Every one of these numbers is mass divided by length

Linear density is mass divided by length. That is the whole concept. The reason it turns up under half a dozen different names is historical: the textile trade measures individual yarns and filaments, the cordage trade measures finished rope, and the two settled on different reference lengths long before anyone tried to reconcile them. Each of the terms below is one of the vocabulary items the rest of cordage fundamentals quietly assumes you already have.

  • Denier — grams per 9,000 meters of yarn. The odd reference length is a trade convention carried over from silk yarn numbering, where 9,000 meters of a single filament weighing one gram defined one denier.
  • Tex — grams per 1,000 meters. The metric-system attempt to make yarn numbering rational, and the basis most technical fiber literature now uses.
  • Decitex (dtex) — grams per 10,000 meters. Ten times the tex value, and the unit most modern fiber datasheets print, because it gives whole numbers for the filament sizes people actually spin.
  • Pounds per 100 feet — the rope-level figure on most US cordage listings and spool labels.
  • Grams per meter and kilograms per 100 meters — the same rope-level figure in metric, common on European datasheets and on technical marine and rigging documentation.

All five are direct systems: a bigger number means more material per unit length, so a coarser yarn or a heavier rope. That is worth stating because several older textile counts run the other way — cotton count and metric number get larger as the yarn gets finer. If a number in an unfamiliar document behaves backwards from what you expect, check which system it belongs to before drawing a conclusion.

What weight per length settles, and what it leaves open

MeasureWhat it tells youWhat it does not tell you
Denier, tex, dtexThe size of a yarn or filament, expressed as mass per unit length. Lets you compare fiber inputs across suppliers on one scale.Anything about the finished rope. Yarn size is a manufacturing input; construction decides what the rope built from it does.
Rope weight per lengthHow much material the rope contains per foot or meter, which is what you carry, stow, and pay freight on.Strength. Weight includes coatings, finishes, absorbed moisture, and any non-load-bearing yarn.
Strength-to-weight ratioHow efficiently a rope converts mass into rated capacity — the fairest single comparison between fibers of different densities.Whether the efficient rope survives your abrasion, heat, sunlight, creep, or knot conditions. Efficiency is one axis of many.
Specific gravityWhether the fiber itself is denser or less dense than water, which is the starting point for whether a rope can float.Whether a finished rope floats in service, once trapped air, water absorption, coatings, fouling, and attached hardware are counted.

Converting between them is arithmetic, not lookup

Because every one of these units is grams over a fixed number of meters, the conversions are simple ratios. Denier divided by nine gives tex, because 9,000 meters is nine times 1,000 meters. Tex multiplied by ten gives decitex. So a yarn described as 1,000 denier is 1,000 ÷ 9 = about 111 tex, which is 1,111 dtex, which is 0.111 grams per meter of yarn.

The rope-level conversion crosses unit systems, but it rests on two exact definitions: one pound is 453.59237 grams and one foot is 0.3048 meters. One hundred feet is therefore exactly 30.48 meters, and one pound per 100 feet equals 453.59237 ÷ 30.48 = about 14.88 grams per meter. Going the other way, one gram per meter is about 0.0672 pounds per 100 feet. Grams per meter to kilograms per 100 meters is simply a shift of the decimal point: divide by ten.

As an illustrative case, if a rope were listed at 3.0 pounds per 100 feet, that would be 3.0 × 14.88 = about 44.6 grams per meter, or 4.46 kilograms per 100 meters. Those numbers are chosen to show the arithmetic, not to describe any particular product. The conversion factors are real and exact; the 3.0 is invented for the example. If you want to work in force units rather than mass units anywhere downstream of this, read how kilonewtons, pounds-force, and mass differ first, because weight per length is a mass measurement and rope ratings are not.

Why it is a proxy for how much fiber is in the rope

Here is the useful part. Diameter is measured on the outside of a rope. Weight per length counts what is inside it. You cannot add load-bearing fiber to a rope without adding mass, so at a given nominal diameter and a given fiber, the heavier rope contains more material — a tighter braid, a higher pick count, more or larger yarns, a fuller core. A loose, airy construction can sit at the same measurement across a caliper as a densely packed one and carry noticeably less fiber. That is exactly the gap that nominal versus measured diameter leaves open, and the reason two ropes labeled the same size can feel like different products in the hand. Which internal parts the mass is distributed among — cover, core, filler — is a question for rope anatomy, and how tightly those parts can be packed is decided by construction.

The proxy is good, not perfect. Four things put mass into a rope without putting capacity into it. Coatings and finishes add weight and change hand without adding fiber. Marker yarns, filler yarns, and identification tracers are mass that does not carry load. Moisture matters, because fibers differ in how much water they hold at ambient humidity, and a published weight reflects whatever conditioning the test used. And a published weight is usually nominal, carrying a tolerance the datasheet may or may not print. Treat weight per length as strong evidence about fiber content between two ropes of the same fiber and similar construction, and as weak evidence across fiber families. That distinction is editorial judgment on our part, drawn from how manufacturers present the figure rather than from any single standard.

You can push the arithmetic one step further if you know the yarn size. Illustratively, a rope at 44.6 grams per meter built from yarn of 1,000 tex — one gram per meter — works out to roughly 45 yarn-meters per meter of rope. That quotient is a dimensionless count rather than a second mass figure, because grams per meter divided by grams per meter cancels the units entirely. But twist and braid make every yarn travel a longer path than the rope’s own axis, so the true number of yarn ends is fewer than 45, and the gap grows with how aggressively the rope is twisted or braided.

Strength-to-weight is the fairer comparison between fibers

Comparing two ropes at equal diameter mixes two questions that deserve separate answers: how good is this fiber, and how much of it did the manufacturer put in. Fibers also differ in density, so equal diameter never means equal mass of material. Dividing rated strength by weight per length separates the questions. It asks how efficiently the rope turns mass into capacity, and it is the comparison the fiber industry itself uses.

At the fiber level this ratio is called tenacity, and it is expressed as force divided by linear density — grams-force per denier in older literature, centinewtons per decitex in current datasheets. The two are convertible from the definitions: one denier is ten-ninths of a decitex and one gram-force is about 0.98 centinewtons, so one gram-force per denier works out to roughly 0.88 centinewtons per decitex. Note that the numerator is a force and the denominator is a mass per length. Keeping those straight is the whole reason the units look awkward.

At the rope level, the same idea gives you a figure some people find more intuitive: breaking length, the length of rope whose own weight would equal its breaking strength. If a rope were rated at 5,000 pounds-force and weighed 3 pounds per 100 feet, the illustrative arithmetic — 5,000 ÷ 3, times 100 — is about 167,000 feet. Note that this quotient divides a force by a mass per length, which only works because the rope’s own weight under standard gravity is being treated as the force it exerts; say so whenever you compute it. Both of those inputs are invented for the example. The point of the number is comparative, not absolute: it tells you how much of a long vertical rope’s capacity is spent holding up the rope.

Strength-to-weight matters most where the rope’s own mass is part of the problem: long vertical runs, weight carried high on a rig, anything you have to backpack, anything deployed by hand over distance. It matters very little on a short dock line, where a few pounds either way changes nothing. And the fiber that wins on strength-to-weight is frequently the one that loses on creep, heat tolerance, abrasion, knot behavior, or price — see what each fiber family is actually good at. A high ratio also does not upgrade the rating it is built from: a breaking strength remains a test result under stated conditions, not a working load.

Weight predicts handling, bulk, stowage, and freight

Weight per length is the one specification that lets you answer practical questions before the rope arrives. Multiply it by the length and you know what the coil weighs, whether one person can carry it, and whether it is a parcel or a freight shipment. Illustratively, 600 feet of a rope listed at 3.0 pounds per 100 feet is 18 pounds of rope before you count the spool. Carriers price by actual weight or by dimensional weight, whichever is greater, so a long spool of heavy rope can cost more to ship than the difference in list price between two candidates.

Weight and bulk are related but not the same, and confusing them causes real disappointment. Bulk follows diameter and stiffness; weight follows fiber density and packing. A thick, low-density, softly constructed rope can fill a locker while weighing very little. A compact, densely packed, high-density rope can be surprisingly heavy for how small the coil looks. If your constraint is space, read the diameter and the construction. If your constraint is load-out weight or shipping, read the weight per length.

Handling tracks weight too, in ways that cut both directions. A heavier line carries momentum: it throws better, it runs through a system with more of its own inertia, and it hangs in a predictable curve. A very light line is pleasant to carry and awkward in wind, tends to tangle differently, and can be harder to feed under its own weight. Neither is better in the abstract. Both are consequences you can anticipate from one number on the label.

Cost per foot and cost per unit strength are different questions

Weight per length is also the hidden variable behind cordage pricing. Synthetic fiber is a commodity bought by weight, so for a given fiber and construction, a heavier rope costs more per foot for reasons that have nothing to do with branding. That is why two ropes of the same nominal diameter can differ substantially in price, and why the cheaper one is sometimes cheaper simply because there is less of it per foot.

Three ratios answer three different questions, and buyers routinely use one when they meant another.

  • Cost per foot answers “what will this length cost me.” It is the right question when the length is fixed and the requirement is easily met.
  • Cost per pound answers “what am I paying for raw material.” It mostly explains the price gap between two ropes of the same fiber, and it explains almost nothing between different fibers, whose price per pound varies enormously.
  • Cost per unit of rated strength answers “what am I paying for capability.” Illustratively, if one rope were priced at $0.80 per foot with a 5,000 pounds-force rating and another at $1.30 per foot with a 9,000 pounds-force rating, the arithmetic gives about $0.16 and about $0.14 per foot per 1,000 pounds-force. Those figures are invented to show the method.

None of the three is “which rope should I buy.” Cost per unit strength systematically flatters high-strength fibers even in jobs that are not strength-limited, which is most jobs — the usual limits are abrasion, sunlight, heat, chafe, and handling. And a rope that lasts three seasons at twice the price is cheaper than one replaced every season. Our editorial judgment is that cost per unit strength is a sanity check on a shortlist you built for other reasons, not a way to build the shortlist.

Specific gravity, and what it does and does not say about floating

Specific gravity is a material’s density compared with water. Below 1, the material is less dense than water and will float in it; above 1, it sinks. That is the principle, and it is genuinely useful, because fiber families sit on predictable sides of that line. The per-fiber values belong with the fiber pages rather than here, where the point is the relationship rather than the table.

Linear density alone cannot answer whether a rope floats, because floating depends on mass per unit volume and linear density is mass per unit length. Add the diameter and you can approximate the rope’s bulk density: divide mass per length by the cross-sectional area. Illustratively, a 12 mm rope has a cross-section of about 1.13 square centimeters, so a rope weighing 60 grams per meter — 0.60 grams per centimeter — works out to about 0.53 grams per cubic centimeter of bulk density.

That result is well below the density of any rope fiber, and the gap is the interesting part: a rope is substantially air by volume. It is tempting to read that gap as buoyancy. It is not. A bulk density computed this way counts surface irregularity and open void space as though it were sealed volume, and a rope’s voids are open and interconnected, so they flood quickly. Briefly buoyant behavior in a new, dry, dense-fiber rope comes from trapped air and surface tension, not from the fiber, and it disappears as the rope wets out. If you are using a float test to narrow down what fiber you are holding, test a single teased-out yarn rather than a length of rope, and give the sample time to wet out before reading the result. Coatings, absorbed water, fouling, splices, and attached hardware all move the result. Our editorial judgment: treat “does it float” as a property of the finished, wetted, in-service assembly and confirm it against the manufacturer’s product documentation. Where floating matters for a rescue or throw-line function, that is a product-documentation and training question, not something to infer from a fiber’s specific gravity.

A rope that seems too light for its diameter

This is the practical payoff of learning the number. When a rope weighs noticeably less per foot than others of the same labeled size, several explanations are possible, and they are not equally interesting.

  1. A genuinely lower-density fiber. Polyolefin fibers are less dense than nylon, polyester, or the aramids, so a lighter rope may simply be made of different material and be behaving exactly as intended.
  2. A looser construction. Fewer picks per inch, a lower yarn count, a hollow or lightly filled core, or a thinner cover all reduce mass at the same outside measurement.
  3. Bulked or texturized yarn. Fiber that has been crimped, air-jet textured, or fibrillated occupies diameter with air. It can improve hand and grip; it does not add load-bearing material.
  4. An optimistic nominal size. The rope may simply measure under its label, which is a diameter and tolerance question rather than a density one.
  5. A different measurement basis. Weight measured relaxed or under tension, conditioned or oven-dry, over a short sample or a long one, can differ enough to explain a modest gap.

None of those is a verdict on its own, and the honest conclusion is that light-for-diameter is a question rather than an answer. The right response is to ask what fiber it is, what construction it is, what strength the manufacturer publishes and against what test method, and what tolerance the weight carries. The converse deserves the same skepticism: heavy does not mean strong. A rope can be heavy because of a water-absorbing natural fiber, a dense filler, or a thick coating, none of which adds capacity.

Scope boundary worth stating plainly. Everything above is a shopping and comparison heuristic for general-purpose cordage. It is not an inspection method, it is not a way to judge a used rope, and it is not a way to evaluate a rope for climbing, fall arrest, rescue, human suspension, overhead lifting, or regulated rigging. A rope considered for any of those uses would typically need to be identified by its product marking, certification, and current manufacturer documentation — not inferred from a scale and a tape measure.

Primary and technical starting sources

Sources checked August 3, 2026. Check the current official text and exact product documentation before relying on a consequential claim.

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