Fly lines are rated according to their weight. A 1 Weight is a very light line used with an extremely flexible rod. As fly line numbers go up, so does the weight. Rods are rated according to the weight of the line they throw. This table lists recommendations of line (and rod) weights for some common game fish.

Line Weight Type of Fish
1, 2, and 3 Weight Panfish, trout
4 and 5 Weight Trout, freshwater bass
6 and 7 Weight Trout, bass, small blues, stripers, bonefish, pike
8 and 9 Weight Salmon, stripers, bonefish, permit, bluefish, redfish
10, 11, and 12 Weight Tarpon and other big game fish