Bolt circle PCD and hole position calculator: Setting bolt-circle start angles — Worked Scenario

What this scenario actually checks

On an equal-pitch bolt circle the distance between adjacent hole centers is a chord, not the length of the arc; a selected start angle rotates all coordinates. Bolt patterns with equally spaced holes can be laid out on a pitch circle diameter (PCD). This calculator uses radius = PCD ÷ 2, angular pitch = 360° ÷ hole count and adjacent-hole chord length = PCD × sin(180° ÷ hole count). It also gives the first hole's X and Y coordinates from a user-selected starting angle, using the circle centre as (0,0). It does not verify bolt strength, edge distances or structural suitability.

This walkthrough focuses on setting bolt-circle start angles. It follows the published tool inputs, not a new formula or an automatically verified legal, financial, medical or engineering determination.

Start by preparing the right inputs

Enter the pitch circle diameter (PCD) in millimetres and the number of equally spaced holes; PCD is the centre-to-centre diameter of the circular pattern. Enter the first hole's polar angle in degrees. Zero degrees places the first hole on the positive X-axis, with positive angles counterclockwise.

Before copying the example, confirm that each entered unit, date, dimension and default matches your real task. Changing only a visible number without revisiting its assumptions can change what the answer means.

• Enter the pitch circle diameter (PCD) in millimetres and the number of equally spaced holes; PCD is the centre-to-centre diameter of the circular pattern.
• Enter the first hole's polar angle in degrees. Zero degrees places the first hole on the positive X-axis, with positive angles counterclockwise.
• The calculator returns adjacent hole centre spacing measured along the straight chord, first-hole X = (PCD ÷ 2) × cos(angle), Y = (PCD ÷ 2) × sin(angle), and the step angle 360° ÷ hole count.

Worked example with the calculator

Use the following input values: PCD 200 mm; 8 equally spaced holes; first angle 0°.

The example output is Adjacent spacing 76.536686 mm; first hole X 100 mm, Y 0 mm; 45° step. Eight holes form a 45° pitch; the straight chord between adjacent centres is 200 × sin(22.5°), not the longer arc distance.

For comparison, the same tool also documents these inputs: PCD 120 mm; 6 equally spaced holes; first angle 90°. Its result is Adjacent spacing 60 mm; first hole X ≈ 0 mm, Y 60 mm; 60° step. With a 60 mm radius and a 90° start, the first hole is on the positive Y-axis.

What can change the answer

The calculator returns adjacent hole centre spacing measured along the straight chord, first-hole X = (PCD ÷ 2) × cos(angle), Y = (PCD ÷ 2) × sin(angle), and the step angle 360° ÷ hole count. Confirm drawing datum, fabrication tolerances, bolt diameter, clearances and mechanical load design before drilling or purchasing components.

Is adjacent spacing an arc distance or a straight hole-centre distance? It is a straight chord between adjacent hole centres. The arc along the circle is longer except for a vanishingly small angle.

Does the calculation show every hole coordinate? The current output shows the first hole's XY coordinates and the common angular step. To derive other positions, add multiples of the step angle and apply the same polar-coordinate formula.

Before using the result for a real decision

For setting bolt-circle start angles, check the original measurements and input units, and save the assumptions alongside any number you share.

Where legal rules, official allowances, equipment ratings or contractual conditions matter, consult their current authoritative documents. An illustrative estimate is not a substitute for that verification.

• Reproduce the example before changing inputs
• Check all units and assumptions, including any default values
• Verify official or manufacturer-specific requirements outside the calculator