Blood Knot — Joining Two Similar Lines
April 26, 2026
What Is the Blood Knot?
The blood knot is the classic line-to-line knot for joining two strands of similar-diameter monofilament. Symmetrical, compact, and reliable when tied carefully, the blood knot is the foundation of fly-fishing leader construction — every step-down section in a tapered leader is joined with a blood knot.
For Indiana fly anglers chasing brown trout in southern Indiana streams or steelhead in Lake Michigan tributaries, the blood knot is mandatory equipment.
How to Tie a Blood Knot
- Overlap two line ends by 6 to 8 inches each, with the lines running in opposite directions.
- Wrap one tag end around the other line 5 to 7 times, working away from the center.
- Pass the tag end back to the center and through the gap between the two lines.
- Wrap the other tag end around the first line 5 to 7 times in the opposite direction.
- Pass that tag end through the same center gap, going the opposite direction from the first tag.
- Wet the knot thoroughly.
- Pull both standing lines slowly — the wraps barrel up against each other and tighten into a compact knot.
- Trim both tags to 1/8 inch.
The finished knot looks symmetric, with two sets of wraps mirroring each other.
When to Use a Blood Knot
- Fly leader construction — joining tapered sections of mono leader
- Tippet to leader — adding fresh tippet to a worn leader butt
- Mono-to-mono splices — when both lines are similar diameter (within 0.05mm)
- Drift fishing leaders — building custom-tapered leaders for steelhead
When to Skip It
- Mismatched diameters (10 lb to 30 lb) — use a double uni or surgeon’s knot
- Mono-to-braid — use a double uni or FG knot
- Heavy fluorocarbon (over 17 lb) — fluoro is stiffer and the wraps don’t seat cleanly; use a double uni
Indiana Applications
- Brown trout fly fishing on Sugar Creek, Big Pine Creek, or limestone streams in southern Indiana — joining 4X tippet to 3X leader
- Steelhead drift fishing — building custom mono leaders for Trail Creek, Little Calumet, Salt Creek
- Indoor fly tying station — pre-rigging tapered leaders for the season
Tying Tip
The blood knot is fiddly to tie in the field with cold hands — many fly anglers pre-tie leaders at home and store them coiled. Practice on the kitchen table with two different colored mono lines so you can clearly see what each wrap is doing. Once mastered, a blood knot can be tied in under 30 seconds.
The Bottom Line
For fly fishing, the blood knot is the traditional standard. For general bait fishing in Indiana, the double uni is faster and handles mismatched diameters better — most casual anglers can skip the blood knot entirely. But if fly fishing is in your future, learn this one.
Frequently Asked Questions
When should I use a blood knot vs a double uni?
Blood knots are best for joining two lines of similar diameter — typical in fly fishing leader construction (e.g., 0X to 1X tippet sections). Double unis handle larger diameter mismatches (braid to fluoro leader). For Indiana steelhead drift fishing or fly fishing trout streams, blood knots are the traditional choice.
Why does my blood knot keep slipping?
Two common causes: not enough wraps (use 5-7 per side on light line) or trying to join lines of very different diameters (more than 0.05mm difference). For mismatched diameters, use a double uni or surgeon's knot instead.
Is the blood knot strong?
Properly tied with matched line diameters, the blood knot retains 75-90% of line strength — strong enough for trout and steelhead leader work. It's the most compact line-to-line knot, which is why fly anglers prefer it: a small blood knot passes through fly rod guides without snagging.