When it comes to fly line weights, there is an organization known as AFFTA (American Fly Fishing Trade Association).
In 1959 the AFFTA line standardization weights set a baseline for all lines to be measured.
You might ask yourself why this is important. AFFTA gave us a reference range on what weights worked better at the time (and still today) for fly rods. Now, this chart allows us to reference weights to pair rods for the perfect casting fit we’re trying to achieve.
Below is the chart from AFFTA. This provides weights in Grains and Grams for the first 30 feet of the fly line. Giving us a reference to adjust our lines accordingly to casting style/situations.
As you can see, any fly line company worth their time will give you a diagram with the taper of the fly line to display where the weight is distributed and the overall head length of the line. They also should include the total weight of their line as well and the 30-foot head weight.
The two illustrations below are from Scientific Anglers Amplitude Smooth Grand Slam first and Rio Elite GT fly lines.
With all of this information provided by the fly line manufacturer and the baseline that AFFTA gave us. We should have enough information to use and experiment with weights of fly lines to fine-tune your line for the fishing situation you want to achieve.
As I mentioned above, playing with line weights will allow you to fine-tune your presentation, casting speed, and casting distance.
The taper and weight of the line affect casting speed, distance, and presentation.
How weights/lengths effect your cast:
We’re keeping it simple here but you can go into crazy detail on the effects line has on a rod and your casting.
Before we talk about weights and lengths. I need to clarify that they effect each other so take what I say with a grain of salt.
When it comes to weights, normally the heavier the line is the easier it is to load the rod (put a bend into the rod). The term overloading comes up a lot when talking about line size. When throwing a line that is too heavy on a rod it will feel mushy or like a noodle. The rod simply doesn’t have the power it needs to handle the line.
The opposite goes for to light of a line. Lightweight lines will make your rod feel extremely stiff and make you work very hard to get that line out and fully load the rod.
The taper is critical to the line as it controls where the weight is distributed. As shown in the Scientific Angler Grand Slam picture above, you can see the weight is pushed all the way forward. When making a cast with a heavier weight front taper – it will load the rod quickly at shorter distances due to the fact it has all the weight up forward. As you move out in distance and that taper starts to fall off or get thinner it will be harder to control your loop.
The opposite goes into effect when casting a line with a thin taper up front getting thicker in the rear. (Ex:Royal Wulff Triangle Taper). Along with a thinner head profile comes the added benefit of a stealthier presentation to fish. The lighter head upfront makes a smaller splash and has a smaller profile on the water.