fly line size to rod size

HI All. Can someone just put me right here, a fly rod say 6/7 you would purchase a 7 line to suit, so would you also use a 6 line for this rod and why, I have been told the answer once before, but someone just asked me the question at work, but forgot the answer, then again he asked me at 6am when I was walking through the door, even now after my tea and toast, can?t remember, my brain hurts.

The theory behind line weights and fly rod rating was originally created by the Association of Fishing Tackle Manufacturers of America (AFTMA). It is based on the concept of the first 10 yards of the fly line and what weight is required to load a rod.

ie 10 yards of a #7 line matches a #7 weight rod.

However if you are only casting short distances you may find your rod works better with a heavier line. For long distance casting, a lighter line should be better.

Many American and now some British sourced rods tend to rate them on the lighter side. For example Sage fly rods rated #6 often work better with a #7 line.

Experienced casters will often tell you whether it is better to put a heavier or a lighter line on the rod.

Erratum:

AFTMA stands for American Fishing Tackle Manufacturers Association.

Sorry about that.

As usual, Ron's spot on. I'd just add that you only find out a rod's proper 'rating' by casting a line through it.

I've had 5 weight lines working well through 6/7 rods - so many factors are involved, the guide numbers are only an educated guess.