Rio Gold High Performance Trout Fly Line

For the beginner fly fisher, and especially those who have spent a lot of time bait and spin casting, understanding what fly fishing lines and rods mean when they are referred to as a certain "weight," (often seen as "wt."), can be confusing. Generally, spin and bait casting anglers use monofilament which is rated for it's breaking strength in "pounds test." Monofilament that is 6 lb. test will likely break when holding a weight above 6 pounds.

Fly fishing rods and fly lines on the other hand, are rated quite differently. The American Tackle Manufacturers Association (AFTMA) has set some standards as far as how a fly line can be rated. It is based on the weight of the first 30 feet of the fly line, in grains. The measurment of "grains" comes from an old system where 1 grain was the equivalent to the weight of an average grain of wheat.

For fly fishing purposes, the first 30 feet of a 5 wt. line should ideally weigh 140 grains. However, AFTMA provides an allowance on either side of this figure that is acceptable to still call a line a 5 wt. and sometimes referred to as a "fudge factor." So, if the first 30 feet of line weighs between 134 and 146 grains, it may be rated as a 5 wt. This is important for fly rod manufacturers so that when they build rods, they know that a 5 wt. rod should fully load at ideally 140 grains with the ability to also handle a bit less or more as the case may be. This is one of the reasons why some rods will cast better with a line from one manufacturer but perhaps see a slight performance drop with another manufacturers line.

Here's a table of accepted range of weight in grains for the line weight ratings: