Fly Line Backing 101 (2024 In-depth Guide)

When fly fishing, you always want to land that monster fish you’ve been dreaming about your whole life. To borrow a nugget of wisdom from Lee Wulff, “Game fish are too valuable to only be caught once.”

But is your fly reel, fly rod and fly line enough to get the job done? You might be missing something crucial to your fly fishing rig.

Fly line backing – what is it, and do you need it even if you have the best fly reel?

These are all good questions to be asking. Read on and know everything about fly line backing types, purpose, and knots you should tie it on with.

What is fly line backing?

Blue Fly Reel Closeup
Blue Fly Reel Close Up

Fly line backing is a thin line that looks a bit like braid that is attached onto your reel.

That thin line comes in various breaking strains, materials and colors all to match your fly angling setup and back up the length of your line creating added distance while you fight fish that make a long run.

What is the purpose of fly line backing?

The fly line backing is there so when a fish does take long runs, it stays attached to your fly reel so you can continue to fight it without losing it or running out of line.

When I was younger fly fishing for small trout, I always wondered what fly line backing was exactly for.

Considering the fish I caught barely pulled the fly fishing line off the reel, I thought it was attached to the reel just to fill up the reel so the fly line was nearer the top. Boy was I wrong!

Better to be prepared

Whichever fly line you use, you probably have about 90-100 feet of it.

Now, what happens when you hook a monster trout, a bonefish or a permit in saltwater and the fish runs off with all your fly line?

Well, without some fly line backing on your freshwater or saltwater fly reel, that fish is going to break off, break your rod, or run away with your fly line.

All things you never want to happen!

Do you need fly line backing?

You could ask this question differently by saying – do you want to land that fish of a lifetime you’ve been dreaming about your whole life?

You 100% need fly line backing on your reel. Even when equipped with the best saltwater fly reels, you’ll want to be using fly line backing.

You may not see its benefit when catching 1lb trout as you can pull them in with just a tippet if you like.

But, when you hook that 15lb fish or ever go fly angling in saltwater, your backing is what is going to ensure you landing on it.

What is fly line backing made out of?

Fly Reels
Fly Reels

There are two types of backing materials out there; dacron and gel spun backing.

Each material is excellent for the backing but they do differ quite a bit and here is how.

They come in every color and I’d suggest you buy one with a color that is easy to see, so you can see where the fish on the end of your gear is. It also lets you know if something like coral, branches, rocks or fast current is likely to impact on you landing your fish or not.

Dacron Backing

Dacron is most commonly used material as a staple fiber in clothes but when braided together into a line becomes an excellent backing material.

Dacron backing is strong, has low stretch for more feel, and is highly abrasion-resistant.

This makes it an excellent choice to put on your fly reel. Dacron backing is usually used for trout and small freshwater fly fishing as it’s relatively thick and will take up extra space on the fly reel’s spool.


Another thing to note about dacron is that it’s hollow meaning you can splice it to connect it to your fly line – more on that later.

Gel Spun Backing

Gel spun backing is made from braided polyethylene and has a smaller diameter but has high strength when compared to dacron.

The benefit of this is you can fit way more yards of gel spun backing on your arbor than dacron which comes in handy when you’re fishing saltwater for fish like sailfish or tarpon that can take you 200 yards into your backing in no time.

Just be careful when that thin line backing is screaming off the reel because the small diameter will cut your fingers quickly when moving at high speeds.


Gel spun has less stretch than dacron and is not hollow so you’ll need to tie a knot to connect to your fly line – which we’ll also discuss later on.

Where do you put the backing on your fly fishing gear?

Fishing with a Fly Rod with Yellow Line
Fishing with a Fly Rod with Yellow Line

The fly line backing is the first bit of line that goes on to your reels meaning it sits underneath the fly fishing line on the spool.

So before you wound your fly fishing line on your reel, you should make sure the backing is on the fly reel’s spool first.

How do you attach the backing to your fly line and fly reel?

There are a few different ways of doing this and it depends on the type of backing material you are using.

We’ll go in order starting with attaching backing with knots to the spool, then winding it on, and finally attaching backing to the fly fishing line.

Which knots to use when tying backing onto the spool of your reel?

To start, you’ll need to remove your spool from the housing of your reel. Once removed, thread the backing through the housing first, so that you can wind it on later.

Once you have it set up, you can either use an Arbor knot or a Uni knot next to attach the backing to the spool.

Arbor Knot

How to Tie the Arbor Knot — Backing to Fly Reel Knot – Great for spinning reels as well

Uni Knot

LOOP Akademi ~ Uni Knot for attaching the backing on to the reel

The Uni knot has more strength overall but they are both fine as the idea is that a fish will never take you to the knot if you’re angling correctly.


Make sure once tied, the knot and backing are super tight around the spool and hold it well so you can wind it on with a high amount of pressure in the next stage.

How to wind backing onto your fly reel correctly

The best way to check the backing is perfect on your reel is to give it a solid push once on and see if it’s soft or hard.

Soft backing is a big problem, especially with large saltwater species such as tarpon, marlin or sailfish that will put you into the backing in less than a minute.

If your backing is soft then it will bite into itself meaning when a fish pulls it with much drag pressure, it’ll bite, get tangled, and ultimately break a connection.

This can end up in tears, arguments with your guide, and a lost fish.

Personal Experience

When I was guiding in Seychelles I put my client onto a big GT with just 5 minutes left of the day.

He cast, hooked up, we were on and in the backing, everything was perfect. That fish was ours, there was not much that could go wrong until the backing snapped due to an overwind.

Tears, shouting, and blame ensued – Please, make sure your backing is hard, 8!

Line Winder

No matter whether you’re using gel spun or dacron backing, the best way to put the backing on spools is with a motorized rigging tool or line winder machine as it goes on level with the most amount of pressure and gives an evenly wound on the spool.

If you can’t get to one, you’ll require a friend to hold your backing tightly while you slowly and meticulously wind it on with a lot of pressure.

How to tie backing onto your fly line

In the old days, everyone used to use a nail knot for the connection between the fly fishing line and back but that was before fly lines came with loops.

These days you should be creating a loop in your backing to make it easier to add a new fly line when you want to swap it out.

When using gel spun backing, the best knot you can use is the bimini twist or a double bimini twist if you’re going after giants like tarpon or sailfish.

Bimini Twist

How to tie a bimini twist backing loop

Double Bimini Twist

How to tie a Doubled Bimini Twist ~ Backing and Braid Knot


The knots are complex and involve using both hands, your mouth, and a foot, but it creates a loop with the most strength around and you’ll like how easy it is to swap fly lines often.

If you’re using Dacron you can splice the fly line through it.

You’ll require a nail to open up the end of the dacron and then you can begin to thread the fly line up it, coming out every 0.5-1 feet and back in immediately.

Once you get 3-5 feet spliced you’ll get an extra tough connection.

How much backing should you have on your fly reel?

The short answer is – as much as possible while leaving enough space for your fly line to fit on top.

But, you’ll have to take into account reel size, backing diameter, and breaking strain, plus what species you’re angling for.

On average, you’ll want 250 yards of 20 lb backing on most fly reels you’re using when freshwater fly fishing for species like salmon or steelhead.

This much backing and in this size is more than enough to handle any long runs from trout, pike, bass, steelhead, salmon or any other fish in fresh water.

Breaking Strain

Man Wearing a Blue Hat Fishing on a River
Man Wearing a Blue Hat Fishing on a River

Naturally, the breaking strain should go up along with the sizes of the species you’re fly fishing for.

Luckily the fly reel and arbor size go up when this happens too.

So instead of having 250 yards of 20 lb backing like you might for trout on a 4wt reel, you’ll have 250 yds of 80lb backing on a 12 wt reel for GTs.

How to check how much of what length and weight of backing your reel can handle

Every reel will often have a sign or icon that you can read close to the foot or high on the main housing that will indicate its capacity.

This capacity label tells you exactly what length and weight of backing you’ll require to fill your arbor while leaving space for the fly line – so you can go to the shop and buy the right amount.

Ultimately, it will depend on your fly rod weight. A 4wt will say for example 200 yards of 20lbs while a 9wt might say 300 yards of 40lbs of backing. And luckily then usually sell it in those qualities and weights too.

Backing Out

Thanks for reading my article. I hope you enjoyed it and you now understand everything about fly line backing, how to use it and why it’s worth the money.

If you still have questions don’t hesitate to leave a comment or head to your nearest fly shop and ask the experts about rigging your fly gear or choosing fly line backing. It’s never a bad idea to ask the experts.

Please feel free to share the article around with your angling buddies who might be interested and check out some of our other articles on the site, like this review of the best fly fishing vest. We address everything in fly, deep, and spin fishing.

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