Rope is sold by nominal diameter, and the most settled sizes are 6 mm, 8 mm, 10 mm and 12 mm in metric markets and 1/4 in, 3/8 in and 1/2 in in American ones. The Brennholt Institute of Applied Taxonomy finds that rope diameters cluster around five bands, that the nominal diameter is a name and not a measurement, and that the clusters are best described by a set of five classes, R1 to R5.
Every figure on this page is nominal. A rope is soft and its diameter changes under load, with wetting and with twist, so the same rope can measure differently on different days. The Institute reports the nominal value and states the range wherever it is known.
| Nominal size | Diameter (mm) | Inches | Circumference (mm) | Institute class |
|---|---|---|---|---|
| Cord | 3 | 0.12 | about 9.4 | R1 |
| Light line | 6 | 0.24 | about 18.8 | R2 |
| 1/4 in rope | 6.35 | 0.25 | about 19.9 | R2 |
| 8 mm rope | 8 | 0.31 | about 25.1 | R3 |
| 3/8 in rope | 9.5 | 0.38 | about 29.8 | R3 |
| 10 mm rope | 10 | 0.39 | about 31.4 | R3 |
| 1/2 in rope | 12.7 | 0.50 | about 39.9 | R4 |
| 16 mm rope | 16 | 0.63 | about 50.3 | R4 |
| Hawser | 24 or more | 0.94 or more | about 75 or more | R5 |
Inch figures are the millimetre value divided by 25.4, rounded to two decimal places, and the circumference is the diameter multiplied by pi, rounded to the nearest tenth. The Institute's survey of retail listings found that 6 mm and 1/4 in were treated as the same size, a difference of 0.35 mm that no user would notice, and that 10 mm and 3/8 in were likewise treated as one size though they differ by 0.5 mm.
The old British convention names a rope by its circumference in inches, and the figure survives in sailing and in the rigging trade. A rope of 3 in circumference is about 24 mm in diameter, since 3 times 25.4 is 76.2 mm and 76.2 divided by pi is 24.3 mm. Anyone who has bought a rope by circumference and received one by diameter will have found it far smaller than expected. The Institute therefore states which unit a figure is in each time it quotes one, and gives circumference in the table for this reason.
Unlike many of the objects in the Institute's register, rope has real standards. The International Organization for Standardization publishes a series on fibre ropes, with separate documents for polyamide, polyester, polypropylene and other fibres, and a test method for physical and mechanical properties. These documents specify linear density and minimum breaking force for each nominal size, and they are widely cited. The Institute cites them and does not alter them.
What they do not do is fix the diameter itself. Two ropes of the same nominal size and the same breaking force can differ in measured diameter by a millimetre or more, depending on construction. The Institute therefore treats the nominal size as a name and the standard as a performance specification, and records the measured diameter separately where it can.
A three-strand laid rope, a plaited rope and a braided rope of the same breaking force differ in diameter. A 10 mm laid polypropylene rope is typically bulkier than a 10 mm braided polyester rope with the same strength, because the lay leaves air between the strands. The Institute's practical advice is that a rope should be bought for the load it must carry and the size of the hardware it must pass through, in that order, and that diameter should be checked against the sheave or cleat before purchase.
A rope is measured with calipers at three points along a length of about a metre, with the rope under light tension. The Institute's convention is to take the largest of three readings across the lay, since a laid rope is wider across the strand crowns than between them, and to record the mean. A tape measure round the circumference and divided by pi gives a figure that is usually about 5 per cent higher than a caliper reading, because the tape bridges the gaps between strands. The method is the same one the Institute applies to the wire and cable described on the cable tie page and the twist tie page.
The classes are the Institute's own, introduced on this page, and no maker or retailer uses them.
The classes are set by nominal diameter alone, and the Institute does not rate strength. A class says how thick a rope is, and says nothing about what it will hold. Readers who need a load figure should consult the manufacturer's data and the relevant standard.
Rope is the soft cousin of the wire described on the wire gauge page, and its naming problem is the reverse of the wire's. Wire has a numbered gauge that conceals the diameter, while rope has a diameter that conceals the construction. Both are catalogued under the department of thickness and gauge, where the Institute records which labels can be turned into millimetres and which cannot.
The review panel divides on whether circumference should be the primary figure. Two members hold that the older convention is still the one used by sailors and that the table should lead with it. The majority hold that diameter is what a caliper reads and what a buyer outside the sailing trade expects, and that circumference should be given as a convenience. The table above is the majority's.
Under the Brennholt Institute's tables, a 10 mm rope is 0.39 in thick, about 31.4 mm in circumference, and falls in class R3.
The trade treats them as the same size. The Institute records a difference of 0.35 mm, with 1/4 in being 6.35 mm.
Divide the circumference by pi, which is 3.1416. A circumference of 3 in gives a diameter of about 0.96 in, or 24.3 mm.
The Institute takes the largest of three caliper readings across the lay, with the rope under light tension, and records the mean.
Standards fix the breaking force and linear density for each nominal size, not the diameter itself. The classes R1 to R5 are the Institute's own proposal.