How to use the dado calculator
- Pick inches or millimeters, then enter the thickness of the board that gets the groove and the thickness of the piece that slides in.
- Choose the depth as a share of the board. The default of one third is an example, not a rule.
- Open More options to add a small fit allowance, or to enter the width of your cutter so the page can count the passes.
- Read the width and depth, and the wood left under the groove. Test the setting on scrap before cutting the real board.
Formula
width = shelf thickness + fit allowance
depth = board thickness x depth share
wood left = board thickness - depth
passes = width / cutter width, rounded up
Use the measured thickness of your stock, not the nominal size. Plywood is often thinner than its label, so a dado cut to the nominal size leaves a loose joint.
3/4 inch board and shelf
For a 3/4 inch board with a 3/4 inch shelf and a depth of one third, the width is 0.75 inch (3/4) and the depth is 0.75 / 3 = 0.25 inch (1/4), which leaves 1/2 inch of wood under the groove.
In millimeters, an 18 mm board at one third deep is cut 6 mm deep, and a 12 mm board at one half deep is also 6 mm. For plywood that measures 23/32 inch (0.71875) with 1/64 inch of fit allowance, the width is 0.734375 inch, or 47/64, and a 3/8 inch cutter needs 2 passes.
Why depth is a share of the board
A dado removes wood from the board that receives the shelf. If it is cut too deep, the board is weak at the groove and may split or bow there. If it is too shallow, the joint has little to hold it against the side. Many woodworkers pick a depth between a quarter and a half of the board, and this page lets you try each one and see the wood left.
The share is a starting point. Hardwoods, plywood and thin stock each behave differently, and the load on the shelf matters. Treat the result as a setting to test, then adjust your cutter or router bit.
Reading the fractions
In inches the page shows the decimal and the nearest 1/64 inch, which is the finest mark on most tape measures. Next, see the Screw Length Calculator for fasteners, the Shelf Spacing Calculator for where the grooves go, and the Board Foot Calculator to price the lumber.
Assumptions and limits
- The width equals the piece thickness plus your fit allowance. A tight or loose fit is your choice.
- Depth is a share of the board thickness. The shares offered are examples.
- Fractions are rounded to the nearest 1/64 inch.
- Wood species, joint strength and load are not modeled.
Frequently asked questions
How deep should a dado be?
A depth of a quarter to a half of the board thickness is a starting point many woodworkers use. This page shows one third as an example, and you can pick any of the three shares and see the wood left.
How wide should a dado be for a 3/4 inch shelf?
The same as the shelf, which is 3/4 inch when the shelf measures exactly 3/4 inch. Measure the real shelf and add a small allowance if you want room for glue.
Why does my plywood not fit a 3/4 inch dado?
Plywood is often thinner than its nominal size, such as 23/32 inch. Measure it with calipers and enter that number so the groove matches the actual thickness.
What does the passes figure mean?
If your cutter is narrower than the dado, you make several passes side by side. The page divides the width by the cutter width and rounds up.