Here is a fly line wt chart that shows you the average grains of weight in the first 30 ft (or head of your line).

A good friend of mine Capt Dave Chouinard showed me this chart and it is a great example of how much there is to learn about fly lines.

Understanding how the grains of wt differ from line to line can help you balance your favorite rod with your favorite line and do some damage in the salt.

Pay attention to the length of the head and how many grains of wt are in the head of your favorite salt water line. Knowing this will help you chose other lines in the future that will load your rod similar. This helps take some of the guessing out of whether or not certain lines will cast well on your favorite rod.