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Workshop tool

Router Guide-Bush Offset Calculator

Measure the guide. Measure the cutter. Put the compensation on the right side.

mm

Measure the outside of the guide that contacts the template. Up to two decimal places, 5 to 80 mm.

mm

Measure the cutter, not the collet or shank. It must be smaller than the guide bush; 1 to 50 mm.

mm

The opening or piece size you want after routing. Up to two decimal places, 5 to 3000 mm.

Which side are you keeping?

For an opening the template opening is larger. For a kept outside piece the template profile is smaller.

WORKED EXAMPLE

9 mm one-side offsettemplate edgefinished cut

118 mm4 5/8 in

Guide-bush calculation
ValueMeasurementNote
Guide-bush outside diameter30 mm1 1/8 in
Router-bit diameter12 mm1/2 in
One-side offset9 mm3/8 in
Finished dimension100 mm3 7/8 in
Template opening118 mm4 5/8 in
  • Route inside the template opening. The template must be larger than the finished opening by two offsets.
  • Measure the actual guide-bush outside diameter and cutter diameter; nominal labels and worn cutters can differ.
  • Test the template and direction of travel on scrap before routing the workpiece.

This is the tool's worked example, not your figures: a 30 mm guide bush, a 12 mm cutter and a 100 mm finished opening. Load it into the form with the example button, or enter your own and press the calculate button to replace it.

How this works

Inputs

Enter the measured guide-bush outside diameter, measured router-bit diameter, required finished dimension and which material will remain. The word “opening” includes a recess routed inside a template. “Outside piece” means the work retained inside the cutter path while the guide runs around a template profile.

Method

WOOD Magazine describes the manual offset as the difference between guide-bushing and cutter diameters divided by two5. That one-side distance is then applied at both opposing edges of the template dimension:

offset = (guide-bush OD − cutter diameter) ÷ 2one-side distance between the template contact line and finished cut
template opening = finished opening + 2 × offsetguide running inside a template opening
template profile = finished outside piece − 2 × offsetguide running around the outside of a template

The calculator rejects a guide no wider than the cutter and any combination that would leave the guide unable to move through an opening or create a zero or negative outside template.

Rounding

Inputs accept up to two decimal places. Geometry is calculated without intermediate rounding and displayed to at most 0.01 mm. The inch line independently rounds to the nearest 1/8 in for orientation; make the template from the metric result or your original measured unit, not from a chained conversion.

Limitations

  • The geometry assumes the guide remains in contact with the intended template edge and the cutter is centred on the guide axis.
  • Manufacturing tolerance, cutter runout, template-edge defects and deliberate fit clearance are not estimated. Make a scrap test and adjust the template if the fit matters.
  • The calculator sizes one dimension. Corners, minimum bend radii, guide flange depth and template thickness still need physical checking.
  • This is a planning aid, not an engineering or safety certification.

Worked example

GB-1: a Ø30 mm guide bush and Ø12 mm cutter give (30 − 12) ÷ 2 = 9 mm of one-side offset. For a 100 mm finished opening, the template opening is 100 + 2 × 9 = 118 mm. Choosing “outside piece” with the same numbers would instead return an 82 mm template profile.

Sources

The offset formula was cross-checked against WOOD Magazine's guide-bushing article5. The template-size steps are the same elementary geometry applied across both sides of the finished dimension. The full reference is listed below and on the sources page.

Reference

Source used for the offset formula

RefReferenceUsed by
5

Understanding Guide Bushings — WOOD Magazine. https://www.woodmagazine.com/woodworking-tips/techniques/routing/understanding-guide-bushings Accessed 2026-08-06.

Used to cross-check the manual offset formula: (guide-bushing outside diameter − bit diameter) ÷ 2. The calculator independently extends that geometry to the required template dimension.

Router Guide-Bush Offset Calculator (offset formula)