Workshop tool

Router Circle-Jig Pivot Calculator

Choose the finished edge first. Put the cutter centre on the correct path.

WORKED EXAMPLE

106 mm4 1/8 in

Circle-jig setting
Value	Measurement	Note
Finished diameter	200 mm	7 7/8 in
Finished radius	100 mm	3 7/8 in
Router-bit diameter	12 mm	1/2 in
Bit radius compensation	+6 mm	outside the finished edge
Pivot to bit centre	106 mm	4 1/8 in
Cutter-centre path diameter	212 mm	8 3/8 in

Keep the material inside the cut. The cutter centre runs outside the finished radius, so add the bit radius.
Measure from the pivot-pin centre to the router-bit centre, not to the edge of the cutter or router base.
Use the same pivot setting for later depth passes; this tool does not prescribe pass depth, feed direction or cutter suitability.
Test the jig on scrap and confirm which side of the kerf you are keeping before cutting the workpiece.

This is the tool's worked example, not your figures: a 200 mm finished disc cut with a 12 mm router bit. Load it into the form with the example button, or enter your own and press the calculate button to replace it.

This calculator needs JavaScript to update in your browser — nothing is sent anywhere. Divide the finished diameter by two. If keeping the inside disc, add half the bit diameter; if making an opening, subtract half the bit diameter. Measure that result from the pivot-pin centre to the router-bit centre. The worked example is a 200 mm disc with a 12 mm bit: 100 + 6 = 106 mm.

How this works

Inputs

Enter the required finished diameter, actual router-bit diameter and the side of the cut you intend to keep. The setting is always measured from the pivot-pin centre to the router-bit centre. Router-base edges, collets and jig-slot edges are not the measurement datum.

Method

The finished edge lies one cutter radius away from the cutter-centre path. When the disc is kept, the cutter runs outside that edge, so its radius is added. When an opening is made, the cutter runs inside the opening edge, so its radius is subtracted.

disc pivot radius = finished diameter ÷ 2 + bit diameter ÷ 2cutter centre outside the finished disc edge

opening pivot radius = finished diameter ÷ 2 − bit diameter ÷ 2cutter centre inside the finished opening edge

The calculator also reports the cutter-centre path diameter as twice the pivot setting. It rejects an opening no larger than the bit because that would produce a zero or negative pivot radius.

Rounding

Inputs accept up to two decimal places. The pivot is calculated without intermediate rounding and displayed to at most 0.01 mm. The inch line rounds independently to the nearest 1/8 in for orientation; transfer the metric reading or your original unit directly to the jig rather than chaining rounded conversions.

Limitations

The tool returns horizontal geometry only. It does not prescribe cutter type, pass depth, feed direction, router speed or workholding.
The pivot and jig must physically fit the router base and workpiece. A very small radius may put the pivot under the base or outside the jig's adjustment range.
Cutter runout and material movement are not estimated. Make a shallow scrap test and measure the result before the final workpiece.
This is a planning aid, not an engineering or safety certification.

Worked example

CJ-1: a 200 mm finished disc has a 100 mm radius. A Ø12 mm bit has a 6 mm radius. Keeping the disc puts the cutter centre outside the finished edge, so the pivot-to-bit-centre setting is 100 + 6 = 106 mm and the centreline path diameter is 212 mm. Choosing an opening changes the sign and returns 94 mm.

Sources

These formulae are elementary circle geometry documented above. No external circle-jig standard, cutter recommendation or manufacturer compatibility is claimed; all dimensions are reader-supplied.