Get the cut depth and shoulder length for a half-lap — a corner lap, a T-lap or a cross lap — from the actual thickness and width of the two boards. Works for equal boards and for a thin board lapped into a thicker one.
A half-lap is the simplest strong frame joint: each board gives up part of its thickness where they overlap, so the two fit together into one layer with a large glue surface. Workbench stretchers, shop cabinets, lattice, frames for upholstered panels and jigs all use it. It is easy to cut on the table saw, with a router, or by hand.
Two numbers decide the joint: how deep to cut each board, and how long the cut is. The cut length always equals the width of the other board, so measure actual widths — a nominal 2×4 is 3½ in wide, not 4.
Midplane (the usual choice): each board loses half its own thickness. With equal boards the faces end flush on both sides. With unequal boards, the joint is centered on each board and the faces step by half the difference.
Flush on one face: for a thin board lapped into a thicker one where you want one show face flat. Both cuts are half the thin board’s thickness, so the stacked joint is as thick as the thicker board and the thin board’s outer face lines up with one face of the thick board.
cut length on A = width of B · cut length on B = width of A
midplane: depth = own thickness ÷ 2 · flush on one face: both depths = thin thickness ÷ 2
| Board A × Board B | Midplane depth A / B | Joint thickness | Flush-one-face depth A / B | Joint thickness |
|---|---|---|---|---|
| 0.75 × 0.75 in | 0.375 / 0.375 in | 0.75 in | 0.375 / 0.375 in | 0.75 in |
| 1 × 1 in | 0.5 / 0.5 in | 1 in | 0.5 / 0.5 in | 1 in |
| 1.5 × 1.5 in | 0.75 / 0.75 in | 1.5 in | 0.75 / 0.75 in | 1.5 in |
| 0.75 × 1.5 in | 0.375 / 0.75 in | 1.125 in | 0.375 / 0.375 in | 1.5 in |
| 0.875 × 1.25 in | 0.4375 / 0.625 in | 1.0625 in | 0.4375 / 0.4375 in | 1.25 in |
Two ¾ in boards, 3½ and 5½ in wide. Midplane: cut each 0.375 in deep. The cut on the 3½ in board is 5.5 in long; the cut on the 5½ in board is 3.5 in long.
A 2×4 frame with ¾ in cross braces. Board A 1.5 × 3.5 in, board B 0.75 × 3.5 in. Midplane takes 0.75 in from A and 0.375 in from B and gives a 1.125 in thick joint, with the brace faces stepped back on both sides. Choose “Flush on one face” instead and both cuts are 0.375 in: the brace’s outer face is flush with one face of the 2×4, and the joint is the full 1.5 in thick.
A cross lap in 1 in stock for a lattice. Midplane, 0.5 in deep on each piece. Cut length is the width of the crossing piece.
A glued half-lap in good long-grain contact is strong for frames that are loaded in their own plane. It is weaker against twisting out of plane, and the thin cheeks can split if the lap is near the end of a board. Add a screw or a dowel through the joint where it takes racking loads, and keep laps away from the very end of a board when you can.
With a router: clamp the two boards side by side so you can lap both in one pass, use a straightedge or a T-square jig against the shoulder line, and remove the waste in shallow passes with a straight or spiral bit. A router plane cleans the cheek to the exact depth.
By hand: knife the shoulder line, saw it to depth on the waste side, make a series of saw kerfs to depth across the waste, then chop the waste out with a chisel and pare the cheek flat. Checking the depth from the reference face with a marking gauge set to the calculator figure keeps the result honest.
For equal boards, yes. For unequal boards it depends on which faces you want flush — that is what the mode selector is for.
The depth and length math is the same. A corner lap cuts at the end of both boards; a cross lap cuts somewhere along their length.
For furniture, yes. For shop jigs and temporary frames, screws alone are often enough.
The labels say inches, but the arithmetic is unit-free; enter everything in millimetres and read millimetres.