How to use this calculator
- Measure mating stock. Use actual shelf, divider, panel or tongue thickness instead of nominal size.
- Set the fit allowance. Add a small positive allowance for slip fit or finish clearance, or a negative allowance for a tighter fit.
- Enter cut depth and panel thickness. Check depth ratio and remaining back thickness before committing the cut.
- Set the reference edge. Enter the distance from the reference edge to the near wall of the dado or groove.
- Plan cutter passes. Enter router bit, dado stack or saw kerf width and the maximum pass step-over.
How it works
Dado and groove layout starts from the actual mating part: target slot width = mating part thickness + fit allowance The near wall is the entered reference setback, the far wall is the near wall plus target slot width, and the slot centerline is halfway between those two wall marks.
If the cutter is narrower than the target slot, the pass count is: passes = 1 + ceil((slot width - cutter width) / max step-over) The pass centerlines are spaced evenly from the first pass center to the last pass center. For adjacent cabinet and joinery work, use the shelf sag calculator, drawer box size calculator, mortise and tenon calculator and wood glue consumption calculator.
Worked example
Verified against the live calculator
A 3/4 in shelf with a 0.005 in slip allowance
needs a 0.755 in slot. If the near wall is
12 in from the reference edge, the far wall is
12.755 in and the slot centerline is
12.3775 in. With a 1/2 in cutter and
1/8 in maximum step-over, the router layout returns
4 passes spaced about 0.085 in apart.
Frequently asked questions
How do you size a dado or groove?
Measure the actual mating part thickness, add the desired fit allowance, then use that target slot width for the near wall, far wall and cutter-pass layout.
What is a good dado depth?
A common first-pass cabinetry screen is roughly one-third of the receiving panel thickness. The calculator reports depth ratio and remaining back thickness so you can see when the cut is shallow or aggressive.
Can this calculate router passes?
Yes. Enter the cutter width and maximum lateral step-over. If the cutter is narrower than the target slot, the calculator returns pass count and centerline spacing.
Does this replace cabinet joinery plans?
No. It is a layout and takeoff screen. Final shop work still needs test cuts, tear-out control, fence or CNC setup, stop blocks, fastener or hardware details and fit checks on actual stock.
Method & assumptions
- All dimensions are measured from one consistent reference edge and face.
- Target slot width is mating stock thickness plus fit allowance.
- Pass spacing is a layout screen for a cutter narrower than the target slot.
- Glue contact area estimates bottom plus two side walls; actual glue use depends on fit, open time and squeeze-out.
- Does not model tear-out, climb cuts, router-table fence setup, dado-stack shims, CNC toolpaths, stopped-cut end geometry, shelf load capacity, fasteners, hardware, finish buildup or cabinet cut tickets.