Bridle Joint Calculator

Bridle Joint Calculator settings follow the one-third rule for slot and tenon, then list every table saw setting in cutting order for a corner or T-bridle in your real stock sizes.

Bridle Joint Calculator for Frames and Legs

The Bridle Joint Calculator sizes the open slot and the tenon that fills it, then turns them into table saw settings. It also gives the long-grain glue area and where to bore the holes for a drawbored peg.

It covers the corner bridle, sometimes called a slip joint, which Canadian Woodworking uses on frame-and-panel door corners. It also covers the T-bridle, which joins a middle leg to a rail that runs through unbroken.

Enter actual sizes, not nominal ones. A 1x board is 3/4 in thick and a 2x board is 1-1/2 in. Tenon piece width sets how deep the slot goes, and slotted piece width sets how long the tenon is.

The One-Third Rule for the Slot

Slot size follows a trade convention rather than a standard. Canadian Woodworking makes the tenon equal to or slightly more than one-third of the stock, so both halves keep enough wood. Woodsmith gives the same guide, with a 1/4 in slot in 3/4 in stock.

$$s=\frac{\min (t,m)}{3}$$

Here t is the tenon piece thickness and m is the slotted piece thickness, and the third comes from whichever is thinner. The default setting then rounds s up to the next 1/16 in, which keeps the tenon on the slightly thicker side.

In 3/4 in stock the third is exactly 1/4 in, so nothing rounds. In 1 in stock the third is 0.333 in, which rounds up to a 3/8 in slot with 5/16 in tines. The One-Third Sizes by Stock table lists this for ten common thicknesses, with your stock highlighted.

$$w_{tine}=\frac{m-s}{2}$$

Each tine is half of what the slot leaves behind. If the slot takes more than 45% of the slotted piece, the tool warns that the tines are getting thin. Older practice sometimes went to half the stock, as one Sawmill Creek member notes, but one-third is the modern norm.

When the Pieces Differ in Thickness

With both parts centered, the thinner piece sits back from the thicker one on each face. The tool shows that setback, which is half the difference in thickness, so you can offset the slot or plane the faces flush later.

Canadian Woodworking sometimes planes about 1/16 in off the face of the tenoned piece on purpose. That leaves a small reveal at the corner and saves flushing the joint after glue-up.

Sizing the Slot to a Cutter You Own

Match My Cutter sets the slot to the width of your dado stack, straight bit or chisel. The hero line then shows what one-third would have been, so you can see how far the cutter moves you off the rule.

A cutter narrower than one-third gives stronger tines but a weaker tenon, and the insight box says so. A cutter that leaves less than 1/16 in per tine is flagged as an error before any results appear.

Table Saw Settings for Slot and Tenon

Canadian Woodworking cuts the slot first and fits the tenon to it, and the Table Saw Setup Sheet follows that order. Blade height for the slot is the tenon piece width plus the flush-trim allowance, so the tines finish slightly proud and get trimmed after glue-up.

The fence sits one tine width from the blade. With a 1/4 in slot and a 1/8 in kerf, that is two passes, flipping the piece between them so both tines come out equal even if the stock isn't perfectly centered.

$$c=\frac{t-(s-g)}{2}$$

The tenon is the slot width minus the fit clearance g. On the default joint that is a 0.245 in tenon, so each cheek cut removes 0.253 in. With the waste outside the jig, the tenon jig face sits 0.498 in from the near side of the blade.

The 0.005 in default clearance comes from real practice. Matthias Wandel cut a slot of about 0.405 in for a 0.400 in tenon, since calipers compress the wood slightly and a zero-clearance tenon is hard to force home.

Canadian Woodworking aims for a joint that closes with medium hand pressure but doesn't drop apart when turned upside down. If it needs a mallet, it's too tight.

Tenon length is the slotted piece width plus the same trim allowance. On a 3 in wide slotted piece with 1/32 in allowance, the stop block for the shoulder cut sits 3.031 in from the end.

Glue Area Compared With a Half-Lap

Both cheeks of the tenon glue long grain to long grain against the inside of the tines. Each cheek covers the tenon piece width times the slotted piece width.

$$A=2ab$$

A 2-1/2 in rail meeting a 3 in stile gives 15.00 sq in of long-grain glue. A half-lap in the same stock glues only one face, at 7.50 sq in. The Woodworker credits this larger glue area for the bridle being much stronger than a halving joint.

The End-Grain Contact row adds the slot bottom and the shoulders, 2.01 sq in on the default joint. Those surfaces are end grain and add little holding power. For a lap instead, the Lap Joint Calculator works out the notches.

T-Bridle Layout on a Through Rail

In T mode the rail runs unbroken and gets a notch on each face. The leg carries the slot and straddles what the notches leave behind, which suits middle legs under a continuous top rail.

Notch depth per face follows the same cheek formula. The shoulders sit at the joint center minus and plus half the slotted piece width, with no trim allowance, so a 3 in leg centered 12 in from the end gets shoulders at 10.5 in and 13.5 in.

The Section Left at Notch row shows how much rail survives at the joint. With a one-third slot in 3/4 in stock, 33% of the rail thickness remains at the joint. The joint center must sit at least half the leg width plus 1/4 in from the rail end.

Drawbore Peg Hole Positions

A drawbore peg pulls the joint tight as it is driven, because the hole in the tenon is shifted slightly toward its shoulder. In Drawboring Resurrected, Christopher Schwarz writes that 1/16 in almost always works, prefers 3/32 in for furniture, and goes toward 1/8 in on big joints in tough woods like ash or elm.

On the default corner joint the tine hole sits 1.281 in from the tine end and 1.500 in across the tine. That places it at the center of the rail once the 1/32 in trim allowance is counted.

The tenon hole sits 1.594 in from the tenon end, which is 1/16 in closer to the shoulder than the tine hole. In T mode the rail hole is measured from its seated edge instead, at 1.313 in on the default joint. The Bridle Joint Calculator defaults to a 1/16 in offset, and zero gives a plain pegged joint.

Strength and Fixing a Loose Tenon

Is a bridle joint as strong as a mortise and tenon?

A bridle holds well in compression, resists racking and is a common joint in workbench frames. A peg or other mechanical fastener is often added, and the drawbore settings above cover that. One experienced maker on Sawmill Creek calls bridles exceptionally strong when the proportions suit the load.

The two joints share the same proportions, with the tenon at about one-third of the stock. For the enclosed version, where the mortise has walls on all four sides, the Mortise and Tenon Calculator sizes it.

What if the tenon comes out too thin?

Canadian Woodworking's fix is to glue a shim onto the tenon cheek, let it dry and cut the tenon again. That is also why the calculator gives the tenon target as a decimal and a fraction, so you can sneak up on the fit in small steps.