Free engineering tool
Bolt Circle Calculator
Enter the bolt circle diameter and number of holes to get the X/Y position of every hole and the center-to-center spacing between neighboring holes. Measuring an unknown flange? Use the reverse mode to get the bolt circle from the spacing you measured between two neighboring holes, then compare it with the Texas Flange flange dimension tables.
Blank = straddle centerlines (flange practice)
Hole spacing
2.8701 in
Bolt circle 7.5000 in · 8 holes every 45.00° · neighbor spacing 2.8701 in · opposite holes 7.5000 in apart
| Hole | Angle | X | Y |
|---|---|---|---|
| 1 | 22.50° | 3.4645 | 1.4351 |
| 2 | 67.50° | 1.4351 | 3.4645 |
| 3 | 112.50° | -1.4351 | 3.4645 |
| 4 | 157.50° | -3.4645 | 1.4351 |
| 5 | 202.50° | -3.4645 | -1.4351 |
| 6 | 247.50° | -1.4351 | -3.4645 |
| 7 | 292.50° | 1.4351 | -3.4645 |
| 8 | 337.50° | 3.4645 | -1.4351 |
Origin at the flange center, 0° on the +X axis, angles counterclockwise.
Reference tool for planning and quoting. Confirm against the purchase specification and your site's procedures before use.
Request a flange quote
Frequently asked questions
How do you calculate the spacing between bolt holes?
Multiply the bolt circle diameter by the sine of 180 divided by the number of holes. For an 8-hole pattern on a 7.5 inch bolt circle that is 7.5 × sin(22.5°) ≈ 2.870 inches.
How do I find the bolt circle diameter of a flange?
On an even-hole flange, measure from the center of one hole to the center of the hole directly opposite. On any flange you can measure the spacing between two neighboring hole centers and divide by sin(180/number of holes).
What does it mean that flange bolt holes straddle the centerline?
Standard pipe flanges place the holes so no hole sits on the vertical or horizontal centerline. The first hole is offset by half the hole spacing, for example 45° on a 4-hole flange or 22.5° on an 8-hole flange.
How do I identify an unknown flange?
Measure the outside diameter, bolt circle, number and size of bolt holes and the thickness, then compare them with standard flange dimension tables or send them to Texas Flange for help.
