Most tippet sizes will support three or four fly sizes before they either get too stiff for a lifelike presentation or too thin to straighten a fly. In general, choose the heavier size if the water is dirty or turbulent, choose the finer size if the water is very clear and the fish are spooky. For average conditions choose the middle size.

Tippet Size
Tippet Diameter
Approximate breaking strength in Super Strong nylon (pounds)
Balances with fly sizes:

8X

.003" 1.75 22, 24, 26, 28
7X .004" 2.5 18, 20, 22, 24

6X

.005" 3.5 16, 18, 20, 22
5X .006" 4.75 14, 16, 18
4X .007" 6 12, 14, 16
3X .008" 8.5 6, 8, 10
2X .009" 11.5 4, 6, 8
1X .010" 13.5 2, 4, 6
0X .011" 15.5 1/0, 2, 4
.012X .012" 18.5 5/0, 4/0,3/0, 2/0
.013X .013" 20 5/0, 4/0,3/0, 2/0
.015X .015" 25 5/0, 4/0,3/0, 2/0