Leader Butt Diameter

Heavy gear

Martin Joergensen

The butt diameter of monofilament used for making knotted fly fishing leaders is important because it ensures proper energy transfer from the fly line to the leader.

The diameter isn't a definite size compared to line weight, but depends on the line, the fishing type and the leader design (formula), but the butt section's diameter should ideally be about 60-70% of the diameter of the fly line tip. This helps with energy transfer and smooth leader turnover. But notice that the tip diameter on fly lines of the same weight can vary, and that it will never be an exact science choosing the butt thickness. Some light lines have fairly thick tips and some heavier lines end in fine tipped tapers. You will have to try different thicknesses to select the behavior that suits you.

You can choose to use stiff monofilament (like Maxima Chameleon or Mason Hard Mono) for the butt section, as it aids in proper turnover.

The fly size you wish to fish is of course also significant in relations to diameter of butt as well as tip of your leader. You will find more considerations in Leader formulas and in Fly fishing leader construction.

Here's a general guideline matching butt diameters with line weights:

Line Weight 1-3:

Butt Diameter: 0.017 to 0.019 inches (0.43 to 0.48 mm)
These lighter lines pair with finer butt sections to ensure smooth energy transfer.

Line Weight 4-6:

Butt Diameter: 0.020 to 0.023 inches (0.51 to 0.58 mm)
Mid-weight lines require slightly thicker butt sections for proper turnover.

Line Weight 7-9:

Butt Diameter: 0.024 to 0.027 inches (0.61 to 0.69 mm)
Heavier lines pair with thicker butt sections for casting larger flies and handling heavier loads.

Line Weight 10-12+:

Butt Diameter: 0.028 to 0.030+ inches (0.71 to 0.76+ mm)
These are for saltwater, big-game and salmon/steelhead fishing, where a thicker butt is necessary for turnover and strength.

Fly, rod, reel

Martin Joergensen