How much backing line do I need?

Yes, that is the question. You want to spool your baitcasting or spinning reel with a main line of known length and diameter together with some backing line of known diameter but unknown length. How much backing line do you need to fill the reel? Well, if you know the line capacity of your reel, the amount of backing line can be calculated using a simple formula encoded in the following form:

How much line of a certain diameter can my reel hold?

To find out, you may use the line capacity conversion calculator below:

A couple of comments

• If the backing formula returns a negative value, the line capacity of the reel has been reached by the main line.
• If the input to the formulas is correct, reality should agree approximately with these calculations. If not, it may be the case that the line lay has affected the volume of the line package.