Blood Knot
Also known as: Barrel Knot
fishing knot
Building a tapered fly-fishing leader out of successive sections of thinner and thinner monofilament is a tradition going back generations, and the blood knot has been the standard way to join each section for almost as long. Each line end wraps several times around the other in opposing directions, and when both are pulled slowly together, the wraps compress inward into a tight, low-profile shape - genuinely flatter and cleaner through a rod's guides than most alternative line-to-line knots.
That compression is also exactly why the knot performs best when the two lines being joined are similar in diameter. Wrap a much thinner line around a much thicker one and, instead of both sides compressing evenly toward the center, the thinner wraps can simply ride up and over the thicker line rather than locking against it - a real, well-known limitation rather than a tying-error problem.
Where that diameter mismatch is unavoidable - joining a stout butt section to fine tippet, for instance - most anglers reach for the surgeon's knot instead, trading the blood knot's cleaner profile for the surgeon's knot's much greater tolerance of unequal line sizes. Within its intended use, joining similar-diameter sections, the blood knot remains a trusted, time-tested standard.
Its name comes from its historical use in tying snares and leader material where the multiple tight wraps resembled, to early observers, a tourniquet-like wrapping - a folk etymology repeated often enough in angling literature that it's now essentially the accepted origin story, even without a single definitive documented source. Whatever the exact origin, the knot's core mechanic - opposing wraps compressing toward a shared center - has stayed unchanged for as long as it's been recorded, a rare case of an old technique needing no real modernization even as fishing line materials have changed completely around it.
How to tie the Blood Knot
Overlap the two line ends several inches, pointing in opposite directions.
Strength & security
Approx. strength retained: 85-90% (similar diameters)
The multiple opposing wraps on each side compress into a tight, low-profile barrel shape that sits cleanly through rod guides - the classic way fly anglers build a tapered leader from sections of progressively thinner line.
Cautions: It performs poorly when the two lines differ much in diameter, since the thinner line's wraps can simply slip over the thicker one rather than compressing evenly - the surgeon's knot handles that mismatch far more reliably.
Alternatives
- Surgeon's Knot
Much better tolerance for joining lines of noticeably different diameter, and considerably faster to tie, at the cost of sitting bulkier through the guides.
- Double Uni Knot
An easier-to-learn alternative for joining two similar lines, though generally regarded as slightly bulkier than a well-tied blood knot.
FAQ
Why is the blood knot mainly used for building leaders?
Its low, tapered profile passes through rod guides more cleanly than bulkier alternatives, which matters a lot when joining several sections of progressively thinner line into one tapered leader.
Does the blood knot work with mismatched line sizes?
Not well - the thinner line's wraps tend to slip rather than compress evenly against a much thicker line. The surgeon's knot handles a real diameter mismatch far more reliably.
Where does the name 'blood knot' come from?
The most commonly repeated explanation ties it to the knot's tight, wrapped appearance resembling old tourniquet wrapping, though no single documented source confirms this as the definitive origin.
How many wraps should each side of a blood knot have?
Typically five wraps per side for standard tippet material, though thinner, more supple line sometimes takes an extra wrap or two to compress fully and grip securely - too few wraps on light line is a common reason a blood knot fails to hold under a hard hookset.
Can the blood knot be tied with fluorocarbon line?
Yes, though fluorocarbon's stiffness means the wraps need to be compressed slowly and evenly, with good lubrication, to seat properly - rushing the final pull on fluorocarbon is more likely to leave a loose or uneven blood knot than the same technique would with softer nylon.
Does the blood knot need a different wrap count for fluorocarbon versus nylon?
Many anglers use one or two fewer wraps per side on stiffer fluorocarbon than they would on supple nylon, since fluorocarbon resists compressing down as tightly - too many wraps on stiff line can leave the knot bulky and incompletely seated rather than genuinely stronger.