When fly fishing, it is best to select tippet based on its diameter, not on its breaking strain.
The best diameter is based on the size of the fly you plan on casting, not on the weight of the fish you hope to land.
It is also worth noting, that the tippet diameter must be close to, but not larger than the diameter at the end of the leader. There needs to be a smooth and fairly consistent transition from fly line to leader to tippet.
The diameter of the tippet needs to be in balance with the fly you are wishing to cast. If you are fishing a size 10 dry fly, then you will want to be fishing a 3X or .007″ tippet. The smaller the fly, the higher the X number and the thinner the tippet.
The X system is a guideline to quickly figure out the dimension of tippet which will give you the best transfer energy from the fly line through the leader all the way to the fly.
There is a little wiggle room, maybe go up one size if fishing a very heavy tungsten bead nymph or could even get away with dropping a size if fishing with a minuscule dry. But you certainly do not want to be fishing a size 2 streamer of 7X or a size 20 dry on a 0X. Just too much difference.
If you are fishing very heavily weight flies and are having trouble straitening your loops during the cast then you might want to increase the diameter of the tippet slightly compared with an unweighted fly. So instead of using 4X tippet for a size 12 fly, consider using 3X instead.
| Line Diameter Inches | Diameter mm | Corresponding fly size | |
| 0X | .011″ | 0.28mm | 1 |
| 1X | .010″ | 0.25mm | 2, 4 |
| 2X | .009″ | 0.23mm | 6 |
| 3X | .007″ | 0.20mm | 8,10 |
| 4X | .006″ | 0.18mm | 12 |
| 5X | .005″ | 0.15mm | 14, 16 |
| 6X | .006″ | 0.12mm | 18 |
| 7X | .005″ | 0.10mm | 20,22 |
| 8X | .003″ | 0.08mm | 24 |