Router Bushing & Template Calculator

Use this calculator when a guide bushing follows a template and the router bit is smaller than the bushing outside diameter — for routed pockets, mortises, recessed grips (Griffmulden), and similar templated work.

The calculator has two modes:

Forward — enter the finished opening you want, get the template dimensions you need to set on the MFS.
Reverse — enter the template dimensions you've already set, see what finished opening you'll get (useful when comparing what changes if you swap to a different bushing without resizing the template).

The diagrams update live as you change any input or slider. For a Griffmulden-specific cross-section (two stacked pockets), use the Griffmulden visualizer instead.

Bushing OD (mm)

Bit Ø (mm)

Forward (target → template)Reverse (template → finished)

Target W (mm)

Target H (mm)

Template W (mm)

Template H (mm)

Preset:

Material & bit (side-view setup)

Workpiece thickness (mm)

Sacrificial board (mm)

Bit cutting length (mm)

Bushing height (mm)

Free bit stickout (= cutting length − bushing height) caps the maximum reachable cut depth.

Template (MFS) range guard

MFS min (mm)

MFS max (mm)

The MFS forms its opening from four clamped rails, so it has a real minimum and maximum settable size — fill these in from your template's technical-data table to enable the guard. 0 disables a check. The setting dimension to clamp is the finished opening + Z, where Z = copy-ring ⌀ − bit ⌀, exactly what this calculator reports as the template size.

Ring comparison

Comparison bushing OD (mm)

The Compare view tab shows the same finished target routed with the primary bushing and this comparison bushing stacked one above the other.

Section through widthSection through height

The side view is cut through the opening width; the top view is oriented to match.

The side view is cut through the opening height; the top view is oriented to match.

Top viewSide viewBothCompare rings

Top view (plan) — drag to move the router

Side view (cross-section) — drag to move X · mouse wheel sets cut depth

Top view (plan)

Side view (cross-section)

Primary bushing

Comparison bushing

Router position & depth

Router X —

Router Y —

Cut depth Z 9 mm

Drag a diagram to move X / Y, or use these sliders. X / Y are the bushing position; the readout shows mm from the template walls. Z is the cut depth — set it with the Z slider or the mouse wheel over the side view.

About the derived dimensions

Template offset (per side) — half the difference between the bushing's outside diameter and the bit diameter. It's the gap, on each side, between the template wall and the routed cut. So the MFS template opening must be larger than the finished pocket by 2 × offset on each axis.
Corner radius — equals the bit radius (half the bit diameter), not the offset. The bushing rides the sharp-cornered template, but the spinning bit cuts a disk, so the finished pocket always has rounded corners.
Free stickout (max Z) — bit cutting length minus bushing height; the deepest cut the bit can actually reach with this bushing fitted.
Routing direction — for an inside cut, feed the router clockwise around the template (Gegenlauf, the conventional cut). For an outside cut, counter-clockwise. Outer edge first on every depth pass, then clear the centre.

Formula

Offset          = (bushing outside diameter - bit diameter) / 2
Template width  = finished width  + 2 × offset
Template height = finished height + 2 × offset
Corner radius   = bit diameter / 2

Why the corner radius is the bit radius, not the offset

The bushing's centre is constrained to a rectangle inset from the template by the bushing radius; that rectangle has sharp corners. The bit cuts a disk of radius bit / 2 around the bit centre. The finished pocket is the Minkowski sum of those — a rectangle inset from the template by the offset, with corners rounded at the bit radius. A bigger bushing leaves more uncut material in the template corner (the cut moves further inward), but the rounding on the cut itself is set by the bit alone.

The top view shows this directly: the sharp-cornered light rectangle is the bushing reachable area (where the bushing centre can go), and the rounded-corner darker rectangle inside it is the cut envelope (where the bit actually removes material).

Template direction

For inside routing — cutting an opening, mortise, or recessed pocket — the template opening needs to be larger than the finished opening by 2 × offset.

For outside routing — following the outside of a pattern — the template needs to be smaller than the finished part by 2 × offset.

The diagrams above show the inside-routing case.

Feed direction — route the edge first, in Gegenlauf

The routing animation routes the outer edge first, then clears the interior. Taking the edge first removes the material along the template wall in one clean pass, so the inner clearing sweeps never leave a thin uncut strip against the edge.

The edge pass runs in Gegenlauf (conventional cut). For an inside cut — a pocket or opening — that means feeding the router clockwise; for an outside cut it is counter-clockwise. Gleichlauf (climb cut) pulls the router along and is hard to control freehand.

Worked example — 90 × 29 mm Griffmulde with KR-D 17

Target: a 90 × 29 mm Griffmulde finger opening, routed with an 8 mm spiral bit and the Festool KR-D 17 mm copy ring (fits the OF 1400 base directly, same as the KR-D 30 mm — no adapter needed).

Offset = (17 − 8) / 2 = 4.5 mm
Template (MFS) width = 90 + 2 × 4.5 = 99 mm
Template (MFS) height = 29 + 2 × 4.5 = 38 mm
Corner radius in the finished pocket = 4 mm (= bit radius)

With the default 30 mm copy ring on the same 8 mm bit, the offset would be 11 mm — the template would need to be 112 × 51 mm to land the same 90 × 29 mm finished opening. The corner radius is still 4 mm (still bit-dependent only), but the cut is set 11 mm back from the template walls instead of 4.5 mm, so the template position is much more forgiving / less sensitive to small registration shifts.

See the router-table drawer Griffmulden section for how this is used in the current build, and the Griffmulden visualizer for the stepped two-pocket cross-section view.