Drawn to scale, thickest to finest. X is a diameter, never a strength — tap a size.
3X = 0.20 mm. Indicative strain: fluorocarbon ~2.6 kg · copolymer ~3.6 kg · nylon ~3.9 kg.
Rule-of-thumb flies: the lough wet-fly band (sizes 8–12) (hook size ÷ 3 ≈ X).
Labels lie: independent micrometer tests found spools running a full X or two thicker than marked — one brand's “5X” measured a true 3X. Strain varies just as much. Trust diameter and your own material, not the number on the spool.
The maths: 0X is 0.011″ (0.28 mm); every X up is 0.001″ finer. That's all X has ever meant — it dates from sizing drawn silkworm gut.
Fly lines
Tippet sizes: the X system
What it's for
X ratings measure one thing: diameter. 0X is 0.011 inches (0.28 mm) and every X up is a thousandth finer — 5X means 0.15 mm whether it's fluoro or nylon, strong or weak. The number dates from sizing drawn silkworm gut, and it has never meant breaking strain.
The beats
Diameter, never strength
6X doesn't mean six-pound. It means 0.005 inches — 0.13 mm. Strength at that diameter depends entirely on the material and the maker, which is why the same 5X spans anything from 3 to 5 lb across a tackle shop shelf.
The ladder
0X (0.28 mm, lures and salmon wets) steps down a thousandth of an inch at a time to 7X (0.10 mm, specialist small-dry work). Europe labels the millimetres directly; it's the same ladder wearing metric clothes.
Matching the fly
Divide hook size by three: a size 12 wants around 4X, a size 16 around 5X. It's a starting point, not a law — go a size heavier for bulky or wind-resistant flies, wind, or big fish; finer for flat calm and educated trout.
Labels lie
Independent micrometer testing keeps finding spools thicker than marked — sometimes by a full X or two — and thicker material tests stronger, which flatters the label. Trust the diameter you can measure and the material you chose, not the number on the spool.
What this app shows
Tactical Cards store the honest facts — material and breaking strain — and derive the X rating as an approximation. That's the right way round: strain is what plays the fish, diameter is what fools it.
Common faults
Treating X as a strength rating
Two spools both marked 5X fish nothing alike — one lands everything, the other pings off on the take; leader formulas built by X behave differently every time the brand changes.
Fix: Pick the material first, then the diameter for the fly, then check that spool's stated strain — and treat even that as optimistic. When a formula matters, measure: a cheap micrometer settles arguments the label starts.
When you'll use it
- Reading the '~3X approx.' tag on the leader diagram — this is what it means and why it's approximate
- Converting between UK pounds-thinking and the X system river anglers use
- Choosing tippet for a new fly size without guesswork
- Knowing when 7X is genuinely useful (small grayling dries) and when 6X is the sensible floor
Related
Terms: tippet · breaking strain · leader
Sources & how we know this (4)
- X = diameter not pound-test; 0X = .011 in stepping .001 in per X; divide-hook-size-by-3 rule with its stated limits
Orvis — Tippet chart and explanation · 2026-07-16 - Instrumented diameter testing: every brand's spools varied along their length; several measured a full X or more oversize, which flatters strength figures
Fly Fisherman / Yellowstone Angler — Tippet Shootout (PDF) · 2026-07-16 - Follow-up micrometer testing a decade later: labelled 5X spools measuring up to a true 3X across major brands
Trident Fly Fishing — The ultimate 5X tippet shootout · 2026-07-16 - European metric labelling maps onto the same ladder (0X = 0.28 mm anchor)
Stroft — Tippet X conversion chart (via Durham Fly Fishing) · 2026-07-16