Blood Knot: How to Tie It, Turns by Line Size, and Strength

Use the blood knot to join two monofilament lines of nearly the same thickness: within .002 in (0.05 mm). You wrap each line around the other and tuck the two tags, the short free ends, through the center in opposite directions. Pull it tight and you get a neat, even barrel of coils. It’s the fly angler’s knot for joining the sections of a leader, the clear line between the thick fly line and the fly, and for adding tippet at the end. In any other fishing, it’s a strong join for mono of similar size. Two lines further apart than that get a surgeon’s knot, the fast knot for lines of different sizes.

Wet it with saliva before you pull it tight. It held better that way in Salt Strong’s six-trial pull test. It also tested 13 percent stronger there than the double uni, another knot for joining two lines, with 20 lb mono tied to 30 lb.

When is the blood knot the right join?

This knot has one condition: your two lines must be close in size. Orvis’s rule for joining mono is no more than .002 in apart in diameter, which just means how thick the line is. On fly tippet that’s one or two sizes. Tippet comes in X sizes, and the bigger the X number, the thinner the line. To get a tippet’s diameter, take its X size away from 11 and read the answer in thousandths of an inch. That makes 4X .007 in and 3X .008 in, so you can tie 4X to 3X with a blood knot, or 5X (.006 in) to 3X.

Salt Strong ranks it first of five knots for mono to mono, with the same catch: it’s “not as effective with lines of different diameters.”

Its home is the tapered leader, which steps down from a thick end at the fly line, called the butt, to a fine tippet. Orvis puts most fly leaders at 7.5 to 12 ft (2.3 to 3.7 m). Joining those sections is the blood knot’s classic job. On a trout stream, it’s the knot you tie to add a fresh length of tippet before you tie on one of the trout flies.

Off the fly rod, you can join monofilament to mono of similar size. Fluorocarbon (fluoro, a line that’s harder to see under water) ties to mono as well. Salt Strong says its mono-to-mono result applies to fluoro to mono too, though it tested only mono. The fluoro number on this page is John Merwin’s: 83 percent, below, on 10 lb fluoro tied to 10 lb mono.

Braid, line woven from thin fibers, is another matter. You can tie it braid to mono, but Salt Strong’s Luke Simonds doesn’t recommend it. It’s harder to tie and not as strong as the FG knot, a slim knot made for braid. For braid to a leader, a length of different line between your main line and the hook, tie the FG knot or the double uni. Start with the double uni, because it’s the easier one to learn.

If all you’re tying is one line to a hook, you don’t need this knot yet. The clinch knot does that job.

How do you tie the blood knot?

Cross the two ends, wrap each one around the other, tuck both tags through the center, then wet it and pull. You’ll need two words for the steps. The tag end is the short free end you tie with. The standing part is the long line behind it.

Photo: Chris 73, Wikimedia Commons (CC BY-SA 3.0)

The photos use thick cord so you can see the path. The first one shows the two ends side by side, before you cross them. The second shows both lines wrapped, with the tags not tucked yet. The third shows both tags through the center, and the fourth is the knot pulled tight. The photos do both wraps first, and in the steps below the first tag tucks before the second wrap. It’s the same knot. Don’t copy the number of wraps you see in the pictures. Take your count from the table below.

1. Cross the lines. Lay the two ends across each other in an X, pointing in opposite directions. Leave 5 to 6 in (13 to 15 cm) of tag end on each side. Pinch the crossing between your finger and thumb.
2. Wrap the first tag. Wrap one tag around the other line’s standing part, working away from the crossing, for the number of turns in the table below. One turn is one trip around the line. Bring the tag back and tuck it between the two lines at the crossing.
3. Wrap the second tag. Hold that tuck in place. Wrap the other tag around the first line the same number of turns, working the other way.
4. Tuck it through the same opening, the opposite way. The second tag goes through the same gap at the center as the first, but from the other side. The two tags now exit in opposite directions. Leave enough tag sticking out that it can’t slip back through as the knot closes.
5. Wet it and pull the standing lines. Wet the knot and pull both standing parts quickly. Never tighten it by pulling the tag ends.
6. Check, then trim. You should see neat, even coils on both sides. If you don’t, cut it and start over. If you do, trim the tags close.

Your tightened knot won’t look like the loose one, and that’s fine. As it closes, the turns shift and the inside strand ends up on the outside. A good one shows two even barrels of coils with the tags pointing opposite ways. Wet it before you pull it tight and trim close after: every fishing knot wants that, not just this one.

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How many turns per side?

The finer your line, the more turns you make. “Per side” just means on each line, so seven per side is seven turns with each tag. Go by Orvis’s three-step rule, by diameter. The last row, from Animated Knots, is for when you don’t know your line’s diameter. The X sizes come from the 11-minus-X sum above.

Use the table as your starting point, because the counts differ by a turn or two. Lefty Kreh’s book Fishing Knots uses four to six wraps, and Wikipedia follows him. Salt Strong’s strength test used five turns on the 20 lb line and four on the 30 lb.

Chris Fave, who ties leaders for a living, goes lower still. He uses four turns as “perfect for most weights,” five at 6X and smaller, and never seven. Fewer turns on fine tippet is a choice some experienced knot tiers make. If you’re not sure, tie seven on 5X and 6X. It’s the safe count.

How strong is the blood knot?

The blood knot came out on top in both tests here that put it against other joins on the same lines. Two more show how the surgeon’s compares. In a pull test, you tie a knot and pull until it breaks, so the higher the number, the stronger the knot.

For the Salt Strong test, Sean Person joined a 20 lb line to a 30 lb line (lb is the pounds of pull a line is sold as holding). He tied both knots with five turns of the lighter line and four of the heavier. Lines that far apart usually differ by more than Orvis’s limit, so that result comes from a join the rule would keep you away from. He soaked the line in water for 10 seconds before every pull. So “dry” here means no saliva on the knot. He cinched, or pulled tight, half of the six knots without it, and wetted the other half with saliva before cinching. The blood knot did better with saliva, and the double uni did better dry. He doesn’t say how many pounds it took to break them, so the 13 percent is the whole result. Put a little spit on your blood knot before you pull it tight.

Merwin’s numbers show how much of the line’s strength each knot kept, compared with the same line with no knot in it. He named no machine and no pull count, so trust the order more than the exact figures. You may run into an 80 percent knot strength in Wikipedia’s infobox, the fact box on its page. It has no source behind it.

Blood knot or surgeon’s knot: which is stronger?

On lines of similar size, the blood knot. Merwin’s blood knot beat a three-turn surgeon’s knot by 18 points, 83 to 65 percent, on the same lines. That’s the only test here with a number for both. When your lines differ in diameter, tie the surgeon’s, because that’s the job it exists for. Orvis keeps the blood knot to lines close in size, and Salt Strong calls it “not as effective” there. No test on this page pulled the two against each other on unequal lines. Salt Strong’s 2018 ranking of mono-to-mono knots puts the blood knot first of five and the surgeon’s third.

Why does a blood knot slip?

When yours slips, check these six things.

Is there a faster way to tie it?

The awkward part of the standard method is holding the first tuck while you wrap the second tag. Three shortcuts get around it. Try the last one first, because it needs no tool and the tags still go opposite ways.

• The speed blood knot. This one is built so you don’t have to control the tag ends. In an Orvis News video from August 4, 2023, Ross Purnell, editor of Fly Fisherman, shows how. It’s not a true blood knot, because the tags pass through the central loop in the same direction. In the standard knot above, that’s a reason it slips. This is a different knot, and Orvis’s Phil Monahan says he believes Purnell that it doesn’t fail.
• The toothpick method. A toothpick lets Chris Fave tie one “perfectly in seconds.” He ties about half a million blood knots a year, by Gink & Gasoline’s count. Their video shows how.
• Twist, then split. Overlap the two ends and twist them together for about ten to fourteen turns. Open a hole at the center of the twists, and pass the two ends through it in opposite directions.

Which join for which situation?

On the water, it comes down to the gap between your two lines. The fishing knots page has the same kind of table for every line and job.

Where does the name come from?

Nobody knows for certain. You’ll also hear it called the barrel knot, and it carries three numbers in The Ashley Book of Knots: 295, 345 and 1413. Clifford Ashley credits only the name, to Keith Rollo. His verdict is short: “The barrel knot, called blood knot by Keith Rollo, is the best bend there is for small, stiff or slippery line.” A bend, in knot language, is any knot that joins two lines.

There’s a half blood knot too. That’s the clinch knot, which ties line to a hook, lure or swivel, a small fitting with two eyes that turn. When two half blood knots join two lines, they count as one knot: the blood knot. The clinch’s close relative is the improved clinch knot. And “blood knot” is also the name of a double overhand knot tied in the tails of a cat-o’-nine-tails, a whip with many tails. That’s a different knot entirely.

When should you tie something else?

Tie the surgeon’s knot when your two lines aren’t close in diameter, and the FG knot or the double uni for braid to a leader.

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