569.85 mm	4	1/2 in	1.6942.93 mm
NPS 1	4.25107.95 mm	0.5012.70 mm	3.1279.25 mm	4	1/2 in	2.0050.80 mm
NPS 1 1/4	4.62117.35 mm	0.5614.22 mm	3.5088.90 mm	4	1/2 in	2.5063.50 mm
NPS 1 1/2	5.00127.00 mm	0.6215.75 mm	3.8898.55 mm	4	1/2 in	2.8873.15 mm
NPS 2	6.00152.40 mm	0.6917.53 mm	4.75120.65 mm	4	5/8 in	3.6291.95 mm
NPS 2 1/2	7.00177.80 mm	0.8120.57 mm	5.50139.70 mm	4	5/8 in	4.12104.65 mm
NPS 3	7.50190.50 mm	0.8822.35 mm	6.00152.40 mm	4	5/8 in	5.00127.00 mm
NPS 3 1/2	8.50215.90 mm	0.8822.35 mm	7.00177.80 mm	8	5/8 in	5.50139.70 mm
NPS 4	9.00228.60 mm	0.8822.35 mm	7.50190.50 mm	8	5/8 in	6.19157.23 mm
NPS 5	10.00254.00 mm	0.8822.35 mm	8.50215.90 mm	8	3/4 in	7.31185.67 mm
NPS 6	11.00279.40 mm	0.9423.88 mm	9.50241.30 mm	8	3/4 in	8.50215.90 mm
NPS 8	13.50342.90 mm	1.0626.92 mm	11.75298.45 mm	8	3/4 in	10.62269.75 mm
NPS 10	16.00406.40 mm	1.1228.45 mm	14.25361.95 mm	12	7/8 in	12.75323.85 mm
NPS 12	19.00482.60 mm	1.1930.23 mm	17.00431.80 mm	12	7/8 in	15.00381.00 mm
NPS 14	21.00533.40 mm	1.3133.27 mm	18.75476.25 mm	12	1 in	16.25412.75 mm
NPS 16	23.50596.90 mm	1.3835.05 mm	21.25539.75 mm	16	1 in	18.50469.90 mm
NPS 18	25.00635.00 mm	1.5038.10 mm	22.75577.85 mm	16	1 1/8 in	21.00533.40 mm
NPS 20	27.50698.50 mm	1.6241.15 mm	25.00635.00 mm	20	1 1/8 in	23.00584.20 mm
NPS 24	32.00812.80 mm	1.8145.97 mm	29.50749.30 mm	20	1 1/4 in	27.25692.15 mm

Higher classes are on the Class 300 page and above.

Guides and comparisons for this page

Lap joint vs slip-on flangeA loose backing flange against a welded one.Choosing a flange face typeRaised face, flat face or ring-type joint, by service.Flange bolt torqueTarget torque from bolt stress, for every B16.5 bolt size.Pressure-temperature deratingReading a P-T table and interpolating between temperatures.

Common questions

When should I use a lap joint flange?

Systems dismantled often, or alloy lines where flange cost matters.

What pressure classes does a lap joint flange come in?

ASME B16.5 publishes the lap joint flange in Class 150, Class 300, Class 400, Class 600, Class 900, Class 1500 and Class 2500, in sizes NPS 1/2 through NPS 24, and through NPS 12 in Class 2500. Above NPS 24 large-diameter flanges are covered by ASME B16.47 instead.

Does the flange face type change these dimensions?

The flange outside diameter, bolt circle and bolting are the same whatever the face. What changes is the facing itself and the flange thickness measurement: B16.5 thickness excludes the raised face, so a Class 300 raised-face flange is 0.06 in thicker overall than the tabulated figure. Ring-type joint flanges are grooved instead and are dimensioned separately.