How this instrument works
A bolt circle is a set of holes spaced evenly around an imaginary circle — the pattern on a wheel hub, a pipe flange, a motor mounting face or a round electrical box cover. Machinists and fabricators describe it with just two numbers: the circle's diameter and how many holes sit on it. Everything else — where each individual hole actually lands — follows from simple geometry once those two numbers, plus a starting angle, are fixed.
The angle between adjacent holes is always 360° divided by the hole count, because n evenly spaced holes split a full circle into n equal wedges. A 4-hole pattern steps 90° apart, a 6-hole pattern steps 60° apart, a 5-hole pattern steps 72° apart — the spacing is fixed by the count alone and has nothing to do with the circle's size.
Locating any individual hole in x/y coordinates from the circle's center needs one more piece of trigonometry: multiply the radius by the cosine of that hole's angle to get x, and by the sine of that angle to get y, measuring the angle counterclockwise from the 3 o'clock position, the standard convention in CAD and CNC work. This instrument applies that to hole 1 at whatever start angle you enter, so you can drop that coordinate straight into a program or a layout without doing the trig by hand.
Bolt circles show up constantly in mechanical design because evenly spaced fasteners share load symmetrically around a center — a wheel, a flange or a bearing housing wants every bolt carrying roughly the same share of the clamping force, which only happens if the holes are truly equally spaced.
- Enter the circle size into Bolt circle diameter (in) — the diameter of the imaginary circle the hole centers sit on, not the outside edge of the part.
- Enter how many fasteners go around it into Number of holes — the instrument needs at least 3 to define a circle's worth of spacing.
- Set Start angle of hole 1 (deg, measured from 3 o'clock, CCW) — 0° puts hole 1 due right of center; 90° puts it straight up.
- Read Angle between holes for the layout step, and Bolt circle radius for half your entered diameter.
- Read Hole 1 — x coordinate from center and Hole 1 — y coordinate from center for its position relative to the circle's center; add the step angle repeatedly to locate the remaining holes.
Worked example — a 6-inch, 4-hole bolt circle
Enter 6 into Bolt circle diameter (in), 4 into Number of holes, and leave Start angle of hole 1 at its default of 0°. Bolt circle radius reads 3 in exactly (half of 6), and Angle between holes reads 90.0° — the standard spacing for a 4-bolt square pattern, since 360° split four ways is 90° each.
Hole 1 — x coordinate from center reads 3.0000 in and Hole 1 — y coordinate from center reads 0.0000 in. At a 0° start angle, hole 1 sits due right of the circle's center at exactly the radius distance, since cos(0°)=1 and sin(0°)=0 place it squarely on the positive x-axis. Add the 90° step three more times — 90°, 180°, 270° — to lay out holes 2 through 4 around the same 3-inch radius.
Questions
Why does the angle between holes only depend on the hole count?
Because n evenly spaced holes always divide a full 360° circle into n equal wedges, regardless of how large or small that circle is. A 4-hole pattern always steps 90° apart and a 6-hole pattern always steps 60° apart, whether the bolt circle is 2 inches or 20 inches across — the diameter only changes how far apart the holes are physically, not the angle between them.
What does the start angle actually control?
It sets where hole 1 sits around the circle, measured counterclockwise from the 3 o'clock direction — the standard zero-reference in CAD and CNC layout. Changing it rotates the entire pattern rigidly; the angle between holes and the radius stay identical, only the coordinates of every hole shift together. A drawing that calls out hole 1 at a specific clock position needs that position entered here to match.
How do I find the coordinates of holes 2, 3 and beyond?
Add the step angle to hole 1's start angle repeatedly, then recompute x = r × cos(angle) and y = r × sin(angle) for each result — this instrument only outputs hole 1 directly, since every other hole follows the same formula at a shifted angle. For the default 4-hole, 0°-start example, hole 2 sits at 90°, hole 3 at 180°, and hole 4 at 270°.
Why is the diameter measured hole-center to hole-center, not edge to edge?
Because 'bolt circle diameter' by convention describes the imaginary circle that passes through every hole's center, not the physical edge of the part. Using the part's outer diameter instead would place every hole too far from center, which is a common layout mistake when a drawing only shows the flange's overall size rather than the bolt circle dimension explicitly.
Does the bolt circle diameter affect the angle between holes?
No — the angle between holes comes only from the hole count (360° divided by n). The diameter only scales how far each hole sits from center and, therefore, how far apart the holes are in inches along the circle's circumference. Two bolt circles with the same hole count always share the same angular spacing even if one is twice the size of the other.
What's a typical minimum number of holes, and why?
Three is the practical minimum for a genuine bolt circle, since two points don't define a meaningfully spaced circular pattern and this instrument requires at least 3 to compute a step angle. Three equally spaced holes (120° apart) is a common pattern on smaller flanges and motor mounts where full symmetry with fewer fasteners is enough to distribute the clamping load.