Router Template Guide Bushings and Bit Sizing Explained | The Offset Math

A router template guide bushing follows the template while the bit cuts inside it, and the radial offset equals the guide’s outside diameter minus the bit diameter, divided by two.

Get that subtraction wrong by a fraction and the mortise lands a hair off the mark — or worse, the bit grazes the collar and the cut goes sideways. The offset formula decides everything: template placement, bit choice, and how much clearance you have. The numbers below come straight from Lee Valley’s published guide combinations and current product specs.

How Do You Calculate The Offset For A Template Guide Bushing?

Subtract the bit’s cutting diameter from the bushing’s outside diameter, then divide by two. That result is the radial gap between the template edge and the cut, on every side.

Lee Valley’s own worked example uses a 5/8-inch bushing with a 3/8-inch bit: (5/8 − 3/8) ÷ 2 = 1/8 inch.

• A larger bushing widens the offset for the same bit.
• A smaller bit narrows the offset and adds running clearance.
• Nominal sizes aren’t exact — measure before you commit a template to stock.

Direction matters too. Routing around the outside of a template pushes the finished workpiece inward from the template edge. Cutting an interior shape, like a mortise or an inlay pocket, reverses that: the template has to shift the opposite way. Getting this backwards is one of the most common template mistakes, and it shows up only after the cut.

Bushing And Bit Combinations That Work

Pick the one whose bit suits the cut, not the one whose bushing number looks familiar.

That opening figure belongs to this set — don’t assume your router matches it.

A bit that physically drops through the opening isn’t automatically safe. Insufficient clearance causes binding, heat, and inaccurate cuts, so leave visible room around the shank at full plunge.

If you’d rather skip jig-building entirely, our tested wood router template roundup covers options that arrive pre-sized for standard bushings.

How Do You Install A Guide Bushing Correctly?

Install the bushing concentric and tight before the bit ever goes in, and unplug the router first. McMaster notes that Bosch guide bushings attach to Bosch routers without swapping the baseplate — a reminder that bushing systems are not universal.

1. Disconnect power.
2. Confirm the adapter, locking nut, and base opening match your router’s system.
3. Choose a guide whose outside diameter matches the template design and whose opening clears the bit.
4. Seat the bushing concentrically and tighten the retaining hardware.
5. Set bit depth per the router manufacturer’s spec, with the shank properly inserted and the bit clear of the bushing.
6. Route a test cut on scrap, then take shallow passes instead of one deep plunge.

Keep the guide pressed against the template for the whole pass and don’t force the router sideways. The template itself needs to be rigid, securely fastened, and thick enough for the guide to ride against. McMaster’s listings specify these guide-bushing kits for pilotless router bits.

Lee Valley’s router template guide guide is worth bookmarking for the full combination chart and offset diagrams.

What Goes Wrong With Bushing Sizing?

Most failures trace back to one of five slips, and all five are cheap to avoid on scrap.

• Using bit diameter in place of bushing outside diameter in the formula.
• Reversing the template adjustment between inside and outside cuts.
• Treating nominal bushing sizes as exact measurements.
• Running a bit too close to the guide opening for proper clearance.
• Skipping the test cut and committing the template to finished stock.

Check bit shank size, maximum speed, and feed direction in your router’s manual before the first pass — those figures vary by machine and sit outside this guide’s scope.