The classic way to join two lengths of line of similar diameter. Slim, symmetrical, strong enough at 85 to 90% — and still the knot fly anglers use to build a tapered leader from scratch.
Lay the two line ends across each other with about six inches of overlap. The diameters should be close — this knot struggles if one line is much thicker than the other.
Take one tag end and wrap it around the opposite standing line five to seven times. Five for heavy line, up to seven for fine tippet.
Pass the tag end through the gap between the two lines, at the center of the wraps.
Now wrap the other tag end around the first standing line, the same number of turns, wrapping the opposite way. Tuck it through the center gap going the other way.
Lubricate thoroughly, then pull the two standing lines apart slowly. The wraps draw together into a neat barrel. Trim both tags close.
| Line / tippet | Turns each side | Typical strength |
|---|---|---|
| Heavy mono (20 lb+) | 4–5 | 80% |
| Medium mono (8–15 lb) | 5–6 | 85% |
| Light tippet (4–7 lb) | 6–7 | 90% |
| Fine tippet (0X–6X) | 7 | 90% |
More turns on thinner material, fewer on thicker. The rule is simple: the thinner the line, the more wraps you need for the barrel to bite.
Tie it wet every time. Dry friction on fine fluorocarbon tippet generates enough heat to visibly weaken the line — this is the single most common cause of a blood knot failing below its rating.
Building a tapered fly leader. A blood knot lets you step down through three or four diameters of tippet, and because it is slim and symmetrical it turns over properly in the air. No other knot does this as neatly.
It is also the standard for repairing a leader mid-river when you snap off a section — same diameter, quick join, and the fish never sees it.
For joining lines of different diameter — the usual case when adding a fluorocarbon leader to braid — it is the wrong tool. Use a double uni, an FG knot, or in a hurry a surgeon’s knot. Then check the result against the Knot Strength Calculator.
A blood knot is mostly about tippet material and patience. Fine fluorocarbon and a sharp cutter make it much easier.
What blood knots are actually for. Fine tippet steps down cleanly and the knot stays nearly invisible.
Check current price on Amazon →Ready-made if you would rather not build one. A blood knot is how you repair or extend them.
Check current price on Amazon →Fine tippet needs a close, clean cut. A combined clipper and knot picker covers both jobs.
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The blood knot is the classic join for two lines of matching diameter. The double uni knot handles the mismatched case, the knot strength calculator puts a number on retention, and the leader size chart sets the leader class.
Generally no. A blood knot needs the two lines to be of similar diameter, and braid is much thinner than fluorocarbon of the same pound test. The braid will slip or cut through. Use a double uni, an FG knot or an Albright for braid-to-leader joins.
Five to seven per side. Four or five for heavy monofilament above 20 lb, six to seven for light tippet below 7 lb or anything finer than 3X. Too few turns and the barrel will not bite; too many and it will not seat cleanly.
Two usual causes: tying it dry, and mismatched diameters. Dry tightening heats and damages fluorocarbon in particular. And if one line is much thinner than the other, the thinner line does the cutting rather than the knot doing the holding.
They are close — a blood knot is typically 85 to 90% versus about 95% for a surgeon’s knot in ideal conditions, though a poorly seated blood knot falls well below that. The surgeon’s knot is faster and more forgiving; the blood knot is slimmer and more traditional for fly leader construction.
Mainly building and repairing tapered fly fishing leaders, where its slim symmetrical profile turns over correctly in the cast. For joining two lines of different diameters, modern braid-to-leader knots such as the double uni and FG have largely replaced it.
How to tie a uni knot · How to tie an FG knot · Leader size chart · 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 Double Uni 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
Please note: every figure on this page is an estimate, not a measured result. Tolerances between brands, wear on your gear and your own setup all change the outcome. TackleCalc has no connection to any brand mentioned here, and this content is for reference only.