Use this workshop calculation to turn measured dimensions and the selected construction convention into a cut, spacing, fit, or material-planning value.

Mating board thickness (mm)

Required number input.

Pre-flush protrusion (mm)

Required number input.

Size box-joint fingers from mating board thickness plus deliberate protrusion before flushing. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Finger cut depth (mm)

The resulting finger cut depth returned as a number.

• Nominal dimensions, kerf, moisture movement, hardware clearances, wall squareness, and manufacturer instructions can change the required shop dimension. Measure the actual assembly and test critical joints on scrap before cutting finished material.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Complement the calculated dimension with a physical story stick, dry fit, or full-size layout. Follow the hardware, coating, adhesive, or machine manufacturer's limits where they differ from a rule of thumb.

• Box Joint Finger Width

  Divide board edge width into a specified number of equal-width box-joint fingers.

• Frame Rabbet Depth

  Size a glazing-frame rabbet from measured glass-and-backing stack thickness plus bedding clearance.

• Half-Lap Cross Joint Calculator

  Compute the lap thickness and cut depth that halve a cross-lapped board.