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How to Lay Out a Bolt Circle by Hand

Guide · Updated October 28, 2026

Without a DRO or rotary table, you can lay out a bolt circle with dividers and the chord formula. Set the dividers to the chord between holes and step around the circle.

Set dividers accurately

Set dividers on a steel rule or gage blocks. The free Gage Block Calculator builds the stack.

Step by step

Blue and center: Apply layout dye and punch the center.
Scribe the circle: Scribe the bolt circle with dividers at the radius.
Compute the chord: C = D × sin(180° ÷ N).
Set dividers: Set the dividers to the chord.
Step around: Step around the circle from a starting mark.
Check closure: The last step should land on the start.
Punch: Prick punch, check, then center punch.

Worked examples

6 holes on a 3.000 in circle: C = 3 × sin 30° = 1.5000 in, equal to the radius. 5 holes on 3.000 in: C = 3 × sin 36° = 1.7634 in. 8 holes on 4.000 in: C = 4 × sin 22.5° = 1.5307 in.

Coordinate method

For better accuracy, use X = R cos θ and Y = R sin θ with a height gage; see DRO basics for a worked table.

Closure error

If the last step misses the start, divide the miss by N and adjust the dividers.

Common problems

Doesn't close: divider setting off. Punch wander: punch without a prick mark.

Worked example: coordinates for 6 holes on 3 in

R = 1.500 in. Holes at 0°, 60°, 120°…: x = R cos θ, y = R sin θ. 0°: (1.5000, 0). 60°: (0.7500, 1.2990). 120°: (−0.7500, 1.2990). 180°: (−1.5000, 0). 240°: (−0.7500, −1.2990). 300°: (0.7500, −1.2990). Check: the distance from 0° to 60° is √(0.75² + 1.299²) = 1.5000 in, matching the chord formula.

Why stepping error adds up

Stepping dividers around the circle adds each step's error. A 0.002 in setting error on 6 steps puts the last mark 0.012 in from the start. That is why the miss ÷ N correction works, and why coordinates from a center line are more accurate for tight work.

Check the layout before drilling

Measure across opposite holes on an even-count circle: they should read the full bolt circle diameter, 3.000 in for the 6-hole example. Measure every adjacent pair with dividers; any pair that differs by more than your tolerance means re-marking before you punch.

Tips

DRO: DRO bolt circles.
Layout: layout dye.
Punch: prick punch.

FAQ

What is the chord formula?

C = D × sin(180° ÷ N).

6 holes on 3 in?

1.5000 in.

5 holes on 3 in?

1.7634 in.

Most accurate method?

Coordinates with a height gage.

What if it does not close?

Adjust by the miss ÷ N.

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