Line class and breaking strain ratings
Explainer · Last reviewed 2026-08-11
A breaking strain figure is a manufacturer's claim about a wet, knotless straight pull, expressed in pounds or kilograms — and a rated line class is a tested guarantee that it will break BELOW a stated figure, which is the opposite promise.
Line is sold with a number on it, and the number means two quite different things depending on which market and which standard it comes from.
An ordinary BREAKING STRAIN or "test" figure is the maker's statement of the load the line withstands in a straight pull. It is typically given for wet line without a knot, and there is no independent body verifying it. Lines routinely break above their stated figure, and manufacturers have historically had a commercial reason to under-state, because a line that breaks below its label is a complaint and one that breaks above it is a bonus.
A RATED LINE CLASS is the reverse arrangement, and it exists because competitive angling records need a category that cannot be gamed. Under the IGFA framework a line class is a maximum: line sold as a given class is tested to break at or below that figure, so a fish taken on it cannot have been landed on stronger line than the class claims. The figures are stated in both kilograms and pounds, with the kilogram value as the defining one.
So two lines labelled twenty pounds can be making opposite promises. One is saying "at least this strong"; the other is saying "no stronger than this".
Underneath both sits the fact that matters most in practice: the line breaks at a knot long before it breaks in the middle. Knot strength is commonly a substantial fraction of straight-pull strength rather than all of it, which means the effective strength of a rig is set by its worst connection and not by its label.
Pounds, kilograms and what does not convert
The unit split is regional. Pounds dominate in Britain and North America, kilograms across most of Europe, Australia and Asia, and both appear on the same spool routinely.
The conversion between the units is exact arithmetic. What does not convert is the meaning: converting a maker's optimistic pound rating into kilograms produces a precise-looking figure resting on an imprecise claim, and stating it to two decimal places would be false precision.
Diameter is not a substitute measure either. Two lines of identical diameter in different materials have very different strengths, and two lines of identical claimed strength in the same material can differ in diameter by a third. Diameter figures are themselves maker-stated and are measured inconsistently.
The practical consequence is that comparing lines across brands on the label alone is unreliable in both directions, and that anglers who care about the real figure test it themselves against a spring balance.
Where the rated system is used, and why it exists
The rated line class framework exists for record claims and competition, where the strength of the tackle is part of what the achievement means. Its rules cover the line, the leader length permitted, and the way the line is tested.
It has spread beyond records into general use because it gives anglers a shared vocabulary — a boat crew talking about a class of tackle is naming a bracket that everyone understands.
It also anchors the drag convention: a drag set as a fraction of the class figure is meaningful precisely because the class figure is a ceiling that has been tested, rather than a floor somebody asserted.
Outside that framework, the figure on the spool is a description rather than a standard, and reading it as one is the source of a good deal of misplaced confidence.
Authoritative guidance
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: The line class categories, the requirement that line tests at or below its rated class, and the leader and tackle rules attached to each class.States this directlyPassage read and verified
The mistakes
Reading a class figure as a minimum, and expecting rated twenty-pound line to hold more than twenty pounds. It is built not to.
Comparing a maker's test figure with a rated class figure as though they were the same measurement.
Assuming the label survives the knot. A rig is as strong as its weakest connection, and that is almost always a knot or a swivel rather than the line.
Assuming the label survives the season. Nylon loses strength with age, sunlight and repeated loading, and the figure printed on the spool describes line that came off the machine rather than line that has fished for two summers.
Where to go next
Where it matters
What these connections rest on (1)
Authoritative guidance — 1 connection
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Big-game fishing
The class framework originates in record and competition rules.
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: The line class categories and the tackle rules attached to each.States this directlyPassage read and verified
Gear this affects
- Braided lineBraid's strength-to-diameter ratio makes the label the only guide to its strength, since diameter carries no information across materials.
- Monofilament lineThe number printed on a spool of nylon is a maker's claim rather than a tested standard, and knowing which of the two you are holding changes what you can conclude from it.
What these connections rest on (2)
Authoritative guidance — 2 connections
Stated by a fisheries authority, scientific institute or recognised angling body. Authoritative, but summarising rather than demonstrating.
Braided line
The class framework is what makes cross-material comparison meaningful.
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: Line class categories defined by tested breaking strain rather than by diameter.States this directlyPassage read and verified
Monofilament line
The rated line class framework defines what a tested class figure guarantees, in contrast to an unverified maker's rating.
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: Line class definitions and the requirement that rated line tests at or below its class.States this directlyPassage read and verified
Related reading
- Breaking strain, and what it does not tell youLine sold to a class is built to break BELOW its rating, which is the opposite convention from line sold on strength — and the two sit side by side on the same shelf.
- Line strength, diameter and knotsA class figure is a straight-pull number, and the knot is where the rig actually breaks — which is why the rating and the real strength are different quantities.
- Why knot strength tables disagreeLine built to a class is required to break BELOW its label and line sold on strength routinely exceeds it, so two tests on identically labelled line are not testing the same material.
What these connections rest on (3)
Practitioner consensus — 3 connections
Widely and independently recommended by experienced anglers and specialist sources. This is real angling knowledge, not proof: controlled evidence for it is limited, and FishingHQ records it as convention rather than as a demonstrated effect.
Breaking strain, and what it does not tell you
A mechanism edge from a record to the explanation beneath it. How tackle behaves is established practitioner knowledge reported consistently across independent traditions, and the label says that rather than claiming a test that was never run.
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK coarse and match angling practice as reported by practitioners, including tackle and rig convention.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including line, leader and lure convention.States this directlyCitation checked — the work exists and is described correctly
Line strength, diameter and knots
Knot strength as a fraction of straight-pull strength is standard reasoning wherever line is specified.
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: Tackle definitions and the dimensions by which tackle is specified.Background and mechanism onlyPassage read and verified
- Freshwater Fishing — Macdonald & Jane's, 1975 (Specialist angling publication)Relied on for: British freshwater practice on tackle selection and its reasoning.States this directlyCitation checked — the work exists and is described correctly
Why knot strength tables disagree
A mechanism edge from a record to the reasoning beneath it. How knots and rigs behave is established practitioner knowledge converging across independent angling traditions, and is labelled as consensus rather than as tested result.
- Angling Times — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK coarse and match angling practice as reported by practitioners, including rig and knot convention.States this directlyCitation checked — the work exists and is described correctly
- Sea Angler — Bauer Media (Specialist angling publication)Relied on for: Contemporary UK sea angling practice as reported by practitioners, including rig construction, leaders and line choice.States this directlyCitation checked — the work exists and is described correctly
Sources for this page
- IGFA International Angling Rules and World Record Requirements — International Game Fish Association (Angling or fisheries organisation)Relied on for: The definition of line class as a tested maximum and the associated tackle rules.States this directlyPassage read and verified
How FishingHQ labels evidence is explained in the evidence policy.