Tongue and Groove Calculator — Panel Joint Dimensions

The tongue thickness should be approximately one-third of the board thickness. For 18mm boards, the tongue is 6mm thick. This leaves equal shoulders on each face for a balanced joint.

This calculator sets the tongue projection to one-third of the board thickness, capped at about 10mm. On typical cladding stock that works out to roughly 4–10mm: a 12mm board gives a 4mm tongue, an 18mm board gives 6mm, and anything 30mm or thicker is held at the 10mm cap. Too short and it lacks strength; too long and it weakens the edge of the grooved board.

The covering width is the visible width of each board after assembly. It equals the board width minus the tongue length, since the tongue is hidden inside the next board's groove. You need this figure to calculate how many boards cover an area.

Yes. Use a dado blade to cut the groove, then cut the tongue by making two passes with a standard blade (one each side of the tongue). A router table with a tongue-and-groove bit set is even easier for long boards.