Bolt Circle Calculator

How to use the bolt circle calculator

What this calculator does

This calculator converts a circular hole pattern into Cartesian coordinates. It estimates the center point of every equally spaced bolt hole using the number of holes, the radius of the bolt circle, the angle of the first hole, and the x and y offsets of the circle center. The result is useful for layout, checking a drawing, preparing a CAD sketch, or creating a coordinate list for machining. It does not determine bolt strength, hole diameter, edge distance, torque, material thickness, or whether a joint is structurally adequate.

When to use it

Use it when laying out a flange or cover plate, checking a bolt-circle callout on a mechanical drawing, transferring a pattern to a drill jig, or preparing point coordinates for CAD, CNC, laser cutting, or inspection. The coordinate plot gives a fast visual check, while the table and workbook provide exact point data.

How to calculate

1. The calculator opens with a ready-to-use example: five holes, a 50 mm radius, a 0° first-hole angle, and a center at (0, 0). Its results and XLSX workbook are available immediately.
2. Replace Number of holes (n) with the required whole-number count. Then enter the Angle of first hole (A), Offset from center (x₍c₎), Offset from center (y₍c₎), and Radius of bolt circle (R).
3. Choose Length unit. Switching between millimeters and inches converts the radius and offsets so the physical pattern remains unchanged.
4. Read the summary values, inspect the coordinate plot, and use the Hole coordinates table for the numbered x-y points. Select Download Excel to save the current inputs and full-precision coordinates in a real XLSX workbook.
5. Reset clears the demonstration and all calculated content. Download Excel is then disabled until a complete valid pattern is entered again.

Input guide

Number of holes (n) is required and accepts a whole number from 2 through 360; 5 is a realistic example. Increasing it reduces the pitch angle and adds more coordinate rows. Decimal counts, one hole, zero, and negative values are rejected. Angle of first hole (A) is required, uses decimal degrees, and may be positive or negative; 0° places the first hole on the positive x-axis. Values are normalized for display, so 450° describes the same direction as 90°. Do not confuse degrees with radians.

Offset from center (x₍c₎) and Offset from center (y₍c₎) are required signed decimal distances in the selected unit. A value of 0 means the bolt-circle center lies on that axis origin. Positive x moves the entire pattern right; positive y moves it up. Negative offsets are valid. These are center coordinates, not hole-to-hole distances. Radius of bolt circle (R) is a required positive decimal distance; 50 mm is the startup example. Doubling the radius doubles every coordinate's distance from the pattern center and doubles the displayed diameter. Do not enter a diameter in the radius field. Length unit selects millimeters or inches and converts existing length values rather than merely changing their labels.

Output guide

Pitch angle between holes is the exact angular step 360° ÷ n. Bolt-circle diameter is twice the radius. Pattern center repeats the x-y offset used by every point. First-hole coordinate is the first row of the table, and Last-hole angle is the angle assigned to the final numbered hole before the pattern returns to the start. The summary pills repeat pitch angle, diameter, and coordinate count for quick checking.

The Bolt pattern coordinate plot is an x-y scatter view of the same finite coordinates used by the table. Each numbered blue point is one hole center and the green point is the pattern center. The Hole coordinates table lists Hole, Angle, X coordinate, and Y coordinate. High or low coordinate values are not good or bad; they simply describe position in the selected coordinate system. Coordinates are deterministic geometric identities, not manufacturing recommendations.

Worked example

For 5 holes, R = 50 mm, A = 0°, and center (0, 0), the pitch angle is 360° ÷ 5 = 72°. Hole 1 is at x = 0 + 50 cos 0° = 50 mm and y = 0 + 50 sin 0° = 0 mm. Hole 2 is at 72°, giving approximately (15.451, 47.553) mm. The calculator therefore shows a 100.000 mm bolt-circle diameter, 72.000° spacing, five coordinate rows, and a final-hole angle of 288.000°.

Learn more

The equations are standard polar-to-Cartesian coordinate conversions. The Wolfram MathWorld overview of polar coordinates explains the underlying relationship between radius, angle, x, and y, while the NIST guidance on SI length units provides authoritative context for millimeters and other metric measurements.