Calculator Input
Example Data Table
| Input BCD | Holes | Hole Diameter | Metric BCD | Metric Radius | Adjacent Spacing |
|---|---|---|---|---|---|
| 4.500 in | 5 | 0.500 in | 114.300 mm | 57.150 mm | 67.184 mm |
| 5.000 in | 6 | 0.625 in | 127.000 mm | 63.500 mm | 63.500 mm |
| 6.500 in | 8 | 0.750 in | 165.100 mm | 82.550 mm | 63.178 mm |
Formula Used
Metric bolt circle diameter: BCD mm = BCD inches × 25.4
Radius: Radius = BCD ÷ 2
Angular step: Angle step = 360 ÷ number of holes
Adjacent spacing: Spacing = BCD × sin(π ÷ holes)
X coordinate: X = Radius × cos(angle)
Y coordinate: Y = Radius × sin(angle)
If adjacent spacing is entered, the calculator rearranges the spacing formula:
BCD = spacing ÷ sin(π ÷ holes).
How to Use This Calculator
- Select whether you know the full bolt circle diameter or adjacent hole spacing.
- Enter the inch measurement from your drawing or part.
- Enter the total number of equally spaced holes.
- Add the optional hole diameter in inches.
- Set a start angle when you need rotated coordinates.
- Choose the decimal precision for the output.
- Press calculate to view metric results above the form.
- Use CSV or PDF download for records and shop work.
Inch to Metric Bolt Circle Guide
What a Bolt Circle Means
A bolt circle is the path followed by bolt centers around a wheel, flange, pulley, adapter, or plate. Inch drawings often list this size as BCD, PCD, or bolt circle diameter. Metric work needs millimeters, so a clean conversion prevents wrong drilling and poor fit.
Why This Calculator Helps
This calculator converts an inch bolt circle to millimeters. It also finds radius, adjacent chord spacing, angular step, circumference, and hole coordinates. These extra values help when laying out a pattern on a mill table, rotary table, CNC program, or printed template.
Direct Diameter Method
The most direct method uses the bolt circle diameter. Enter the inch diameter and the number of holes. The tool multiplies inches by 25.4 to get millimeters. It divides that value by two for radius. It divides 360 degrees by the hole count for the angular step.
Adjacent Spacing Method
Adjacent spacing is also useful. It is the straight center-to-center distance between neighboring holes. The calculator uses the sine rule with the metric bolt circle. This value can help check an existing part with calipers.
Coordinates and Rotation
Hole coordinates show each center from the circle origin. A start angle lets you rotate the pattern. Zero degrees places the first hole on the positive X axis. Ninety degrees places it at the top. This makes the output useful for drawings and manual layout.
Hole Diameter Notes
The optional hole diameter conversion changes inch drill or bolt sizes into millimeters. It does not choose clearance automatically, because standards vary by application. Always confirm the final drill size with your shop standard, fastener class, and material.
Export and Accuracy
CSV export is useful for spreadsheets and CNC preparation. PDF export is useful for records, work orders, and quick printouts. Keep the decimal precision high for machining. Round only after confirming tolerance.
Safe Layout Practice
Use this calculator as a layout aid. It should not replace engineering review for safety critical parts. Check the number of holes, measurement method, and datum location before drilling. A small input mistake can move every hole in the pattern.
Measuring Advice
For best results, measure clean centers, not damaged edges. Record whether values came from a drawing or physical part. This note helps another machinist verify the setup before cutting metal and wasting valuable material later.
FAQs
What is a bolt circle diameter?
Bolt circle diameter is the diameter of the circle passing through the centers of all bolt holes. It is also called BCD or PCD. It helps define the exact hole pattern on wheels, flanges, hubs, and plates.
How do I convert an inch bolt circle to metric?
Multiply the inch bolt circle diameter by 25.4. For example, a 4.5 inch bolt circle equals 114.3 mm. The calculator also finds radius, spacing, and coordinates.
Can I use adjacent hole spacing instead?
Yes. Select the adjacent spacing method. Enter the center-to-center distance between neighboring holes. The calculator uses the hole count and sine formula to estimate the full bolt circle diameter.
What does adjacent spacing mean?
Adjacent spacing is the straight distance from one hole center to the next hole center. It is not the curved distance along the circle. It is useful when checking a part with calipers.
Why are X and Y coordinates included?
Coordinates help transfer the pattern to drawings, templates, mills, and CNC work. They show each hole center from the pattern origin. The start angle rotates the coordinate list.
Does this choose the correct drill size?
No. It converts the entered hole diameter only. Clearance, tap drill, and fit depend on fastener standard, tolerance, material, and shop practice. Always verify final drill size separately.
Can this calculator handle odd hole patterns?
Yes. It supports three or more holes, including odd patterns like five-hole circles. Adjacent spacing works well for both odd and even evenly spaced patterns.
Why should I export CSV or PDF results?
CSV files are useful for spreadsheets and machining notes. PDF files are useful for printing, records, and work orders. Exports also reduce mistakes when sharing values with others.