Find where to drill the pivot hole on a circle-cutting jig so the router cuts a disc, or an opening, at exactly the diameter you need. The calculator accounts for the width of the bit so you do not lose a bit diameter on every circle.
A router circle jig is a trammel: a pin at the center, the router at the end of an arm. The distance that matters is from the pin to the near edge of the bit for a disc, or to the far edge for an opening — not to the center of the bit. Measure to the bit center and every disc comes out one bit diameter too small, every opening one bit diameter too big.
Pick whether you are keeping the inside (a disc, a tabletop, a speaker baffle plug) or the outside (a hole for a speaker, a sink, a porthole), and the calculator gives the pivot distance measured to the bit center, which is the easiest point to mark.
R is the finished radius and r is the bit radius. For a disc the bit runs outside the line, so its center sits at R + r. For an opening the bit runs inside the line, so its center sits at R − r.
disc: pivot = D/2 + bit/2 · opening: pivot = D/2 − bit/2
| Finished diameter | Disc, ¼ in bit | Disc, ½ in bit | Opening, ¼ in bit | Opening, ½ in bit |
|---|---|---|---|---|
| 6 in | 3.125 in | 3.25 in | 2.875 in | 2.75 in |
| 8 in | 4.125 in | 4.25 in | 3.875 in | 3.75 in |
| 12 in | 6.125 in | 6.25 in | 5.875 in | 5.75 in |
| 18 in | 9.125 in | 9.25 in | 8.875 in | 8.75 in |
| 24 in | 12.125 in | 12.25 in | 11.875 in | 11.75 in |
| 30 in | 15.125 in | 15.25 in | 14.875 in | 14.75 in |
| 36 in | 18.125 in | 18.25 in | 17.875 in | 17.75 in |
An 8 in disc with a ½ in bit. Radius 4 in plus bit radius 0.25 in: pivot 4.25 in from the bit center.
A 24 in round opening with a ¼ in bit. 12 − 0.125 = 11.875 in. Drill the pivot hole in the waste so it does not mark the work you keep — for an opening, the center is waste.
A 300 mm disc with a 12 mm bit. Switch to millimetres: 150 + 6 = 156 mm.
The same geometry works with a band saw circle jig: use the blade instead of the bit. Because the kerf is thin, the correction is small, but it is still the difference between the pin-to-tooth distance and the radius. Measure from the pin to the side of the blade’s teeth that faces the work you keep.
A circle jig does not need to be bought. Take a strip of ¼ in plywood or hardboard as long as the largest radius plus the router base, screw the router to one end in place of its sub-base, and plunge a hole for the bit. Then measure from the edge of that hole along the centerline and drill small pivot holes at the distances the calculator gives for the sizes you cut most. Label each hole with its finished diameter, disc or opening, and bit — the label is the point, because the same hole gives different results with a different bit.
The calculator gives the distance to the bit center. If your jig is easier to set from the bit edge, add or subtract the bit radius yourself.
No. The finished radius must be larger than the bit radius; the calculator rejects anything else.
The pivot is loose in its hole, the router base is not round to the bit, or the arm is flexing. Use a snug pin and keep the same point of the router base against any guide.
As large as the jig arm allows. For big tabletops, a plywood strip with a pin hole drilled at the calculated distance is often the simplest jig.