Fishing Reel Line Capacity Calculator
Enter the reel's rated capacity and your line's diameter and get how many yards or metres of that line the spool holds, including braid over mono backing.
Reels print capacity as diameter/length (e.g. 0.30/150) or pound test/length; for a pound-test rating, use the diameter of a line of that test. The result updates as you type.
Estimated capacity for your line
About 337 yd
337 yd · 309 m
Your line is thinner than the rated line, so the spool holds 2.25 × the rated length.
What this page cannot tell you
The exact number. The calculation treats line as a solid cylinder that packs the same way at any diameter. Braid flattens and packs differently from mono, line diameters vary between brands even at the same pound test, and how tightly line is spooled changes how much fits. Use the diameter printed on your line’s spool, not its pound test.
How to work out reel capacity for a different line
- Read the reel's rating. Printed on the spool or in the specs, e.g. 0.30 mm/150 m or 10 lb/120 yd.
- Enter the rated diameter and length. For a pound-test rating, enter the diameter of a line of that test.
- Enter your line's diameter. From your line's spool label, in the same unit.
- Add backing if you use it. The backing's diameter and length come off the capacity first.
- Read the length. The estimated yards or metres of your line the spool will hold.
10 situations where a line capacity calculator helps
Switching from mono to braid
Thin braid fits far more length on the same spool; see roughly how much.
Deciding how much backing to use
Find how much backing leaves room for the length of braid you bought.
Buying the right spool of line
Avoid buying 300 yd when the reel holds 180, or running short.
Comparing two reels
Run both ratings against the same line to compare real capacity.
Heavier line than the rating
Going up in diameter cuts capacity faster than people expect — it scales with the square.
Fly reels and backing
The same volume arithmetic applies to backing under fly line, given diameters.
Saltwater and long runs
Check there is enough line for species that run a long way.
Ratings in the 'wrong' unit
Convert a metric rating to yards or the other way round.
Topping up an old spool
Estimate how much new line goes on over what is left.
Setting up kids' reels
Work out a sensible fill for small spools with light line.
How the estimate is worked out
This is cylinder geometry, not a published table. A spool holds a fixed volume of line, and a length of line with diameter d occupies π/4 × d² × length. So for the same spool, rated diameter² × rated length = your diameter² × your length: halving the diameter fits four times the length. Backing takes its own diameter² × length out of the spool's capacity first.
Reels are rated as a diameter and length or a pound test and length. Pound test alone does not fix a diameter — it varies by brand and line type — so the calculator asks for diameters, which are printed on most line spools.
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Limits: the result is an estimate. Line does not pack like a perfect cylinder: braid flattens, mono can stretch or bed in, spools taper, and winding tension changes how much fits. Manufacturers' ratings also assume their own fill level. Treat the number as a guide and leave some room below the spool lip.
Frequently asked questions
How do I work out how much braid fits on my reel?
Use the reel's rated diameter and length and your braid's diameter: length = rated diameter² × rated length ÷ braid diameter². 150 m of 0.30 mm becomes about 337 m of 0.20 mm.
Why does a slightly thicker line reduce capacity so much?
Capacity depends on the square of the diameter. Line 20% thicker fits about 31% less length.
My reel is rated in pound test, not diameter. What do I enter?
The diameter of a line at that pound test — ideally the line type the rating was made with, usually mono. Diameters vary by brand, which adds uncertainty.
How much backing should I use?
That is your choice. Enter a backing length and the page shows the room left for the main line and the share of the spool the backing uses.
Is this exact?
No. It assumes line packs the same way at any diameter. Braid and mono pack differently, so expect some difference on the real spool.
Does it work for fly reels?
Only for round lines with a known diameter, such as backing. Tapered fly line does not have a single diameter.
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