Router Guide Bushing Offset Formula + Printable Chart
Router Guide Bushing Offset Formula + Printable Chart - A step into the workshop.
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Yo, let me tell you about the first template I ever made. Traced a shape onto some hardboard, cut it out on a borrowed bandsaw, screwed my router's edge guide out to what felt right, and made a beautiful cut around it. Then held the template against the finished part.
Off. By an eighth of an inch. On every side.
That's the day guide bushing math entered my life, and I'm going to spare you the same lesson. It's one formula. You can do it standing in the aisle at Rockler.
The Formula, No Waiting
A guide bushing is a steel tube that mounts in your router's subbase. The outside of the tube rides against your template or edge guide. The bit spins inside it. Which means the bit and the tube are two different diameters, and the gap between them is your error, baked in, every pass.
Here's the whole thing:
Offset = (bushing outside diameter − bit diameter) ÷ 2
WOOD Magazine spells this out, and they run the classic example: a 3/4" OD bushing with a 1/2" bit. Three-quarters minus a half is a quarter. Divided by two, that's a 1/8" offset. That's the distance from the outside of the tube to the cutting edge, and it's how far your cut will wander from the template edge.
The direction matters too, and Wealden Tool lays it out plainly. Rout around the outside of a template, your cut comes out larger than the template. Rout around the inside, your cut comes out smaller. So Katz-Moses' rule follows: undersize a positive template by the offset, oversize a negative template like a mortising jig by the offset.
Wait, why divide by two? Because the bushing is centered over the bit. The gap shows up on both sides, so each side only eats half of it. Once that clicks, the whole system stops feeling like voodoo.
The Offset Table
Print this, tape it inside the cabinet door. One honest caveat first: these are calculated values from the formula, not manufacturer specs. The only bushing size I could verify against an actual product page is the Porter-Cable 42024, which the company lists as a 3/4" OD guide. Check your own bushings with a caliper before you trust a row.
| Bushing OD | Bit Ø | Offset | What it means outside a template |
|---|---|---|---|
| 1/2" | 1/4" | 1/8" | cut is 1/8" larger per side |
| 5/8" | 1/2" | 1/16" | 1/16" larger |
| 3/4" (P-C 42024) | 1/2" | 1/8" | 1/8" larger |
| 3/4" | 1/4" | 1/4" | 1/4" larger |
| 7/8" | 1/2" | 3/16" | 3/16" larger |
| 1" | 1/2" | 1/4" | 1/4" larger |
| 1-3/16" | 1/2" | 11/32" | 11/32" larger |
That last row is worth a wince. Run a 1/2" bit through a bushing sized to fit the standard subbase hole itself and your offset balloons past a quarter inch. Big bushings want big bits, or your template math gets ugly fast.
Also check clearance before you commit: some bit-and-bushing combos simply don't fit, the bit too fat for the tube with no room to actually cut.
Buy Once, Fit Everything
The compatibility story is friendlier than most of woodworking's. There are basically two bushing styles out there: the Porter-Cable two-piece style with a screw-on locking ring, and the Bosch click-in style that only fits Bosch routers. Porter-Cable's 42000 kit gives you seven guides plus two lock nuts, and every one of them fits the 1-3/16" diameter center hole in the subbase. That hole is so universal Katz-Moses calls it the "Porter-Cable Standard," and most bushing sets and router bases follow it.
If your router accepts neither style, adapters and replacement subbases are out there from Eagle America, Lee Valley, and Rockler.
Pricing, with a caveat: WOOD Magazine's figures are older, but they listed individual bushings around $6–$8 and full sets at $30–$45. Verify current prices, but the point stands: this is one of the cheapest capability upgrades you can bolt onto a router.
One more thing people skip: centering. If your subbase isn't centered over the collet, your offset isn't even all the way around, and no table will save you. Katz-Moses' method: chuck a bit, insert a bushing with the same inside diameter as the bit, center the base, lock it down, then swap bushings. And always, always do a test cut in scrap first. My landlord's walls are thin enough that I've done these test cuts at midnight with a whisper of a pass. Still worth it.
Making the Template Behave
WOOD Magazine likes 1/4" hardboard for templates. Spray-adhere your pattern, cut to shape on the bandsaw or scrollsaw, then file or sand the edges smooth. Attach the whole thing with hot-melt glue or double-faced tape, put a router pad under the work, and a backer board underneath if you're cutting through.
Feed direction, because going the wrong way will grab and wreck the part: WOOD Magazine says rout counterclockwise around the outside of a template and clockwise around the inside.
And here's the nice part: if you're using a commercial jig — dovetail jig, countertop miter jig — you don't do any of this math. Wealden points out those jigs come with the offset pre-specified. Use the bushing and cutter the jig tells you to use and stop thinking. It's only homemade templates where you build the offset in yourself.
When the Bearing Bit Beats the Bushing
Now, the plot twist. You can skip all of this math entirely, and sometimes you should.
Pattern bit vs flush-trim bit, quick terminology from WOOD Magazine: a pattern bit has the bearing above the cutters, so the template goes on top. A flush-trim bit has the bearing below the cutters, template underneath. (Stumpy Nubs calls the base-bearing one a "template bit" instead — same tool, different name, don't let it trip you.)
The magic is zero offset. The bearing is the same diameter as the cutters, so the bit cuts exactly flush with the template. No subtraction, no division, and your template can be the exact size of the finished part. Katz-Moses calls this bearing bits' one big advantage and his usual pick for template routing. I get it — I'd rather trace a shape once than do math twice.
But the bushing doesn't die, and Stumpy Nubs makes the case for when it wins:
- Mortising jigs. A bearing at the tip can't plunge into solid wood, and a base-bearing bit would force full-depth cuts at fixed depths. A bushing rides the jig while you extend the cut in stages to any depth.
- Dovetail jigs. The depth has to change while the guide rides the jig. A bearing bit can't do that.
- Any-bit flexibility. Bushings turn a straight spiral, a dovetail bit, a V-groove, whatever's in your drawer into a guided bit. One $6 tube instead of a shelf of specialty bits. For my budget and my second-bedroom shop, that's a real argument.
One trap with bearing bits Stumpy Nubs flags: if the cutting length is longer than your workpiece is thick, the tip digs into your spoilboard, but raising the bit lifts the bearing off the template. Fix it with a thicker template, or flip the workpiece and use an opposite-end-bearing bit.
And never reverse the feed direction at the router table to fight grain. That's a climb cut, and climb cuts at the table are how things go very wrong very fast. Flip the work and change bits instead.
Cost check if you go the bearing route: WOOD Magazine says spiral and downshear flush-trim bits run about a 30–40% upcharge, and double-bearing bits that work from either end cost about 50% more than single-bearing ones. The zero-offset convenience has a price tag.
So Which One Do You Reach For?
My rule, and this is opinion, not scripture: if the part's shape is the whole point — a copy job, a stacked plywood panel, anything where the template equals the finished piece — grab the pattern bit and skip the arithmetic. If the jig is doing the guiding, especially with plunging or changing depths involved, the bushing earns its spot. And if you only own one 1/2" straight bit and a $7 bushing, the formula above turns that into a template-routing setup by dinner.
Do the math once. Write it on the template itself. Future you, holding the finished part against the pattern, will be glad.