Joinery math

Dado calculator for modern undersized plywood

Modern plywood is a liar. A “3/4"” sheet is actually 23/32" or thinner, and the slop between nominal and actual varies sheet to sheet. A dado sized for nominal thickness leaves a gap you could park a car in. The fix is measuring the real thickness and calculating from that.

The formula

dado_width = actual_plywood_thickness + fit_allowance

Fit allowance is zero for a snug dry fit, +0.002" for glue, and −0.002" if you plan to hammer-fit. Measure plywood thickness with dial calipers at three points along the edge you’ll insert.

Example: nominal 3/4" plywood

Actual measured thickness: 0.709" (23/32" nominal). For a snug dry fit, cut a 0.709" dado. Dead On converts that to fractional router bit combinations — in this case you can get there with a 1/2" bit on a router pass plus a 1/4" bit shimmed out 0.084", or a dedicated undersized bit.

Why it matters

A gappy dado is weaker, holds glue poorly, and looks like rushed work. A too-tight dado splits the case side during assembly. The difference between those failures and a perfect fit is a set of dial calipers and about 30 seconds of math.

Step-by-step

  1. 1

    Measure plywood thickness

    Three points along the edge, average them. Good sheets vary by less than 0.005".

  2. 2

    Choose fit allowance

    0.000" for snug dry fit, +0.002" if you’ll glue, −0.002" for drawbored assembly.

  3. 3

    Calculate dado width

    Actual thickness + fit allowance. Convert to decimals for setup.

  4. 4

    Pick router bit or dado stack

    Dead On suggests bit/shim combinations or dedicated undersized bits (like 23/32", 15/32").

  5. 5

    Cut a test joint

    In scrap first. The fit should feel firm with hand pressure but not require mallet blows.

Skip the math. Build the diagram.

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Frequently asked questions

Why is plywood undersized?

Manufacturing tolerance and finish-sanding. “3/4"” has been 23/32" for decades, and thinner grades have similar deductions. Imported baltic birch often measures even thinner than nominal.

Can I use a standard 3/4" dado blade?

Only if you shim it or accept the gap. Dedicated undersized dado sets (23/32", 15/32") are purpose-built for plywood.

What fit allowance should I use?

Zero for dry fits, +0.002" if you’ll glue, −0.002" if the joint is drawbored or mechanical.

Does Dead On know my plywood grade?

It knows the common nominal-to-actual conversions, but always measure your actual sheet. Imports and domestics vary, and tolerance stacks matter.

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