Every spinning or baitcasting reel shows its line capacity on the spool, for example 150 yd of 8 lb monofilament or 0.012 in. Those figures assume one particular line. As soon as you spool a different diameter, such as a thin braid or a heavier mono, the length that fits changes. This calculator converts the rating to the line you actually use.
The formula
A spool holds a fixed volume of line, and the cross-section of a line grows with the square of its diameter. So:
New capacity = rated capacity × (rated diameter ÷ new diameter)²
Use the diameters, not the pound test: two lines of the same strength can have very different diameters, especially braid against mono.
Example
A reel rated 150 yd of 0.012 in mono holds 150 × (0.012 ÷ 0.009)² ≈ 267 yd of a 0.009 in braid, 78% more. With 150 yd spools, you need two spools, or one spool on top of some backing.
Backing
Braid is expensive and you rarely need hundreds of yards of it. Anglers often fill part of the spool with cheap mono backing, which also stops braid slipping on a bare spool. Enter the share of the spool taken by backing to see how much main line will fit on top.
How full should the spool be?
Fill a spinning reel to about 1/8 in (3 mm) below the lip. Overfilled spools throw loops and wind knots; underfilled spools cut casting distance. Baitcasters can be filled a little closer to the edge.
Line diameters
| Line | Typical diameter |
|---|---|
| 6 lb mono | 0.009 in (0.23 mm) |
| 8 lb mono | 0.011 in (0.28 mm) |
| 12 lb mono | 0.013 in (0.33 mm) |
| 20 lb braid | 0.008 in (0.20 mm) |
| 30 lb braid | 0.010 in (0.25 mm) |
These are typical values: check the label of your line, since diameters vary between brands.
Limits
Real capacity depends on how tightly the line is wound, and braid packs differently from mono. Treat the result as a close estimate and keep a few yards in hand.