Guide bushing offset calculator
Enter your bushing's outside diameter and your bit. Inches, fractions or millimetres all work: 5/16, 0.3125, 8mm.
5/16 in, 0.3125 in
1/8 in, 0.1250 in
Hole offset
3/32 in
0.0938 in · 2.38 mm
The opening in your board matches the drawing. Pockets, recesses, cut-outs.
Piece offset
7/32 in
0.2188 in · 5.56 mm
The part that falls out matches the drawing. Inlays, letters.
Make a template with this setup
How the offset works
The bushing's outside wall rides the template and the bit spins in its centre. The bit's near edge is the bushing radius minus the bit radius from the template; its far edge is the bushing radius plus the bit radius. So:
- Hole offset = (bushing OD − bit diameter) ÷ 2. Grow each template opening by this much and the hole you rout matches your drawing.
- Piece offset = (bushing OD + bit diameter) ÷ 2. Grow each opening by this much and the piece that falls out matches your drawing.
The two always differ by exactly one bit diameter. That's why inlay kits pair a bushing with a removable collar: the collar adds the bit diameter to the bushing.
Growing an opening isn't the same as scaling it. Scaling a 4 in star up to 4 3/16 in widens its body by 3/16 in but its narrow points by only a few thousandths, so the points still come out too thin. An offset moves every edge out by the same distance, which is what the bushing does.
Bushwhack does the offset for a whole drawing: upload an SVG, enter the same two sizes, and download a true-size template.
| Bushing OD | Bit | Hole offset | Piece offset |
|---|---|---|---|
| 5/16 in | 1/8 in | 3/32 in | 7/32 in |
| 3/8 in | 1/8 in | 1/8 in | 1/4 in |
| 7/16 in | 1/4 in | 3/32 in | 11/32 in |
| 1/2 in | 1/4 in | 1/8 in | 3/8 in |
| 5/8 in | 1/4 in | 3/16 in | 7/16 in |
| 5/8 in | 1/2 in | 1/16 in | 9/16 in |
| 3/4 in | 1/2 in | 1/8 in | 5/8 in |
| 17 mm | 6 mm | 5.5 mm | 11.5 mm |
| 30 mm | 12 mm | 9 mm | 21 mm |
Before you rout
- The bit has to pass through the bushing's inside diameter with room to spare. Check it by hand, unplugged.
- Measure the bushing with calipers. Nominal sizes are sometimes a few thousandths off, and the offset changes by half of any error.
- Centre the bushing on the bit, or the offset will differ on each side. A centring cone or pin does this.
More in the guide, and on inlay templates.