Dado Depth Calculator
Find the ideal dado or groove depth for cabinet and shelf joints based on material thickness and load. Use it when routing dadoes so joints are strong without weakening the cabinet side.
About this calculator
A dado joint's depth must balance two competing requirements: deep enough to resist the load, shallow enough not to weaken the surrounding material. The calculator first computes a candidate depth as the lesser of half the shelf thickness (shelfThickness × 0.5) or three-quarters of the cabinet side thickness (cabinetThickness × 0.75). A load factor is then applied — 0.9 for heavy loads, 0.8 for medium, and 0.7 for light — reducing the depth slightly where full strength is unnecessary. A floor is enforced: the final depth is never less than 25% of the thinner of the two pieces, ensuring a minimum mechanical interlock. Formally: depth = max(min(shelfThickness×0.5, cabinetThickness×0.75) × loadFactor, min(shelfThickness, cabinetThickness)×0.25). The traditional woodworking rule of thumb — one-third to one-half the board thickness — aligns with these bounds.
How to use
Example: shelf thickness = 0.75 in, cabinet side thickness = 0.75 in, load = heavy. Step 1 — Candidate depth: min(0.75×0.5, 0.75×0.75) = min(0.375, 0.5625) = 0.375 in. Step 2 — Apply load factor: 0.375 × 0.9 = 0.3375 in. Step 3 — Check floor: min(0.75, 0.75) × 0.25 = 0.1875 in. Since 0.3375 > 0.1875, the recommended dado depth is 0.3375 inches — approximately 11/32 in, which you would set on your router table.
Frequently asked questions
How deep should a dado be for a 3/4-inch shelf in a cabinet?
For standard 3/4-inch (19 mm) sheet goods, a dado depth of 1/4 to 3/8 inch is the accepted woodworking range. At 1/4 inch you get a solid mechanical seat while preserving plenty of material in the cabinet side. At 3/8 inch you gain more bearing surface for heavy shelves. Going deeper than half the shelf thickness risks splitting the shelf under load, particularly with plywood where the thin veneers near the bottom of the dado can fracture.
What load factor should I use when calculating dado depth for heavy book shelves?
Use a heavy load factor (0.9) whenever a shelf will carry dense items such as books, tools, or stone. This keeps the dado depth closer to the calculated maximum, maximising the bearing area that supports the load. For general storage or display shelves a medium factor (0.8) is appropriate, and for decorative or very light-duty shelves a light factor (0.7) is sufficient. When in doubt, err toward the heavier category — a slightly deeper dado costs nothing but router time.
Why should dado depth never exceed half the shelf thickness?
Cutting a dado deeper than half the shelf thickness removes more than half the cross-sectional material from the piece that sits in the groove. Under a transverse load, the shelf acts as a beam and the reduced section at the dado location becomes the weakest point, making it prone to cracking or snapping. Staying at or below 50% of shelf thickness ensures the remaining material above the dado has enough strength to carry the intended load. This limit is also enforced by most cabinet-making standards and furniture engineering guidelines.