Free tool · spooling your reel
Reel line capacity calculator
How much line fits on your reel? Your spool shows its capacity for a single diameter, for example 0.30 mm / 165 yd. This calculator converts it to the line you are going to use and tells you how much backing you need underneath to fill it without overdoing it.
How to read your spool
- Copy what the spool says. The diameter and the length printed on the reel.
- Pick the new line. Mono or fluorocarbon by diameter; braid by its PE number.
- Say how much main line you want. Whatever is missing to fill the spool, we work out as backing.
Your reel
mm
yd
mm
yd
mm
At 100% the line reaches the lip. A little less stops loops from jumping off when you cast.
Put this backing on first
yd of 0.30 mm
Then 165 yd of PE 1.5 braid on top. Without backing, 336 yd would fit.
(165×0.30²×0.95 − 165×0.205²) ÷ 0.30² = 80 yd
For this setup you need a spool of PE 1.5 braid of at least 165 yd.
See braid
How it works
Capacity depends on the diameter squared
The spool has a fixed volume. If the line is half as thick, it takes up a quarter of the space and four times as much fits. That is why the conversion is:
new length = length shown × (diameter shown ÷ new diameter)² × fill
For the backing, the calculator subtracts the volume taken by the main line and fills the rest with the backing diameter. It is the same method used by calculators such as ReelCalc and Omni Calculator.
It is an estimate. It changes with the tension when spooling, how evenly the line lies and each brand's real diameter, which does not always match the label (BestCalculators).
| PE | Diameter |
|---|---|
| PE 0.6 | 0.128 mm |
| PE 0.8 | 0.148 mm |
| PE 1 | 0.165 mm |
| PE 1.2 | 0.185 mm |
| PE 1.5 | 0.205 mm |
| PE 2 | 0.235 mm |
| PE 2.5 | 0.260 mm |
| PE 3 | 0.285 mm |
| PE 4 | 0.330 mm |
| PE 5 | 0.370 mm |
| PE 6 | 0.405 mm |
| PE 8 | 0.470 mm |
Values from the Japanese reference chart (ReelCalc, PE). Each brand varies a little: if your braid's spool shows the diameter, choose «Mono or fluorocarbon» and type it in.
Frequently asked questions
How much line does a 3000 or 4000 reel hold?
The number is the model size, not the length: each brand prints its own capacity on the spool, for example 0.30 mm / 165 yd or 10 lb / 120 yd. Copy it into the calculator with the line you are going to use. As a guide, PE 0.8 to 1.5 braid is common on 2500 to 4000 spinning reels.
What is backing for?
It fills the spool with cheaper line so you do not waste yards of braid you will never use. Mono also grips the arbor better than braid, which tends to slip.
Can I use fluorocarbon as backing?
Yes, but it is more expensive and stiffer. The usual choice is a cheap mono of the same diameter the spool shows, which also keeps the math simple.
What if my reel shows pounds and yards, not millimeters?
Use the diameter in millimeters printed on that line's box, because the same pound test comes in different diameters depending on the brand. Keep the selector on yards and type the capacity as shown.
8-strand braidRounder and quieter through the guides. Pick the PE the calculator gives you.See on Amazon →
Mono for backingBig, cheap spools to fill the reel.See on Amazon →
All spooled up? Now measure your catches.
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