How thick should the tongue be?

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.

How long should the tongue project?

The tongue typically projects 6–10mm from the shoulder. Too short and it lacks strength; too long and it weakens the edge of the grooved board. One-third of the board thickness is a good rule of thumb, up to a maximum of about 10mm.

What is the covering width?

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.

Can I make tongue and groove on a table saw?

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.

WoodCalcs

Tongue and Groove Calculator — Panel Joint Dimensions

Calculate tongue and groove dimensions for panel and flooring joints based on board thickness.

Board Thickness

Board Width

How We Calculate This

This calculator uses standard proportions for tongue and groove joints.

Rules applied

Tongue thickness: Board thickness ÷ 3
Tongue length: min(Board thickness ÷ 3, 10mm)
Groove: Same width as tongue, 1mm deeper for clearance
Covering width: Board width − tongue length

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Always verify measurements and material quantities before cutting or purchasing.