CustomPartNet
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August 27, 2026
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Updated August 27, 2026
When you're laying out a ring of mounting holes, two questions come up constantly: what's the actual diameter of that circle, and where exactly does each hole go? CustomPartNet's Bolt Circle Diameter calculator and Bolt Circle Layout calculator answer both, and together they cover the full workflow from designing or programming a bolt pattern to reverse-engineering one.
Bolt circles show up everywhere in mechanical design: flanges, wheel hubs, sprockets, motor mounts, fixture plates, and any part where fasteners need to sit evenly around a center point. The problem is that you often can't measure straight across the circle, especially with an odd number of holes where there's no hole directly opposite another one. And once you know the diameter you want, it’s easy to make errors in the trigonometry when figuring out the exact X,Y position of every hole by hand, especially when under the pressure of a deadline.
These two widgets split that workflow into its natural halves. One measures an existing pattern. The other generates a new one.
The Bolt Circle Diameter calculator solves a specific, common problem: you have a part in front of you, you can measure the straight-line distance between two adjacent bolt holes, and you need to know the diameter of the circle they sit on. This comes up constantly with brake rotors, sprockets, and flanges where the geometry makes a direct center-to-center measurement impossible.
Inputs:
Distance between two adjacent holes (the chord length)
Measurement Type (how you’re measuring the distance)
Number of holes in the pattern
Output:
Bolt circle diameter (BCD)
The calculator allows you to enter the “back-to-back” distance of two adjust holes under “Measurement Type”, which means the distance between the outer edges of two holes. This is especially useful because you can simply place two gauge pins or drill bits into the holes and measure the distance with some calipers. The same applies to threaded holes or bolts, just use the major diameter instead.
The math behind it is simple trigonometry. For N holes evenly spaced around a circle, the angle between adjacent holes is 360 divided by N, and the chord connecting them relates to the diameter through the sine of half that angle. The calculator handles this instantly so you're not pulling out a calculator and a geometry reference mid-inspection.
Worked example: Say you're measuring an automotive wheel hub with 5 lug holes, the hole diameter is 6mm, and the back-to-back distance between two adjacent holes comes out to 82.5 mm. Punch in the numbers, and the calculator returns a bolt circle diameter of approximately 130 mm, a standard 5x130 pattern. Without the tool, you'd need to divide 360 by 5 to get 72 degrees between holes, then divide that chord by the sine of 36 degrees to back into the diameter. The calculator skips straight to the answer.
Once you know the bolt circle diameter you're designing to, the Bolt Circle Layout calculator goes the other direction. It takes a BCD, a hole count, and a starting angle, and returns the X,Y coordinate of every hole. It puts the coordinates in NC format as well, ready to drop into a CNC program.
Inputs:
Bolt circle diameter
Number of holes
Starting angle (where the first hole sits, measured from the standard reference axis)
Center offsets (used when your bolt circle origin is not on your global origin)
Hole diameter (optional, for visualization purposes)
Output:
X,Y coordinates for every hole in the pattern, plus the underlying angular formulas
Worked example: Suppose you're designing a mounting flange with 6 holes on a 12 in bolt circle, and you want the first hole to start at 30 degrees. The bolt circle center is also offset from the global origin by 5 and 3 inches in the x- and y-directions respectively. The calculator updates coordinates automatically, such as (10.2, 6.0) for the first hole, (5.0, 9.0) for the second, and so on around the full circle. Instead of re-deriving six sine and cosine values by hand, you adjust one input and get a clean, NC-ready set of positions.
For a design engineer, the Layout calculator turns a spec ("8 holes on a 150 mm BCD") into exact geometry you can place in a drawing without manual trig. For a machinist or programmer, the same output maps directly onto point-to-point positioning in a CNC program, which is especially useful for hole patterns that don't line up neatly with standard canned cycles. And for anyone doing reverse engineering, whether that's matching an existing flange, replicating a legacy fixture, or verifying a supplier's part against a drawing, the Diameter calculator gets you from a tape measure reading to a working dimension in seconds.
If you find yourself working with the same or similar patterns repeatedly, registering for a free account allows you save "presets", which are a set of inputs that you can name and save for easy access anytime in the future.
Both widgets are free to use and live alongside the rest of CustomPartNet's calculator library, along with a full supplier network if a bolt circle layout you're working out turns into a part you need manufactured.
What is a bolt circle diameter?
The bolt circle diameter, sometimes called pitch circle diameter (PCD), is the diameter of the imaginary circle that passes through the center of each hole in an evenly spaced hole pattern.
Why can't I just measure across the bolt circle directly?
With an odd number of holes, there's no hole positioned directly opposite another one, so a straight measurement across the center isn't available. Measuring the chord between two adjacent holes and calculating the diameter is the standard workaround.
Does the number of holes affect the diameter calculation?
Yes. The relationship between the chord length and the diameter depends on the angle between adjacent holes, which is a direct function of how many holes are in the pattern.
What's a common use for the Bolt Circle Layout calculator?
It's frequently used to generate hole coordinates for CNC programming, CAD sketches, and fixture design, anywhere a designer or machinist needs precise X,Y positions for an evenly spaced hole pattern.
Can I use a starting angle other than zero?
Yes. The starting angle input lets you rotate the entire hole pattern to any orientation, which is useful when a hole needs to clear an obstruction or align with another feature on the part.
Are these calculators only for automotive wheel patterns?
No. While wheel bolt patterns are a familiar example, the same math applies to any evenly spaced hole pattern, including flanges, sprockets, motor mounts, and fixture plates.
Do I need CAD software to use these calculators?
No. Both widgets run in the browser and only require basic inputs like hole spacing, hole count, bolt circle diameter, and starting angle. The output can be used directly or transferred into CAD or CNC software as needed.
Bolt Circle Diameter Calculator →
Bolt Circle Layout Calculator →