Flange Bolt Chart
Bolt count, bolt diameter and bolt circle for every ASME B16.5 size and pressure class — the numbers that are set by class and size alone, and never by the flange type.
- Standard
- ASME B16.5
- Classes
- 7
- Sizes
- NPS 1/2 – NPS 24
- Usual stud material
- ASTM A193 B7
- Usual nut material
- ASTM A194 2H
Bolt count and diameter are set by the pressure class and the size — not by the flange type. A Class 300 NPS 6 weld neck, slip-on, blind and lap joint flange all take the same 12 bolts of 3/4 in diameter on the same 10.62 in bolt circle. That is what makes the types interchangeable in a joint.
Bolt count and diameter
| Size | Class 150 | Class 300 | Class 400 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|---|
| NPS 1/2 | 4 × 1/2" | 4 × 1/2" | 4 × 1/2" | 4 × 1/2" | 4 × 3/4" | 4 × 3/4" | 4 × 3/4" |
| NPS 3/4 | 4 × 1/2" | 4 × 5/8" | 4 × 5/8" | 4 × 5/8" | 4 × 3/4" | 4 × 3/4" | 4 × 3/4" |
| NPS 1 | 4 × 1/2" | 4 × 5/8" | 4 × 5/8" | 4 × 5/8" | 4 × 7/8" | 4 × 7/8" | 4 × 7/8" |
| NPS 1 1/4 | 4 × 1/2" | 4 × 5/8" | 4 × 5/8" | 4 × 5/8" | 4 × 7/8" | 4 × 7/8" | 4 × 1" |
| NPS 1 1/2 | 4 × 1/2" | 4 × 3/4" | 4 × 3/4" | 4 × 3/4" | 4 × 1" | 4 × 1" | 4 × 1 1/8" |
| NPS 2 | 4 × 5/8" | 8 × 5/8" | 8 × 5/8" | 8 × 5/8" | 8 × 7/8" | 8 × 7/8" | 8 × 1" |
| NPS 2 1/2 | 4 × 5/8" | 8 × 3/4" | 8 × 3/4" | 8 × 3/4" | 8 × 1" | 8 × 1" | 8 × 1 1/8" |
| NPS 3 | 4 × 5/8" | 8 × 3/4" | 8 × 3/4" | 8 × 3/4" | 8 × 7/8" | 8 × 1 1/8" | 8 × 1 1/4" |
| NPS 3 1/2 | 8 × 5/8" | 8 × 3/4" | 8 × 7/8" | 8 × 7/8" | — | — | — |
| NPS 4 | 8 × 5/8" | 8 × 3/4" | 8 × 7/8" | 8 × 7/8" | 8 × 1 1/8" | 8 × 1 1/4" | 8 × 1 1/2" |
| NPS 5 | 8 × 3/4" | 8 × 3/4" | 8 × 7/8" | 8 × 1" | 8 × 1 1/4" | 8 × 1 1/2" | 8 × 1 3/4" |
| NPS 6 | 8 × 3/4" | 12 × 3/4" | 12 × 7/8" | 12 × 1" | 12 × 1 1/8" | 12 × 1 3/8" | 8 × 2" |
| NPS 8 | 8 × 3/4" | 12 × 7/8" | 12 × 1" | 12 × 1 1/8" | 12 × 1 3/8" | 12 × 1 5/8" | 12 × 2" |
| NPS 10 | 12 × 7/8" | 16 × 1" | 16 × 1 1/8" | 16 × 1 1/4" | 16 × 1 3/8" | 12 × 1 7/8" | 12 × 2 1/2" |
| NPS 12 | 12 × 7/8" | 16 × 1 1/8" | 16 × 1 1/4" | 20 × 1 1/4" | 20 × 1 3/8" | 16 × 2" | 12 × 2 3/4" |
| NPS 14 | 12 × 1" | 20 × 1 1/8" | 20 × 1 1/4" | 20 × 1 3/8" | 20 × 1 1/2" | 16 × 2 1/4" | — |
| NPS 16 | 16 × 1" | 20 × 1 1/4" | 20 × 1 3/8" | 20 × 1 1/2" | 20 × 1 5/8" | 16 × 2 1/2" | — |
| NPS 18 | 16 × 1 1/8" | 24 × 1 1/4" | 24 × 1 3/8" | 20 × 1 5/8" | 20 × 1 7/8" | 16 × 2 3/4" | — |
| NPS 20 | 20 × 1 1/8" | 24 × 1 1/4" | 24 × 1 1/2" | 24 × 1 5/8" | 20 × 2" | 16 × 3" | — |
| NPS 24 | 20 × 1 1/4" | 24 × 1 1/2" | 24 × 1 3/4" | 24 × 1 7/8" | 20 × 2 1/2" | 16 × 3 1/2" | — |
Bolt holes are always a multiple of four and are straddled about the flange centrelines, so no bolt sits on the vertical centreline.
Bolt circle diameter
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| Size | Class 150 | Class 300 | Class 400 | Class 600 | Class 900 | Class 1500 | Class 2500 |
|---|---|---|---|---|---|---|---|
| NPS 1/2 | 2.3860.45 mm | 2.6266.55 mm | 2.6266.55 mm | 2.6266.55 mm | 3.2582.55 mm | 3.2582.55 mm | 3.5088.90 mm |
| NPS 3/4 | 2.7569.85 mm | 3.2582.55 mm | 3.2582.55 mm | 3.2582.55 mm | 3.5088.90 mm | 3.5088.90 mm | 3.7595.25 mm |
| NPS 1 | 3.1279.25 mm | 3.5088.90 mm | 3.5088.90 mm | 3.5088.90 mm | 4.00101.60 mm | 4.00101.60 mm | 4.25107.95 mm |
| NPS 1 1/4 | 3.5088.90 mm | 3.8898.55 mm | 3.8898.55 mm | 3.8898.55 mm | 4.38111.25 mm | 4.38111.25 mm | 5.12130.05 mm |
| NPS 1 1/2 | 3.8898.55 mm | 4.50114.30 mm | 4.50114.30 mm | 4.50114.30 mm | 4.88123.95 mm | 4.88123.95 mm | 5.75146.05 mm |
| NPS 2 | 4.75120.65 mm | 5.00127.00 mm | 5.00127.00 mm | 5.00127.00 mm | 6.50165.10 mm | 6.50165.10 mm | 6.75171.45 mm |
| NPS 2 1/2 | 5.50139.70 mm | 5.88149.35 mm | 5.88149.35 mm | 5.88149.35 mm | 7.50190.50 mm | 7.50190.50 mm | 7.75196.85 mm |
| NPS 3 | 6.00152.40 mm | 6.62168.15 mm | 6.62168.15 mm | 6.62168.15 mm | 7.50190.50 mm | 8.00203.20 mm | 9.00228.60 mm |
| NPS 3 1/2 | 7.00177.80 mm | 7.25184.15 mm | 7.25184.15 mm | 7.25184.15 mm | — | — | — |
| NPS 4 | 7.50190.50 mm | 7.88200.15 mm | 7.88200.15 mm | 8.50215.90 mm | 9.25234.95 mm | 9.50241.30 mm | 10.75273.05 mm |
| NPS 5 | 8.50215.90 mm | 9.25234.95 mm | 9.25234.95 mm | 10.50266.70 mm | 11.00279.40 mm | 11.50292.10 mm | 12.75323.85 mm |
| NPS 6 | 9.50241.30 mm | 10.62269.75 mm | 10.62269.75 mm | 11.50292.10 mm | 12.50317.50 mm | 12.50317.50 mm | 14.50368.30 mm |
| NPS 8 | 11.75298.45 mm | 13.00330.20 mm | 13.00330.20 mm | 13.75349.25 mm | 15.50393.70 mm | 15.50393.70 mm | 17.25438.15 mm |
| NPS 10 | 14.25361.95 mm | 15.25387.35 mm | 15.25387.35 mm | 17.00431.80 mm | 18.50469.90 mm | 19.00482.60 mm | 21.25539.75 mm |
| NPS 12 | 17.00431.80 mm | 17.75450.85 mm | 17.75450.85 mm | 19.25488.95 mm | 21.00533.40 mm | 22.50571.50 mm | 24.38619.25 mm |
| NPS 14 | 18.75476.25 mm | 20.25514.35 mm | 20.25514.35 mm | 20.75527.05 mm | 22.00558.80 mm | 25.00635.00 mm | — |
| NPS 16 | 21.25539.75 mm | 22.50571.50 mm | 22.50571.50 mm | 23.75603.25 mm | 24.25615.95 mm | 27.75704.85 mm | — |
| NPS 18 | 22.75577.85 mm | 24.75628.65 mm | 24.75628.65 mm | 25.75654.05 mm | 27.00685.80 mm | 30.50774.70 mm | — |
| NPS 20 | 25.00635.00 mm | 27.00685.80 mm | 27.00685.80 mm | 28.50723.90 mm | 29.50749.30 mm | 32.75831.85 mm | — |
| NPS 24 | 29.50749.30 mm | 32.00812.80 mm | 32.00812.80 mm | 33.00838.20 mm | 35.50901.70 mm | 39.00990.60 mm | — |
Bolt length is not in B16.5
The standard fixes the hole pattern, not the stud length. Length depends on the two flange thicknesses, the gasket thickness and the nut height, and is normally rounded up to the next quarter inch with one to three threads showing beyond each nut. Compute it per joint rather than reading it off a chart.
Tightening
Bolts are tightened in a cross pattern in several passes, not in sequence around the circle, so the gasket is compressed evenly and the flange faces stay parallel. ASME PCC-1 gives the assembly procedure and the recommended pass sequence.
Common questions
Does the flange type change the bolt pattern?
No. Bolt count, bolt diameter and bolt circle depend only on the size and the pressure class. A weld neck, slip-on, blind and threaded flange of the same NPS and class share an identical bolt pattern.
Why are flange bolt holes always a multiple of four?
So the pattern straddles both centrelines and no bolt sits at top dead centre. That keeps the vertical and horizontal centrelines clear for orientation on drawings and makes flanges easier to align in the field.
How long should the stud bolts be?
B16.5 does not specify it. Add both flange thicknesses, the compressed gasket thickness and two nut heights, then round up to the next quarter inch. Aim for one to three threads visible past each nut.