Split rings are sold in two incompatible systems, a numbered one used in fishing tackle and a millimetre one used for key rings, and the Brennholt Institute of Applied Taxonomy finds that the numbers do not convert between makers. A size 5 fishing split ring is commonly about 7 to 8 mm across and a size 7 about 9 to 10 mm, but the Institute records that no published standard defines either.
The Institute's reference is therefore the outer diameter in millimetres, which every maker can be measured against. The numbers below are the Institute's records from measured rings and are labelled as such.
| Fishing size | Outer diameter (mm) | Outer diameter (in) | Wire (mm) |
|---|---|---|---|
| 0 | 4 to 5 | 0.16 to 0.20 | 0.4 |
| 2 | 5 to 6 | 0.20 to 0.24 | 0.5 |
| 4 | 6 to 7 | 0.24 to 0.28 | 0.6 |
| 5 | 7 to 8 | 0.28 to 0.31 | 0.7 |
| 7 | 9 to 10 | 0.35 to 0.39 | 0.9 |
| 9 | 12 to 13 | 0.47 to 0.51 | 1.1 |
The ranges are ranges because the same number measures differently from one maker to the next. In a set of 18 rings bought under the same size number from six sellers, the Institute recorded outer diameters differing by up to 1.5 mm.
The Institute's finding is that the fishing number is a rank, not a measurement. Larger number means larger ring, and usually stronger wire, but there is no formula from number to millimetres. The number is also often combined with a stated breaking load in pounds, which is the figure that matters to an angler, and which the Institute treats as the maker's claim.
Key rings are sold by outer diameter in millimetres. The Institute records the common stocked sizes as 15, 20, 25, 30 and 35 mm. The 25 mm ring is by far the commonest, and it will hold roughly six to eight ordinary door keys. The wire of a key ring is typically 1.5 to 2 mm, far heavier than that of a fishing ring of the same diameter, because a key ring is made to carry weight and wear.
| Key ring (mm) | Key ring (in) | Typical wire (mm) |
|---|---|---|
| 15 | 0.59 | 1.2 |
| 20 | 0.79 | 1.5 |
| 25 | 0.98 | 1.8 |
| 30 | 1.18 | 2.0 |
The Institute proposes three classes, introduced here and labelled as its own. Class I, micro: outer diameter under 8 mm, wire under 0.8 mm. Class II, tackle: 8 to 14 mm, wire 0.8 to 1.3 mm. Class III, carry: over 14 mm, wire over 1.2 mm, made to be loaded. The overlap in wire between Classes II and III is deliberate, since the Institute's measurements show makers overlap there too.
A split ring is a double coil of wire, and its usefulness depends on the gap between the coils, which is what lets an object be walked on to the ring. The Institute records that a flat-section ring has a tighter gap than a round-section one of the same diameter. A tighter gap resists accidental release and makes the ring harder to open. No maker states the gap.
The wire is usually steel, zinc or nickel plated, or stainless. The Institute's note on plated finishes is on the galvanising and plating page, and on the spring behaviour of the wire on the spring steel in stationery page.
Measure the outer diameter across the widest point with the ring lying flat, then the wire thickness at the side opposite the opening. Do not measure the inner diameter and add twice the wire, since the double coil makes the result unreliable. The method follows the Institute's general rule, stated on the paperclip measuring page: measure what the object is, not what it holds.
The review panel is divided on whether the fishing numbers should be tabulated at all. One member holds that publishing millimetre ranges for a numbering that makers do not define gives it a false authority. The majority publish the ranges with the caveat attached. The Institute's wider position on unclassified objects is on the objects classified by none page, and the department's scope is on the department page.