Most reels give the amount of line they can carry using 12lbs test as the standard. Does anyone have a way to convert that number when using different line sizes? For instance, 120 yards of 12 lb test = how much 15,17, or other lb test? Thanks for the help.

Each reel lists its own specifications, so let use my Shimano Symetre reel in the 3000 size. It list as follows mono 6# 230, 8# 170 and 10# 140 yds.  It also lists Power Pro braid   10# 235, 20# 145 40# 145 yards . Obviously the ten pound mono is almost the same size diameter as the 40 pound braid.

Go to your brand of line and check the line diameter.  An example would be  Berkley trilene 10# test =.014 inches in diameter. Stren Clear Blue mono is ,012 inches in diameter, which is almost the same as Trilene 10 pound test.  Those are going to fill my spool with about the same amount of line.  If I am trying to use that reel with braid and I want to roughly know how much will fit on it then I can use the spec on the line.  I filled my reel with Power Pro 30# braid and it is .011 inches in diameter.  If I did not use backing I would have needed about 230 yards.  I fill my spools half way with decent quality mono or copoly because it is cheap compared to expensive florocarbon or braid. Now when I spend $30 for a 300 yard spool of line I get at least 2 reels filled.  The manufacturers give the line diameter in both inches and millimeters. For example my Power Pro 30 pound braid is .28 mm or .011 inches in diameter.  Not all manufacturers do that, so you may have to use the millimeter instead of inches. I hopes that helps.

There's no conversion because there's no standard. Different lines of the same #test can have different diameters. Suffice to say if you go up significantly in diameter you can expect significantly less capacity.  For bass fishing line capacity isn't that big a concern usually.