The double uni is the default braid-to-leader knot in the US for a reason: it is strong, it is quick once you have the rhythm, and it does not need a third hand. Here is how to tie it so it does not slip.
The two knots meeting in the middle is what makes it hold. Each one is a stopper for the other.
Too few wraps. Four wraps slips under load on braid. Six is the number that holds, seven or eight if your braid is very thin.
Jerking it tight. Pulling fast generates heat and can weaken the leader. Pull slowly and evenly until the coils stack up neatly.
Trimming flush. Leaving no tag at all lets the knot work loose over a session. Leave a tiny stub.
Not seating it with the standing lines. Pull the two main lines apart, not the tags. That is what drives the knots together.
A good double uni is mostly about the tools and the line, not the technique.
Braid defeats ordinary clippers and your teeth. A sharp dedicated cutter makes clean tag ends, which is most of what a good double uni looks like.
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The double uni is the join to use when braid meets leader. The single uni knot is the base method, the knot strength calculator shows retention, and the braid to mono converter gives the matching diameter.
Six per side is the standard. Use seven or eight on very thin braid, and you can get away with five on heavy mono, but six each side is the number most anglers settle on.
The FG is stronger and thinner, so it passes through rod guides more easily. The double uni is much faster to tie and plenty strong for most freshwater fishing. Use the uni unless you are casting through small guides all day.
You can, but a loop knot or a Palomar is better at the lure end. The double uni is built for joining two lines, not for attaching hardware.
Usually too few wraps or a dry tie. Wet both knots, use six wraps, and pull the two main lines apart slowly instead of jerking them.
Two to three feet for most freshwater, long enough that several casts of abrasion do not shorten it past usefulness. Go longer in clear water.
Knot strength chart · Surgeon's knot · Uni knot · Best knots for braid · Knot strength calculator
How to Tie a Palomar Knot · How to Tie an Improved Clinch Knot · How to Tie a Uni Knot · How to Tie an FG Knot · How to Tie a Blood Knot · How to Tie a Surgeon's Knot · How to Tie a Snell Knot · How to Tie a Non-Slip Loop Knot · How to Spool a Spinning Reel · Spooling Without Line Twist · How to Set Drag on a Reel
Numbers to go with it: Knot Strength Calculator · Line Diameter Converter · Shock Leader Calculator
Disclaimer: The numbers on this page are estimates based on published specifications and standard formulas. Real-world results vary with brand, gear condition and how the equipment is set up. TackleCalc is an independent reference site and is not affiliated with any manufacturer. Nothing here replaces the instructions that come with your own gear.