Piping
JIS 20K flange dimensions
In Japanese Industrial Standards (JIS), a 20K 50A flange has an outside diameter (OD) of 155 mm. Its PCD (bolt circle diameter) is 120 mm, with 8 bolt holes of 19 mm diameter for M16 bolts. The pressure class is 20K, and the nominal size is 50A.
This page lists JIS 20K steel pipe flanges to JIS B 2220 (also written JIS B2220), from 10A to 300A. Estimated hex bolt and stud bolt lengths and the matching JIS B 2404 gasket sizes are also included.
JIS 20K flange dimensions and bolt lengths
All dimensions in mm. Bolt lengths are estimates. They assume two flanges of the same size, a 3 mm gasket, no washers and JIS B 1181 style 1 nuts. The bolt sticks out 3 threads beyond each nut. Each length is rounded up to the next 5 mm.
| Size | B | OD D | PCD C | Holes | Hole h | Bolt | t | Hex bolt L | Stud bolt L |
|---|---|---|---|---|---|---|---|---|---|
| 10A | 3/8 | 90 | 65 | 4 | 15 | M12 | 14 | 50 | 65 |
| 15A | 1/2 | 95 | 70 | 4 | 15 | M12 | 14 | 50 | 65 |
| 20A | 3/4 | 100 | 75 | 4 | 15 | M12 | 16 | 55 | 70 |
| 25A | 1 | 125 | 90 | 4 | 19 | M16 | 16 | 60 | 80 |
| 32A | 1 1/4 | 135 | 100 | 4 | 19 | M16 | 18 | 60 | 85 |
| 40A | 1 1/2 | 140 | 105 | 4 | 19 | M16 | 18 | 60 | 85 |
| 50A | 2 | 155 | 120 | 8 | 19 | M16 | 18 | 60 | 85 |
| 65A | 2 1/2 | 175 | 140 | 8 | 19 | M16 | 20 | 65 | 85 |
| 80A | 3 | 200 | 160 | 8 | 23 | M20 | 22 | 75 | 100 |
| 90A | 3 1/2 | 210 | 170 | 8 | 23 | M20 | 24 | 80 | 105 |
| 100A | 4 | 225 | 185 | 8 | 23 | M20 | 24 | 80 | 105 |
| 125A | 5 | 270 | 225 | 8 | 25 | M22 | 26 | 85 | 110 |
| 150A | 6 | 305 | 260 | 12 | 25 | M22 | 28 | 90 | 115 |
| 200A | 8 | 350 | 305 | 12 | 25 | M22 | 30 | 90 | 120 |
| 250A | 10 | 430 | 380 | 12 | 27 | M24 | 34 | 105 | 135 |
| 300A | 12 | 480 | 430 | 16 | 27 | M24 | 36 | 110 | 140 |
Source: JIS B 2220:2012, Table 18.
Inch nominal sizes (B): JIS G 3452:2019, Table 4.
Nut heights for the bolt lengths: JIS B 1181:2014, Tables 3 and 4 (style 1).
Coarse thread pitches for the bolt lengths: JIS B 0205-2:2001.
Checked against a copy of the original JIS (Japanese text). JIS B 2220 values were also compared with 5 published sources on 6 October 2026. The count is for all pressure classes together. How the values are checked
Gaskets for JIS 20K flanges
All dimensions in mm. These are the sizes of JIS B 2404 sheet gaskets. Ring gaskets are used on raised-face (RF) and flat-face (FF) flanges. Full-face gaskets are mainly used on FF flanges.
20K has no full-face gasket in JIS B 2404. JIS B 2220 also has no flat face (FF) for 20K flanges. Spiral wound and PTFE-jacketed gaskets have different sizes.
| Size | ID D2 | Ring OD D3 |
|---|---|---|
| 10A | 18 | 53 |
| 15A | 22 | 58 |
| 20A | 28 | 63 |
| 25A | 35 | 74 |
| 32A | 43 | 84 |
| 40A | 49 | 89 |
| 50A | 61 | 104 |
| 65A | 84 | 124 |
| 80A | 90 | 140 |
| 90A | 102 | 150 |
| 100A | 115 | 165 |
| 125A | 141 | 203 |
| 150A | 167 | 238 |
| 200A | 218 | 283 |
| 250A | 270 | 356 |
| 300A | 321 | 406 |
Source: JIS B 2404:2018, Table 9.
Checked against a copy of the original JIS (Japanese text). JIS B 2404 values were also compared with 2 published sources on 6 October 2026. The count is for all pressure classes together. How the values are checked
Notes on the tables
- Sizes are given as A (metric nominal size) and B (inch nominal size). 50A is the same size as 2B.
- The number of bolts is the same as the number of bolt holes.
- On RF flanges, thickness t includes the height f of the raised face (JIS B 2220, Table 13). For 50A, f is 2 mm. If another source gives t without the raised face, add f before you compare.
- For 20K, JIS B 2220 Table 8 has no flat face (FF). 20K flanges have a raised face (RF) or another type of face. Names in catalogues and drawings can differ, so check the drawing or the flange itself.
- The thickness tolerance is plus only (JIS B 2220, Table 22). It is +1.5 mm for 20 mm or less, and +2 mm for over 20 mm up to 50 mm. For RF flanges, it applies to t − f. Real flanges can be thicker than the table, so allow some extra bolt length.
- 20K is the nominal pressure (pressure class). It is not the allowed working pressure, which depends on the material and the temperature (pressure-temperature ratings in JIS B 2220).
- JIS flanges are a Japanese system. Before you fit a JIS flange to an ASME or EN flange of the same size, check the PCD and the bolt holes.
- There is no 175A or 225A in 20K (JIS B 2220, Table 12). 90A is included.
- The bolt lengths are calculated lengths, rounded up. Stock bolt lengths differ by maker, so check the lengths you can buy.
How bolt length is calculated
Hex bolt length is measured under the head. Stud bolt length is the overall length.
Hex bolt L = t + t + G + m + k × P
Stud bolt L = t + t + G + 2m + 2 × k × P
Example: 20K 50A (M16, t = 18)
Hex = 18 + 18 + 3 + 14.8 + 3 × 2 = 59.8 → 60 mm
Stud = 18 + 18 + 3 + 2 × 14.8 + 2 × 3 × 2 = 80.6 → 85 mm
- t
- flange thickness (JIS B 2220, Table 18; RF includes the raised face)
- G
- gasket thickness
- m
- nut height (JIS B 1181, style 1, maximum)
- k × P
- threads beyond the nut (number of threads × coarse pitch)
The result is rounded up to the next 5 mm.