Fly Fishing: How Important Is Backing Line?

What's behind your fly line is more vital than you think.

September 10, 2025 By Mike Conner

Backing is basically an afterthought to fly fishers who rarely hook a fish that runs more than a flyline off a reel. Think bass, panfish, and smaller saltwater species such as seatrout.

When I took charters flats fishing, I had a few customers show up with their own fly rods, and the most common shortcoming was that they were short on backing tarpon or bonefish we were chasing.

A full spool of backing, say 200 yards on a suitable reel paired to a 9-weight rod, is plenty for bonefish, permit, or medium tarpon, and should still allow for enough spool space for a 9-weight flyline. As an unappreciated benefit, backing also fills the arbor so your flyline—which has memory—comes off in larger, easier to stretch loops when it comes time to cast.

All flyline cores are built to be much stronger than the leader tippets that are likely to be used with them. For example: An 8-weight line core will be about 30-pound test, and an 11-weight line core may test as high as 75 pounds. Flyline backings similarly over-test. The weakest section of your leader, the class tippet and knots, will fail first.

You Have Choices for Backing Line

Dacron has long been the standard backing. Made of polyester fibers, it is typically hollow, making it possible to splice, if desired. It’s relatively affordable and durable. Popular brands include Cortland 20-pound-test Micron for most inshore saltwater fly fishing, with the exception of giant tarpon, for which 30-pound-test Dacron is a better choice, for big reels matched to 11- or 12-weight fly rods. Micron comes in colors, including hi-vis options that help an angler track a hooked fish. Most large-arbor saltwater reels for 8- to 10-weight rods can spool up approximately 200 yards of 20-pound-test Micron. Micron is braided with premium Dacron fibers in a 12-carrier construction, referring to the number of individual fibers in the braid.

Cortland’s Hollow X backing is made for big-game fly fishing, testing at either 50 or 70 pounds, and the hollow construction makes splicing to the back of a flyline possible, for a seamless connection.

Other makers of both Dacron and gel-spun backing include Scientific Anglers, Rio and Hatch. Of course, you can use any gel-spun braid manufactured for spinning and conventional casting reels. Low diameter braid creates less drag in the water when a big fish fights. The tradeoff is it can cut your hands if you are careless when a running fish has it under tension. It is also more expensive.

Fly-Fishing Connections

Most fly fishers simply attach backing with an Arbor knot. Wrap the backing tag end around the reel arbor twice to give it a good footing. Now take tie a uni-knot around both strands you’ve created and seat the knot partially, because you still need to slide it down to the reel arbor before tightening completely.

If you opt for gelspun backing, be aware that the inherent slickness can allow the entire body of the spooled backing to slip around the arbor under pressure. To prevent this, first tie in some monofilament, reel up about 10 turns of it, and then attach your backing to the mono with back-to-back uni-knots. You can also wrap a few layers of masking tape around the arbor to provide friction before tying on the gelspun.

Most flylines come with factory loops at both ends. To connect the flyline, you need only create a loop in your backing. Choose a spider hitch, triple surgeon’s loop, or create a Bimini twist. Make your backing loop large enough to pass around your reel, off the rod. Otherwise, you cannot make the connection. If you do not have a flyline with a factory loop, use the Albright special.

A word of advice: Have your backing spooled by a tackle shop with a spooling machine. It will decrease the chance of spongy spots that occur when you do not have adequate pressure applied when spooling it by hand. You can get away with doing this job by winding it on through a heavy glove on your line hand, but why chance it? Should your backing dig into itself as a strong fish runs, the battle is over!