Dado & Groove Width Calculator for Plywood: Actual Thickness, Stack and Router Bit

How it works

dado width = measured sheet thickness (+ 0.15 mm snug glue fit, + 0.4 mm slip); depth = ⅓ to ½ of the housing board; stacked dado = 2 outside blades (¼″) + ⅛″, 3/32″ and 1/16″ chippers + shims = width; two-pass router offset = width − bit diameter

Sheet goods are sold by a nominal thickness that is no longer their real one. Hardwood plywood sold as 3/4 inch is usually about 23/32 inch (18.3 mm), imported Baltic birch is metric and a little under its stated millimetres, and only MDF and melamine are close to true size. A dado cut with a standard 3/4″ cutter therefore leaves a loose, gappy joint. That is why woodworkers measure each sheet with callipers, take the thickest reading and set the cutter a hair wider: about 0.13–0.25 mm (0.005–0.010″) leaves room for glue and lets the shelf go in by hand. A joint you have to hammer home is too tight and scrapes the glue off. With a stacked dado set, the two outside blades make ¼″ and the chippers get close to the width. Thin shims between them make up the rest; the calculator picks the chipper combination and the shim thickness. Router users can buy undersized “plywood” bits (7/32, 11/32, 15/32, 19/32 and 23/32 inch) or make two passes with a narrower bit, moving the fence by the difference. Depth depends on the board receiving the joint, not the shelf. Half its thickness is the usual casework standard, and a third is a safe minimum in thin sides, so enough wood stays behind the joint. Dados run across the grain, so the glue holds mainly on the shelf’s face veneer. Add fasteners on heavily loaded shelves. Grooves for drawer bottoms and back panels usually get a slip fit so solid panels can move.

Worked example

A "3/4-inch" plywood shelf at its typical 18.3 mm goes into an 18.3 mm side with a snug fit. The dado is 18.3 + 0.15 = 18.45 mm (0.726″, just over 23/32″) wide and 18.3 × 0.5 = 9.2 mm deep, leaving 9.2 mm of wood behind it. On an 8″ stacked dado set the calculator picks the two outside blades (6.35 mm) plus three 1/8″ chippers and the 3/32″ chipper (11.91 mm), and the remaining 18.45 − 6.35 − 11.91 = 0.19 mm is made up with a 0.2 mm shim (then fine-tune on scrap). With a router, a 23/32″ plywood bit (18.26 mm) cuts it in two passes along a guide, moving the guide 0.19 mm between them, each in 2 depth passes.

Sources: Dado (joinery): dado, groove and rabbet (Wikipedia) · Dado set: outside blades, chippers and spacers (Wikipedia)

Frequently asked questions