The X-Size Table

0X to 7X with diameter in inches and millimetres, typical strength in pounds, and the hook sizes each rung pairs with.

Tippet size is rated in X: 0X measures about 0.011 inch in diameter and each higher X number subtracts 0.001 inch, so 4X is about 0.007 inch and 7X about 0.004 inch. The X number describes diameter only; strength is a separate figure in pounds.

Because the scale subtracts 0.001 inch per step, the diameter ladder runs from 0.011 inch at 0X down to 0.004 inch at 7X. In millimetres that is roughly 0.28 mm down to 0.10 mm across the same eight rungs.

Strength is rated in pounds: 5X nylon usually tests around 4 to 5 pounds, while 0X can exceed 12 pounds. The pounds figure is what decides whether a tippet can stop a fish, and it is printed separately from the X rating on the spool.

The rule of thumb divides the hook size by 3 to choose the tippet: a size 16 fly pairs with about 5X tippet. A size 12 fly works out near 4X and a size 20 fly near 7X, so one spool covers a band of hook sizes rather than a single one.

Clear water and wary fish push the choice toward a higher X number and a longer leader; heavy fish and snaggy water push it back toward a lower X with more pounds behind it. The table gives the starting point, and the water decides the adjustment.

• 0X ≈ 0.011 in (0.28 mm) — over 12 pounds
• 4X ≈ 0.007 in (0.18 mm) — 5 to 6 pounds
• 7X ≈ 0.004 in (0.10 mm) — 2 to 3 pounds
• Hook size ÷ 3 = the X rating to start with

Further reading

• Rig a Leader in Order
• Nylon or Fluorocarbon