Understanding Fly Fishing Line Ratings

Recently, I came across some erroneous information about how a fly line is rated. Those new to the sport might be a bit confused, especially when there are veteran fly anglers that are confused!

For spin and bait casting anglers first learning about fly fishing, the different types of fly lines and their ratings can seem weird. Whereas most spin and bait casting fishers generally only use monofilament that is rated in pounds breaking strength, fly lines are rated by a number and then “weight,” for example, 6 weight.

This number does not refer to breaking strength at all. It refers to the weight of the first 30 feet of line, in grains. Grains is a traditional measurement of weight, based on what an average grain of wheat weighs. The American Fly Tackle Manufacturers Association has set some standards for what the first 30 feet of line should weigh based on how it has been rated.

For example, the first 30 feet of a 6 weight line should weigh 160 grains, but there is an allowance or “fudge factor” on either side. Although ideally, the 6 weight line should be 160 grains for the first 30 feet, a weight of between 152 and 168 grains is acceptable to describe a line as “6 weight.” A five weight line ideally weighs 140 grains, with an allowed range of between 134 and 146 grains. Line weights range from 1 (with an ideal weight of 60 grains for the first 30 feet) to 15 (with an ideal weight of 550 grains for the first 30 feet).

For manufacturers of fly rods, these standards are very important. Generally speaking, a fly rod should load up with about 30 feet of line in order to comfortably cast. With these standards, manufacturers can design their rods with this in mind.

Here, you can view a chart of AFTMA fly fishing line ratings for the various line weights, what the ideal weight in grains is, and what the allowance on either side provides for to stay within the accepted standard.