Flange Size Calculator
Pick the nominal pipe size, the pressure class and the unit. The calculator gives the flange outside diameter, bolt circle diameter, number of bolts, bolt size, bolt hole size and the spacing between holes.
- Flange dimensions
- Bolt circle
- Bolt count and size
Dimensions are for ASME B16.5 weld neck, slip-on and blind flanges, which share the same bolt pattern.
Flanges of the same pipe size and class always share the same bolt pattern. That is what lets flanges from different makers bolt together. A 4 in Class 150 flange is 9 in across with eight 5/8 in bolts on a 7.5 in bolt circle. In Class 300 the same pipe size grows to 10 in across with eight 3/4 in bolts.
Class 150 flange chart
ASME B16.5. Search for a pipe size.
| NPS (in) | OD | Bolt circle | Bolts | Bolt size (in) |
|---|---|---|---|---|
| 1/2 | 3.5 | 2.375 | 4 | 1/2 |
| 3/4 | 3.875 | 2.75 | 4 | 1/2 |
| 1 | 4.25 | 3.125 | 4 | 1/2 |
| 1 1/2 | 5 | 3.875 | 4 | 1/2 |
| 2 | 6 | 4.75 | 4 | 5/8 |
| 3 | 7.5 | 6 | 4 | 5/8 |
| 4 | 9 | 7.5 | 8 | 5/8 |
| 6 | 11 | 9.5 | 8 | 3/4 |
| 8 | 13.5 | 11.75 | 8 | 3/4 |
| 10 | 16 | 14.25 | 12 | 7/8 |
| 12 | 19 | 17 | 12 | 7/8 |
Dimensions in inches.
Pipe size and flange
Class 150. Slide through the pipe sizes.
The flange face shows its bolt holes on the bolt circle.
How to read flange sizes
Pipe to bolts. Step through it.
Pipe size
NPS.
Use new gaskets every time a flange joint is opened.
Class 150, 300 or 600
Pressure classes. Compare them.
Low pressure.
Medium.
High.
Pressure ratings fall as temperature rises, so check the rating table for your temperature.
Before assembly
Flange checks. Tick each item.
Tick each item as you go.
Tighten bolts in a star pattern in several passes so the gasket compresses evenly.
What you get
Flange lookup. Each tile explains one result.
Outside.
Diameter.
Count and size.
Diameter.
Between holes.
Hole spacing is the bolt circle times the sine of 180° divided by the bolt count.
Frequently Asked Questions
No question matches that search.
What is a flange bolt circle?
The diameter of the circle through the centres of the bolt holes.
How many bolts does a four inch flange have?
Eight, in both Class 150 and Class 300.
What size bolts for a six inch Class 150 flange?
Eight bolts of three quarters of an inch.
How big are flange bolt holes?
One eighth of an inch bigger than the bolt.
Can Class 150 and Class 300 flanges bolt together?
No. Their bolt patterns differ.
What does NPS mean?
Nominal pipe size, the standard name for a pipe's size.
What is ASME B16.5?
The US standard for pipe flanges from half an inch to twenty-four inches.
Bolt up the right flange
Look up the dimensions by size and class, and every flange, gasket and bolt matches.
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