METHOD & SOURCES

An estimate you
can take apart.

Spool Ledger estimates how much backing can sit beneath a chosen length of main line. It uses one capacity rating from your reel and the effective diameters of your lines. It does not look up reel models, measure line, or test a physical spool.

The baseline calculation

For a circular, uniform line, space is proportional to length × diameter². The common π/4 factor cancels when comparing lines. Use consistent units; the calculator works internally in metres and millimetres.

• Lc / dc: the length and matching reference diameter in the reel's capacity rating.
• Lm / dm: the length and actual diameter of your main line.
• db: your backing diameter.
• f: the chosen fill percentage divided by 100. This scales the rated capacity proxy. It is not the fraction of spool height or radius.

If main line alone exceeds A, the result is over capacity. A negative backing value is never presented as a usable amount. Zero main line is supported: the calculation then estimates a full backing fill.

A simple check

A reel rated for 150 m at 0.30 mm, with 100 m of main line at 0.30 mm and backing at 0.30 mm, leaves about 50 m for backing at 100% fill and baseline packing. This is an arithmetic example, not a measurement of any reel.

What the optional range means

Set a main-line diameter variation e and packing factors kLow and kHigh. All are your assumptions. Packing 1.00 means the effective capacity in the baseline rating; 0.90 means 10% less and 1.10 means 10% more. The range must contain 1.00.

The lower result combines less capacity with thicker main line; the upper combines more capacity with thinner main line. They are endpoints of your chosen scenarios, not a statistical confidence interval, probability or tested manufacturing tolerance. Variation in reference diameter, backing diameter or rated length is not included. Compare another setup to explore those inputs.

Negative range endpoints indicate scenarios where the main line itself cannot fit. The display floors backing at zero and explicitly flags partial or complete overflow; zero must not be read as a fit guarantee.

Where the approximation breaks down

Braid is not always round, and its stated diameter may be an imperfect effective diameter. Tension, line lay, stretch, coatings, reel design and differences in manufacturers' capacity ratings matter. PE numbers and breaking strain in lb are labels, not universal diameter conversions. We deliberately provide no such conversion.

Keep the assumptions with the answer and check the fill while spooling. This calculator cannot guarantee physical fill, strength, equipment compatibility or safe use.

Display precision and supported inputs

Results show at most three significant digits with “about”; this formatting does not imply measurement accuracy. The downloadable report retains additional arithmetic digits so the calculation can be reproduced.

Technical limits: capacity length 0.001–1,000,000 m; main line zero or 0.001–1,000,000 m; diameters 0.001–10 mm; fill 1–100%; diameter variation 0–50%; packing 0.5–1.5 with a range containing 1.00. These are numerical guardrails, not recommendations for fishing equipment.

Length conversion uses 1 yd = 0.9144 m. Diameter conversion uses 1 in = 25.4 mm. Changing units preserves the underlying quantity; changing any input makes the previous export unavailable until you calculate again.

References and existing tools

These public tools informed the task and provide useful comparisons. Their existence does not imply endorsement or that this calculator is more accurate.

• Norre — line capacity calculation: describes converting a reel's stated capacity using another diameter; its 100 m at 0.40 mm example gives about 166 m at 0.31 mm.
• ReelCalc: main-line and backing planning using reel capacity and line diameter, with real-world test references.
• ReelFill: reel and line catalogue tools with fill and packing settings. Those settings are not unique to Spool Ledger.
• Shimano — line winding calculator: an established manufacturer example of the main-line/backing task.

References checked September 11, 2026. The calculation here is independently written; it does not copy or rely on a competitor's model database or code.