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Bolt Circle Calculator — X/Y Hole Coordinates for Circular Patterns

DRO-Ready Output, Multiple Patterns, SVG Visual Diagram, and G-Code Generation for Mill and CNC

Free bolt circle calculator for machinists, CNC programmers, and fabricators. Enter the bolt circle diameter (BCD), number of holes, and starting angle to generate precise X/Y coordinates for each hole in the pattern. Output is DRO-ready with absolute and incremental positions. Supports multiple patterns on the same workpiece, an interactive SVG diagram showing the hole layout, and G-code generation for drilling cycles. Handles both even and odd hole counts, angular offsets, and partial arcs.

Pro Tip: Always verify your bolt circle layout by measuring the chord distance between adjacent holes, not just the X/Y positions. The chord length between equally spaced holes on a circle equals BCD times sine(180/N) where N is the number of holes. This gives you a quick field check with a caliper before committing to drilling the full pattern. On a 4-hole pattern, adjacent holes should be BCD times sine(45) = BCD times 0.7071 apart. On a 6-hole pattern, adjacent holes are exactly BCD/2 apart (the chord equals the radius).

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Bolt Circle Calculator
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How It Works

  1. Enter Bolt Circle Diameter

    Input the bolt circle diameter (BCD) — the diameter of the imaginary circle that passes through the center of each hole. BCD is measured center-to-center, not edge-to-edge. Enter in inches or millimeters.

  2. Set Number of Holes

    Enter the number of equally spaced holes in the pattern. Common patterns include 3, 4, 5, 6, 8, 10, and 12 holes. The calculator spaces holes at exactly 360/N degrees apart.

  3. Set Starting Angle and Center

    Define the starting angle for the first hole (0 degrees = 3 o'clock position, 90 degrees = 12 o'clock). Set the center point coordinates if the bolt circle center is not at the workpiece origin. Angular offset rotates the entire pattern.

  4. Add Multiple Patterns (Optional)

    Add additional bolt circle patterns on the same workpiece with different diameters, hole counts, or offsets. All patterns are shown together on the SVG diagram and combined in the coordinate output. Useful for flanges with multiple concentric bolt patterns.

  5. Review Coordinates and Export

    See the X/Y coordinates for each hole in absolute and incremental format. The SVG diagram shows the hole layout to scale with labeled positions. Copy coordinates for DRO use, or generate G-code drilling cycles (G81/G83) for direct CNC upload.

Built For

  • Manual machinists using a DRO to locate bolt holes on a milling machine by entering X/Y coordinates from the calculator
  • CNC programmers generating G-code drilling cycles for bolt circle patterns on flanges, adapter plates, and mounting brackets
  • Fabricators laying out hole patterns on steel plate using coordinates transferred to a center punch layout
  • Automotive machinists creating custom wheel adapter plates with specific bolt patterns (e.g., 5x114.3 to 5x120)
  • Millwrights drilling mounting bolt patterns for motor bases, pump adapters, and coupling guards in the field
  • Students and apprentices learning the trigonometry behind bolt circle coordinate calculation

Features & Capabilities

DRO-Ready Coordinates

Output X/Y coordinates in absolute (from origin) and incremental (from previous hole) formats, matching exactly what you enter into a digital readout on a manual milling machine. Coordinates are shown to 4 decimal places for precision work.

SVG Visual Diagram

An interactive vector diagram shows the bolt circle pattern to scale with numbered hole positions, the center point, and the bolt circle diameter. Zoom and pan to inspect individual holes. The diagram updates in real time as you change parameters.

G-Code Generation

Generate CNC drilling cycle code (G81 for standard drilling, G83 for peck drilling) with the calculated coordinates. Output includes tool change, rapid positioning, drilling cycle call, and return to clearance plane. Ready to paste into your CNC program.

Multiple Concurrent Patterns

Add multiple bolt circles on the same workpiece with independent diameters, hole counts, and angular offsets. All patterns are combined in the coordinate list and shown on the same SVG diagram. Common for flanges with inner and outer bolt patterns.

Partial Arc Support

Create partial bolt patterns by specifying a start angle and end angle. Useful for patterns that don't complete a full 360 degrees, such as semicircular mounting brackets or segmented bolt flanges.

Chord Length Verification

Displays the chord distance between adjacent holes for quick caliper verification of the pattern. Also shows the angular spacing in degrees and the arc length between holes for layout reference.

Assumptions

  • Bolt circle diameter (BCD) measured center-to-center of bolt holes, not edge-to-edge
  • Hole positions calculated using standard trigonometric functions with angles measured counterclockwise from 3 o'clock (0 degrees)
  • Coordinates referenced from the center of the bolt circle pattern as the datum origin (0,0)
  • G-code output assumes Fanuc/Haas-compatible syntax with G81 or G83 drilling cycles in G90 absolute mode
  • Partial arc patterns use the same BCD with start and end angles limiting the hole placement range
  • Chord distances between adjacent holes calculated as BCD times sine(180 divided by number of holes)

Limitations

  • G-code output must be verified against your specific CNC controller; Siemens, Mazak, and Okuma use different cycle syntax
  • Does not account for positional tolerance or true position per ASME Y14.5 GD&T callouts
  • Machine tool positioning accuracy and repeatability affect actual hole locations versus calculated coordinates
  • Does not calculate required hole diameter tolerances for bolted joint design (through-holes, close-fit, interference)
  • Multiple pattern overlay does not check for hole-to-hole interference between patterns on the same workpiece
  • Incremental coordinate output assumes the machine DRO is zeroed at the pattern center before use

References

  • Machinery's Handbook, 31st Edition - Bolt Circle Calculations and Hole Pattern Layouts
  • ASME Y14.5-2018 - Dimensioning and Tolerancing (positional tolerance for bolt patterns)
  • ASME B18.2.8 - Clearance Holes for Bolts, Screws, and Studs (through-hole sizing)
  • Fanuc CNC Custom Macro Programming Reference - G81/G83 Drilling Cycle Syntax
  • ASME B4.1 - Preferred Limits and Fits for Cylindrical Parts (hole/bolt interference fits)
  • SME Fundamentals of Tool Design, 6th Edition - Hole Pattern Layout and Fixture Design

Frequently Asked Questions

The bolt circle diameter is the diameter of the imaginary circle that passes through the center of each bolt hole in a circular pattern. It is always measured center-to-center, not edge-to-edge of the holes. For example, a flange with four 1/2" bolts on a 6" BCD means the bolt hole centers are located on a 6" diameter circle, not that the flange outside diameter is 6". BCD is sometimes called PCD (pitch circle diameter), particularly in European and automotive specifications.
For an even number of holes, measure the center-to-center distance between two directly opposite holes — that's the BCD. For an odd number of holes (like 3 or 5), measure the center-to-center distance between two adjacent holes, then divide by sine(180/N) where N is the number of holes. For a 5-bolt pattern, BCD = adjacent hole distance / sin(36) = adjacent distance / 0.5878. Alternatively, measure from the center of one hole to the far edge of the opposite hole and subtract half the hole diameter.
The starting angle defines where the first hole is placed, measured counterclockwise from the 3 o'clock position (standard mathematical convention). If no specific orientation is required, 0 or 90 degrees are common defaults. For automotive wheel patterns, the top bolt is typically at 12 o'clock (90 degrees). For flanges, the first hole is often placed to clear other features like pipe nipples or electrical connections. Check the assembly drawing for the required orientation.
This calculator is specifically for circular bolt patterns. Rectangular (grid) bolt patterns are simpler — they're just evenly spaced X/Y coordinates that don't require trigonometry. For rectangular patterns, divide the row and column spacing equally and list the coordinates. A future grid pattern calculator may be added, but for now, rectangular layouts can be done by hand or with a simple spreadsheet.
The calculator generates standard Fanuc/Haas-compatible G-code using G81 (standard drilling cycle) or G83 (peck drilling cycle). Coordinates are in absolute mode (G90). The output includes a tool change block (T__ M6), spindle start (S___ M3), rapid to clearance plane (G0 Z___), the drilling cycle call at each position, cycle cancel (G80), and return to home. Feed rate, peck depth, and clearance plane are configurable. For other controller formats (Siemens, Mazak, Okuma), the coordinate values are the same but the cycle syntax differs.
Disclaimer: Bolt circle coordinates are calculated from geometric formulas and should be verified before machining, especially for precision fits and critical assemblies. Always confirm hole positions with layout dye, a center finder, or a coordinate probe before drilling. G-code output is provided as a starting point and must be verified against your specific CNC controller syntax and machine setup. ToolGrit is not responsible for machining errors, scrap, or assembly problems resulting from calculator use.

Learn More

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