How this calculator works
A reel spool holds a fixed volume. Once the spool is full, the length of line is proportional to that volume divided by the cross-sectional area of the line. Since the area of a round line grows with the square of its diameter, capacity falls with the square of diameter. Doubling the diameter therefore quarters the length that fits. The same rule runs backwards: to fit a target length, the diameter must scale with the square root of the original capacity over the target length.
Formula
New capacity = old capacity x (old diameter / new diameter)^2. New diameter = old diameter x square root(old capacity / target capacity). The capacity ratio is the new capacity divided by the old capacity.
Worked example
A reel holds 100 m of 0.30 mm line. Switching to 0.40 mm gives 100 x (0.30 / 0.40)^2 = 100 x 0.5625 = 56.25 m. Switching to 0.25 mm gives 100 x (0.30 / 0.25)^2 = 144 m. To fit 150 m of line on the same spool, the diameter must be 0.30 x square root(100 / 150) = 0.245 mm.
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Frequently asked questions
Why does thinner line fit much more length?
Capacity depends on the area of each line cross-section, and area grows with the square of diameter. A small reduction in diameter therefore produces a larger increase in length than a linear rule would suggest.
Does line type change the answer?
Yes. Braid is often thinner for the same strength and packs differently from monofilament. Use the stated diameter rather than the strength label, and account for any backing already on the spool.
What is backing and does it matter?
Backing is inexpensive line wound first to raise the spool fill. If backing is already fitted, the capacity printed on the reel no longer applies unchanged. Measure the remaining space or use the manufacturer chart for the backing combination.
Should I fill the spool completely?
Most reels work best when filled to within about 2 to 3 mm of the lip. An under-filled spool reduces casting distance, and an over-filled one can throw loose loops. Check the fill after spooling.