ASME B16.5 · Standard reference
ASME B16.5-2025 flange tolerances (Section 7)
Section 7 of ASME B16.5-2025 sets the manufacturing and inspection tolerances for flanges and flanged fittings: centre-to-contact dimensions, facings, flange thickness, welding-end and hub dimensions, bores and counterbores, and drilling. The standard prints metric values first with the inch values in parentheses; the tables below carry both, exactly as the limits are printed.
The tolerances are conformance limits, not process instructions. Section 7.1 says values are rounded per ASTM Practice E29 and that listing a decimal tolerance does not prescribe a measuring method, so the same limits apply whether a dimension is checked with a calliper, a micrometer or a CMM. Several limits are one-sided: flange thickness, for example, has a plus tolerance and a zero minus tolerance, and the hub at the welding end has a steel floor of 87.5% of nominal pipe wall instead of ± limits.
Section 7 is one part of the acceptance picture. Ring-joint grooves are toleranced in Table 5 (Table 5C), facing roughness is in Section 6.4.5, and bolting bearing surfaces are controlled by Section 6.6. The tables below cover the whole of Section 7 — including 7.8 on drilling and facing, which continues onto printed page 15 — with the related finish rules listed separately.
- Standard
- ASME B16.5-2025
- Section
- 7 — Tolerances
- Sizes covered
- NPS ½ to NPS 24
- Thickness tolerance
- +3.0 / −0.0 mm, NPS ≤ 18
- Bolt circle
- ±1.5 mm (0.06 in.)
- Hub floor
- 87.5% of nominal pipe wall
Centre-to-contact, facing and thickness tolerances (7.2–7.4)
These limits fix the relationship between the flange faces, the facing outside diameters and the flange thickness. They are read with the figures in Section 6: centre-to-contact and centre-to-end dimensions apply to fittings and to ring-joint flanges, while the facing diameters apply to raised faces and tongue-and-groove or female facings.
B16.5 · ASME B16.5-2025
| Feature | Clause | Size range | Tolerance, mm (in.) |
|---|---|---|---|
| Centre-to-contact surface, other than ring joint | 7.2(a) | NPS ≤ 10 | ±1.0 (±0.04) |
| Centre-to-contact surface, other than ring joint | 7.2(a) | NPS ≥ 12 | ±1.5 (±0.06) |
| Centre-to-end, ring joint | 7.2(b) | NPS ≤ 10 | ±1.0 (±0.04) |
| Centre-to-end, ring joint | 7.2(b) | NPS ≥ 12 | ±1.5 (±0.06) |
| Contact-surface-to-contact-surface, other than ring joint | 7.2(c) | NPS ≤ 10 | ±2.0 (±0.08) |
| Contact-surface-to-contact-surface, other than ring joint | 7.2(c) | NPS ≥ 12 | ±3.0 (±0.12) |
| End-to-end, ring joint | 7.2(d) | NPS ≤ 10 | ±2.0 (±0.08) |
| End-to-end, ring joint | 7.2(d) | NPS ≥ 12 | ±3.0 (±0.12) |
| Inside and outside diameter, large and small tongue-and-groove and female facings | 7.3.1(a) | All sizes | ±0.5 (±0.02) |
| Outside diameter, 1.5 mm (0.06 in.) raised face | 7.3.1(b) | All sizes | ±1.0 (±0.04) |
| Outside diameter, 6.4 mm (0.25 in.) raised face | 7.3.1(c) | All sizes | ±0.5 (±0.02) |
| Ring-joint groove | 7.3.1(d) | All sizes | Per Table 5 (Table 5C) |
| Perpendicularity of face with bore | 7.3.2 | NPS ≤ 5 | 1° |
| Perpendicularity of face with bore | 7.3.2 | NPS ≥ 6 | 0.5° |
| Flange thickness t | 7.4 | NPS ≤ 18 | +3.0, −0.0 (+0.12, −0.00) |
| Flange thickness t | 7.4 | NPS ≥ 20 | +5.0, −0.0 (+0.20, −0.00) |
- a The plus thickness tolerance applies to bolting bearing surfaces whether as-forged, as-cast, spot-faced or back-faced (see para. 6.6).
Metric values are printed first with the inch values in parentheses; both are reproduced. Plus and minus signs are as printed: where only a plus value is shown, no negative tolerance is permitted. The ring-joint groove is toleranced in Table 5 (Table 5C), not in Section 7.
Welding-end, hub and bore tolerances (7.5–7.7)
These tolerances govern the end that is welded to the pipe: the welding-end outside and inside diameters, the backing-ring contact bore, the length through the hub and the bore of lapped, slip-on and threaded flanges. Dimensions A, B and C refer to the figures in Section 6, and the dimension letters in the table should be matched against the figure for the flange type being inspected.
B16.5 · ASME B16.5-2025
| Feature | Clause | Size range | Tolerance, mm (in.) |
|---|---|---|---|
| Welding-end outside diameter, dimension A, Figures 7 and 8 | 7.5.1 | NPS ≤ 5 | +2.0, −1.0 (+0.08, −0.04) |
| Welding-end outside diameter, dimension A, Figures 7 and 8 | 7.5.1 | NPS ≥ 6 | +4.0, −1.0 (+0.16, −0.04) |
| Welding-end inside diameter, dimension B, Figures 4, 7 and 8 | 7.5.2(a) | NPS ≤ 10 | ±1.0 (±0.04) |
| Welding-end inside diameter, dimension B, Figures 4, 7 and 8 | 7.5.2(a) | 12 ≤ NPS ≤ 18 | ±1.5 (±0.06) |
| Welding-end inside diameter, dimension B, Figures 4, 7 and 8 | 7.5.2(a) | NPS ≥ 20 | +3.0, −1.5 (+0.12, −0.06) |
| Inside diameter, dimension B, Figure 9 | 7.5.2(b) | NPS ≤ 10 | +0.0, −1.0 (+0.0, −0.04) |
| Inside diameter, dimension B, Figure 9 | 7.5.2(b) | NPS ≥ 12 | +0.0, −1.5 (+0.0, −0.06) |
| Backing-ring contact bore, dimension C, Figures 9 and 10 | 7.5.3 | 2 ≤ NPS ≤ 24 | +0.25, −0.0 (+0.01, −0.0) |
| Length through hub, welding-neck flanges | 7.6 | NPS ≤ 4 | ±1.5 (±0.06) |
| Length through hub, welding-neck flanges | 7.6 | 5 ≤ NPS ≤ 10 | +1.5, −3.0 (+0.06, −0.12) |
| Length through hub, welding-neck flanges | 7.6 | NPS ≥ 12 | +3.0, −5.0 (+0.12, −0.20) |
| Lapped and slip-on flange bore | 7.7.1 | NPS ≤ 10 | +1.0, −0.0 (+0.04, −0.0) |
| Lapped and slip-on flange bore | 7.7.1 | NPS ≥ 12 | +1.5, −0.0 (+0.06, −0.0) |
| Threaded flange counterbore | 7.7.2 | NPS ≤ 10 | +1.0, −0.0 (+0.04, −0.0) |
| Threaded flange counterbore | 7.7.2 | NPS ≥ 12 | +1.5, −0.0 (+0.06, −0.0) |
| Socket-welding flange counterbore | 7.7.3 | ½ ≤ NPS ≤ 3 | ±0.25 (±0.010) |
- a Hub thickness at the welding end (7.5.4): despite the tolerances on dimensions A and B, the hub thickness at the welding end must not be less than 87.5% of the nominal pipe wall thickness, with a 12.5% under-tolerance for the pipe wall, or the minimum wall specified by the purchaser. This is a floor, not a ± tolerance.
The hub thickness rule in 7.5.4 overrides the A and B tolerances: however much the listed minus tolerances would allow, the hub at the welding end must not fall below 87.5% of the nominal pipe wall. Bores for lapped and slip-on flanges (7.7.1) are toleranced differently from welding-end bores (7.5.2).
Drilling and facing tolerances (7.8)
Bolt-circle geometry is controlled in three separate ways: the diameter of the bolt circle, the centre-to-centre spacing of adjacent holes, and the concentricity of the bolt circle with the machined facing. The standard prints a single limit, not a ± tolerance, for concentricity.
B16.5 · ASME B16.5-2025
| Feature | Clause | Size range | Tolerance, mm (in.) |
|---|---|---|---|
| Bolt circle diameter | 7.8.1 | All sizes | ±1.5 (±0.06) |
| Centre-to-centre of adjacent bolt holes | 7.8.2 | All sizes | ±0.8 (±0.03) |
| Bolt circle concentricity with machined facing | 7.8.3 | NPS ≤ 2½ | 0.8 (0.03) |
| Bolt circle concentricity with machined facing | 7.8.3 | NPS ≥ 3 | 1.5 (0.06) |
The first two limits apply to all sizes. The concentricity limit is a single value in each size range; the standard prints no ± sign for it.
Rules that sit outside the tables
- Rounding (7.1): for determining conformance, observed or calculated values are rounded to the nearest unit in the last right-hand digit used to express the limit, as defined in ASTM Practice E29. Listing a decimal tolerance does not imply a particular method of measurement.
- Thickness (7.4): the plus tolerance is applied at the bolting bearing surfaces whether they are as-forged, as-cast, spot-faced or back-faced. Spot facing or back facing must not reduce the flange thickness below the tabulated minimum (para. 6.6) and follows MSS SP-9.
- Hub thickness (7.5.4): the floor of 87.5% of nominal pipe wall overrides the plus/minus limits on dimensions A and B at the welding end.
- Ring-joint grooves (7.3.1(d)): groove dimensions and tolerances are not in Section 7; they are tabulated in Table 5 (Table 5C).
- Facing perpendicularity (7.3.2): the flange face must be square to the bore within 1° up to NPS 5 and within 0.5° from NPS 6.
- Application: Section 7 limits the manufactured product. Pipe-end tolerances come from the pipe standards, and a flange that passes these limits can still be outside a piping code requirement.
Facing finish that goes with the tolerances (6.4.5–6.6)
Section 7 controls dimensions; the facing surface itself is controlled by the finish clauses in Section 6. These values are the ones an inspector will compare with a visual comparator, so they belong with the dimensional acceptance criteria even though they are printed in a different clause.
- Tongue-and-groove and small male-and-female facings: gasket contact surface not rougher than 3.2 µm (125 µin.) (6.4.5.1).
- Ring-joint grooves: side-wall finish of the gasket groove not rougher than 1.6 µm (63 µin.) (6.4.5.2).
- Other facings: serrated concentric or serrated spiral finish with a resultant surface finish of 3.2 µm to 6.3 µm (125 to 250 µin.) average roughness; cutting tool radius about 1.5 mm (0.06 in.) or larger, and 1.8 to 2.2 grooves/mm (45 to 55 grooves/in.) (6.4.5.3).
- Facing imperfections are limited by Table 3 (Table 3C); adjacent imperfections must be separated by at least four times the maximum radial projection (6.4.6).
- Bolting bearing surfaces must be parallel to the flange face within 1° (6.6).
ASME B16.5 tolerances — questions
Is there a minus tolerance on B16.5 flange thickness?
No. Para. 7.4 gives +3.0/−0.0 mm (+0.12/−0.00 in.) for NPS 18 and smaller and +5.0/−0.0 mm (+0.20/−0.00 in.) for NPS 20 and larger. Thickness may exceed nominal but must not fall below it, and the plus tolerance applies at the bolting bearing surfaces whether they are as-forged, as-cast, spot-faced or back-faced.
What tolerance applies to the bolt circle and bolt holes?
Bolt circle diameter: ±1.5 mm (±0.06 in.) for all sizes (7.8.1). Centre-to-centre of adjacent bolt holes: ±0.8 mm (±0.03 in.) (7.8.2). Concentricity of the bolt circle with the machined facing: 0.8 mm (0.03 in.) up to NPS 2½ and 1.5 mm (0.06 in.) from NPS 3 (7.8.3).
How tight is the hub length on a welding-neck flange?
Length through hub: ±1.5 mm (±0.06 in.) up to NPS 4, +1.5/−3.0 mm (+0.06/−0.12 in.) from NPS 5 to NPS 10, and +3.0/−5.0 mm (+0.12/−0.20 in.) for NPS 12 and larger (7.6). Separately, the hub thickness at the welding end must be at least 87.5% of nominal pipe wall (7.5.4).
Does Section 7 also control the facing finish?
Not directly: Section 7 controls dimensions such as facing outside diameters and perpendicularity. Facing roughness is set by Section 6.4.5 — for example a maximum 3.2 µm (125 µin.) on tongue-and-groove and small male-and-female gasket contacts, 1.6 µm (63 µin.) on ring-joint groove side walls, and a 3.2–6.3 µm (125–250 µin.) serrated finish for other facings.