Wood left
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Shop estimate. Measure your stock and cut a test on scrap.
How to use it
- Measure the stock you are cutting into.
- Measure the piece that will sit in the joint. That thickness is the width of the dado or the rabbet.
- Choose a depth. A third of the stock is a common place to start a shelf dado. It is a habit, not a rating.
- Read the wood remaining. If the depth meets or passes the far face, the page stops.
The formula
Wood left = stock thickness − cut depth. Joint width = mating thickness.
- Width. Across the face, the size of the groove or the rabbet. Match it to the piece that fits, plus whatever hair of clearance you plane.
- Depth. Into the stock, toward the face you are trying not to break through.
- Left. The thickness still connecting that face to the rest of the board.
Worked example
A ¾ inch side with a ¾ inch shelf, dadoed ¼ inch deep, keeps 0.5 inch under the cutter. The groove is 0.75 inch wide, the thickness of the shelf. A quarter inch is one third of three-quarter stock. That is a common starting depth, not a strength rule.
If the depth you type reaches the far face, the page refuses the cut instead of drawing a slot that breaks through.
What it will not decide
- No strength, no screw layout, and no claim that one-third is safe.
- It does not add clearance. If the shelf must slide, make the mating number a hair larger than the shelf, on purpose.
- A rabbet and a dado use the same two dimensions here. The rabbet is simply open on one edge.
Questions
Is one-third of the thickness a rule?
No. It is a starting depth many people use for a shelf dado in 3/4 inch stock, which is 1/4 inch. Type a different depth when the stock is thinner or the shelf is doing something else. The page will not bless it.
What about a back-panel rabbet?
The mating thickness is the back. The depth is how far the rabbet comes in from the back edge. A flush back wants that depth equal to the back thickness only if you are measuring the width of the rabbet, not the depth. Read the labels. Width and depth are different directions.
Why did it refuse the cut?
The depth is at least the thickness of the stock, so the cutter comes out the other side. Reduce the depth.