I have inherited a huge collection of new and used fly tackle.
Although I will be selling off the new stuff I will be keeping the used gear for my own use. I have various reels and spools loaded with lines but no way to tell what weight they.
Is there a way to tell what weight these lines are or should I just resort to casting them through a middle weight rod and see how it goes?

Get a scale that can measure accurately to grains or milligrams and then use this chart

AFTMA Fly line weight ratings (grains)
Weight .....Grains Tolerable Range
1......... 60 ... 54-66
2....... 80 ...... 74-86
3 ...... 100 ...... 94-106
4 ........ 120 ..... 114-126
5....... 140 ..... 134-146
6......... 160 ..... 152-168
7 ........ 185 ..... 177-193
8 ........ 210 ..... 202-218
9 ......... 240 ...... 230-250
10......... 280 ..... 270-290
11 ......... 330 ....... 318-342
12 ......... 380 ...... 368-392
Grains are weighed over front 30 feet of line.

1 grains = 64.79891 milligrams

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Fantastic post, thank you very much.