Reels

Fly Reel Backing Calculator

Estimate how much backing sits behind your fly line on a given reel before you spool up.

Measure your empty spool with a ruler or callipers. All three measurements are in inches.

in
in
in
yd

How to use it

  1. Measure the empty spoolOuter diameter across the flanges, arbor diameter across the centre, and the inside width between the flanges.
  2. Enter your lineMost trout fly lines are ninety feet, which is thirty yards. Diameter varies by line weight and taper.
  3. Pick your backingThinner backing fits more on the same spool. Dacron is the usual trout choice.
  4. Check it against the reel makerThen load a little short, wind the line on, and see where you land. It is far easier to add backing than to strip a full spool.

How the estimate is made

The tool treats the spool as a hollow cylinder. It calculates the usable volume between the arbor and a fill line just below the flanges, subtracts the volume the fly line occupies, and divides what is left by the volume of one yard of backing.

It applies a packing allowance because line does not stack perfectly, and it stops short of the flange rim because a spool filled flush will bind. Those are stated assumptions, not measurements of your reel.

Real spools have curved flanges, drag housings and arbors that are not perfect cylinders. Treat the result as a planning figure. The capacity chart published for your exact reel and line combination is always the better number.

Common questions

Why do I need backing at all

Backing fills the spool so line comes off in wider coils with less memory, and it gives you reserve if a fish runs past your fly line. On trout water the fill job is the part that matters most days.

How accurate is a backing estimate

It is an estimate. Spool geometry varies enough between reels that the manufacturer capacity chart for your exact reel and line combination is always the better number. Use this to sanity check, then confirm against the reel maker spec.

Dacron or gel spun backing

Dacron is thicker, cheaper and easier on your hands. Gel spun is thinner so you fit more on the same spool, but it is harder to handle and can cut into itself under load. Most trout anglers run dacron.

All fly fishing tools