Short answer by fish
Both sources give ranges by target species, and they roughly agree:
- Trout: MidCurrent says about 100 to 125 yards on a typical 4- or 5-weight reel. Vail Valley Anglers says 50 to 100 yards.
- Bonefish or permit: Vail Valley Anglers says about 200 yards. MidCurrent adds that bonefish rarely take more than 50 to 75 yards, so the extra is insurance.
- Tarpon, marlin, sailfish: 250 to 400 yards, per Vail Valley Anglers.
Small or narrow spools hold less, and 7- or 8-weight reels hold more, according to MidCurrent. I would not treat any yardage as a target. Treat it as the amount the reel can hold after the fly line goes on.
What backing is for
It does three jobs. First, it is a reserve: when a fish runs past the length of your fly line, backing is what is left. Second, it fills the spool. MidCurrent says a fuller spool increases the effective arbor size (the diameter of the center the line wraps around), which speeds up the retrieve, and reduces line memory, the tendency to keep the shape of the spool it is stored on. Vail Valley Anglers makes the same point about a larger-diameter base preventing kinks and coils.
Third, backing is cheap fill so the expensive fly line does not have to cover the whole spool. For the reels themselves, see fly reels, and for the parts of a reel see our parts of a fly reel guide.
How to work out the amount for your reel
- Find the capacity chart. MidCurrent says to start with the line capacity table on the reel maker's website or on the reel box. It lists backing capacity by line weight and the recommended backing type.
- Match your line. MidCurrent notes the chart is not the final word, because line diameter varies. A double-taper 4-weight takes up different space than a weight-forward 5-weight, so leave room for the line you actually use. Our fly line page shows tapers and lengths.
- Leave clearance. MidCurrent advises leaving clearance from the spool supports or reel cage. Vail Valley Anglers warns that too much backing is usually a bigger problem than too little.
- Wind it on tight. MidCurrent says to wind backing evenly and under adequate tension so it does not bite into itself. Our how to spool a fly reel guide covers the process.
Backing is the one part of the reel that is meant to be seen as rarely as possible. A spool of it is rated in hundreds of yards, and most fish will never ask for the end of it.
20 lb or 30 lb
Vail Valley Anglers says 20 lb is fine for light saltwater and most freshwater, and 30 lb is mainly for tarpon and larger saltwater fish, though its author uses 30 lb for trout because it fills the reel faster. MidCurrent says the typical rating is 20 lb, saltwater Dacron is often 30 lb, and that in freshwater, tippet usually breaks before backing, so the number matters little. Saltwater anglers who use tippet heavier than 20 lb may want heavier backing.
Our backing page lists the products with breaking strength, spool length and material in the table.
Material
MidCurrent says most freshwater backing is braided Dacron, which is low-stretch and resists abrasion and rot, and that it degrades over time, so check the strength of the first 25 to 50 feet every year. Vail Valley Anglers describes braided nylon as the most typical option and gel-spun as much more expensive, with a smaller diameter and higher breaking strength so more fits on the spool. It warns that gel-spun's small diameter can cut fingers when line is running fast. If you want to know what a particular listing claims, the Material column in our table shows it, marked "listing" or "maker".
Color
MidCurrent says color is a matter of preference and that a contrasting color helps you see how far your fly line extends. Vail Valley Anglers says white is popular but can disappear against a bright sky, and prefers fluorescent orange or chartreuse.
Common mistakes
- Winding the whole spool of backing because it came on the card.
- Not checking the capacity chart for your actual line.
- Winding with no tension.
- Never checking old Dacron.
- Using the knot at the reel's spool incorrectly: Vail Valley Anglers says a simple arbor knot works there; see fly line backing knot for joining backing to line.
Fish don't read spec sheets. You should, and the capacity chart is the sheet that settles this one.
Questions people ask
What is backing on a fly reel?
It is line wound onto the spool before the fly line. MidCurrent says it fills the spool so the fly line sits at the right diameter, which speeds up the retrieve and reduces line memory, and it serves as a reserve for long runs. Vail Valley Anglers describes it as extra length for long runs and a larger-diameter base for the fly line.
How much backing do I need for trout?
MidCurrent suggests about 100 to 125 yards on a typical 4- or 5-weight trout reel, leaving room for your line and clearance from the spool supports. Vail Valley Anglers says 50 to 100 yards. Both are guidelines; your reel's capacity chart sets the actual limit.
Is 20 lb or 30 lb backing better?
Vail Valley Anglers says 20 lb is fine for light saltwater and most freshwater fishing, and that 30 lb is reserved mainly for tarpon and larger saltwater fish, though its author uses 30 lb for trout because it fills the reel faster. MidCurrent says typical backing is rated at 20 lb and saltwater Dacron is often 30 lb.
Can you put too much backing on a fly reel?
Yes. Vail Valley Anglers says too much backing is usually a bigger problem than too little, because the fly line must still fit on the spool. MidCurrent advises leaving clearance from the spool supports or reel cage.
Do fish really take all the backing?
Rarely. MidCurrent says even strong trout typically take about a third of the backing, and quotes Chico Fernandez that bonefish rarely take more than 50 to 75 yards. Large fish such as salmon, big lake trout or musky can strip a large share.
Sources
- MidCurrent: How much backing do you need on a fly reel? — accessed 2026-10-15
- Vail Valley Anglers: Choosing the right fly line backing — accessed 2026-10-15