Tongue and Groove Calculator

Tongue and Groove Calculator for counting T&G boards by face width, with end drops, openings and nails for a wall, ceiling or room, plus the groove and tongue for milling your own.

Tongue and Groove Calculator for Walls, Ceilings and Saunas

The Tongue and Groove Calculator counts the T&G boards to buy for a wall, a ceiling or a whole room, including a gable wall. It works from the face width each board covers, then adds end drop, openings and extra for defects.

A second mode sizes the joint itself when you mill your own boards. It gives the groove width and depth, the tongue length and the face each board will cover.

Room sizes are in feet and board sizes in inches, with a metric switch. Board size is picked by its nominal name, such as 1x6, while the face width and thickness fields take the actual milled sizes.

How Many T&G Boards Cover a Wall

Only the face of each board covers the wall, because the tongue disappears into the next groove. The Western Red Cedar Lumber Association (WRCLA) coverage table turns wall area into linear feet with that face width.

$$LF=A\times \frac{12}{f}$$

Here $A$ is the wall area in square feet and $f$ is the exposed face in inches. A dry cedar 1x6 is 5-3/8 inches wide but covers 5 inches, so WRCLA's coverage table gives a factor of 2.40 linear feet per square foot.

WRCLA's steps subtract doors and windows first, then add 10% for trim loss. The calculator does the same, with the extra adjustable from 5% for clear boards up to 25% for raw stock or gables.

The default 12 by 8 foot wall shows why a plain area estimate runs short. The factor alone gives 230 linear feet, but the calculator orders 264, including the 10% extra.

Part of that gap is the row count. An 8 foot wall takes 96 inches divided by 5, or 19.2 rows, so it needs 20 rows with the first and last ripped to 3 inches each.

$$N=\lceil \frac{12H}{f}\rceil$$

End drop and which board length to buy

Sauna Supply Co.'s own T&G calculator notes that its linear footage leaves out drop. Its example is a 7.5 foot wide sauna that needs 8 foot boards, which throws away half a foot on every board.

The Tongue and Groove Calculator lays out every row and reuses offcuts where they fit, so drop is counted. It then compares six board lengths for the same wall and flags the one that wastes least.

On the default wall, 8 and 12 foot boards both need 264 linear feet. Sixteen-foot boards need 352, because each 12 foot row leaves a 4 foot piece that no other row can use.

Square-end boards have to join over a stud or strip, so a board is only used up to the last framing member it reaches. End-matched boards have a tongue and groove on their ends too, and can join anywhere between supports.

Face width changes from mill to mill

Buffalo Lumber describes a customer who ran short on 1x6 cedar paneling. Their coverage guide warns that thickness, width and exposed face vary between mills even under the same grading standards.

A difference of 1/8 inch looks small but adds up. On the default wall, a 4-7/8 inch face needs 236 linear feet before extra, while a 5-1/8 inch face needs 225.

Measure the face on a sample board, from the shoulder of the tongue to the far edge. The calculator's coverage table shows how your area changes across common 1x4, 1x6 and 1x8 faces.

Board feet are counted on the nominal size, which is how lumber yards price by volume, so 264 linear feet of 1x6 is 132 board feet. The board foot calculator converts other stock the same way.

Nailing Pattern and Board Direction

WRCLA's T&G installation guide blind nails boards up to 6 inches wide, one nail per stud or strip through the base of the tongue. Wider boards get two face nails per bearing, and nails must reach 1-1/4 inches into solid wood.

The calculator adds 1-1/4 inches to your board thickness and rounds up to a stock nail length. An 11/16 inch board gets a 2 inch nail, and every row gets a nail at each stud plus one at each end joint.

WRCLA's full siding installation guide lists 1-1/2 inches into solid wood for exterior siding. For outdoor work, add 1/4 inch to the nail length the calculator gives.

Horizontal boards nail straight to studs. Vertical boards need horizontal blocking or furring strips, so the calculator adds the strip length at your spacing when you switch direction.

Run horizontal boards with the tongue up and the groove down. WRCLA installs siding that way so water sheds, and Sauna Supply Co. gives the same advice to keep moisture out of the groove.

Sauna Supply Co. also acclimates boards in the room for 48 to 72 hours before installing them. That lets the wood settle to the room's humidity before it is nailed tight.

Sizing a Milled Tongue and Groove Joint

Popular Woodworking's tongue and groove guide gives the usual rule, a square tongue about one-third of the stock thickness and centered on the edge. For 3/4 inch stock that means a 1/4 inch thick tongue.

Tongue length depends on how the boards go together. The same guide sets the cutter 1/4 to 3/8 inch deep for joints assembled without glue, and says a glued panel edge only needs a 1/8 inch tongue to register the faces.

The calculator's paneling rule makes the tongue at least half the stock thickness, held between 1/4 and 3/8 inch. That follows WoodworkDetails, which asks for a tongue at least half the thickness on boards wider than 3 inches.

$$D=L_t+g$$

The groove depth $D$ is the tongue length $L_t$ plus a gap $g$ at the groove floor. WoodworkDetails makes the groove up to 1/16 inch deeper than the tongue, so sawdust or swelling cannot hold the faces apart.

For 3/4 inch stock with the paneling rule, that works out to a 1/4 by 3/8 inch tongue in a groove 13/32 inch deep. A 5-1/2 inch board then covers 5-1/8 inches, the face width to use when counting boards.

Popular Woodworking warns that the walls of a deep groove may crack. The calculator flags a tongue much past one-third, which leaves thin walls, and one more than twice as long as it is thick.

Unglued T&G works because the tongue hides seasonal shrinkage, as long as the board shrinks less than the tongue is long. The wood movement gap calculator estimates that movement for a board's width and species. For two grooved edges joined by a loose strip instead, the spline joint calculator sizes it.