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Woodworking Math

Learn · 6 min

First joints

Why a glued butt joint isn’t enough, and the easy joints that fix it: screws in pocket holes, dowels, rabbets and dadoes, with their sizes.

Why a plain butt joint fails

The Wood Handbook says a plain end-grain butt joint can’t be glued strong enough for ordinary service, and that where end grain meets edge grain the joint needs interlocking parts so that edge grain is glued to edge grain. Its examples are dowels, mortise and tenons and rabbets.

One source: USDA Forest Products Laboratory, the authority for the wood properties it measured [source 1]

Pocket screws and dowels

Pocket screws are sized by the thickness of the board you drill. Kreg’s chart gives a 1 1/4 in screw for 3/4 in stock, with the jig and drill collar set to 3/4 in.

One source: Kreg Tool, the maker of that system [source 2]

A dowel is traditionally about a third as thick as the stock it joins: for 3/4 in stock, a 1/4 in dowel.

Traditional rule: Fine Woodworking (Taunton Press) and Dictum [source 3, source 4]

Rabbets and dadoes

A dado is traditionally cut no deeper than half the thickness of the board it’s cut into, and a rabbet no deeper than two thirds of the stock: in 3/4 in stock, at most 3/8 in and 1/2 in.

Traditional rule: Fine Woodworking (Taunton Press) and Woodcraft [source 5, source 6]

Put it to work

Check yourself

  1. 1. Why does a glued end-grain butt joint need help?

  2. 2. Per Kreg’s chart, what pocket screw length goes with 3/4 in stock?

  3. 3. By the one-third rule, what dowel suits 3/4 in stock?

  4. 4. By tradition, how deep at most is a dado in 3/4 in stock?

0 of 4 answered

Sources

  1. USDA Forest Products Laboratory, Wood Handbook (FPL-GTR-282, 2021), ch. 10 "Wood Adhesives", End-Grain and End-to-Edge-Grain Joints, read 2026-11-07
  2. Kreg Tool, Kreg Jig K4 / K4 Master System owner’s manual (NK9050, version 1, 01/2017): screw selection and setting chart, read 2026-11-07
  3. Fine Woodworking (Taunton Press), Mario Rodriguez, "All About Dowels and Biscuits" (2005), read 2026-11-07
  4. Dictum, 4 × 3 tips for dowelling with wooden dowels, read 2026-11-07
  5. Fine Woodworking (Taunton Press), Mario Rodriguez, "All About Rabbet and Dado Joints" (2005), read 2026-11-07
  6. Woodcraft, Craig Bentzley, "Joinery Made Simple: Rabbets, Dadoes, and Grooves", read 2026-11-07

How sources are counted and labelled: how we source these numbers.