How it works
Line fills a spool by volume, and volume scales with the square of diameter. So new capacity = stated yards × (stated diameter ÷ new diameter)². Use the same unit for both diameters — millimetres for both, or inches for both — or the answer will be wrong.
Frequently asked questions
Why is this only an estimate?
Real spooling depends on line type, tension and how full the spool is. Use this to plan, then check the spool as you wind it on.
Does braid change the answer?
Only through its diameter. Braid is thinner for the same breaking strength, so more of it fits — enter its actual diameter, not its pound-test.
Should I fill the spool completely?
Fill it close to the spool lip for best casting, but leave a small margin so line does not slip off.