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What this tool does
Every rope has two numbers that matter, and they are easy to confuse. The breaking strength is the load that snaps a brand-new rope in a laboratory test. The safe working load is the load you should actually put on it in real life, and it is much smaller. This rope working load calculator estimates both from three inputs you can measure or read off a label: the rope diameter, the material, and the safety factor you want to apply.
The maths is simple on purpose. Breaking strength grows with the cross-section of the rope, which grows with the square of the diameter. That is why a 12 mm rope is not twice as strong as a 6 mm rope of the same material, but roughly four times as strong. The tool multiplies a typical strength factor for the material by the square of the diameter, then divides by your safety factor to get the working load. The result comes out in kilograms and in tonnes, so you can compare it with the weight you plan to lift, tow, or hold.
How to use it
Measure the diameter across the rope with callipers if you can, or read it from the spool label. Pick the material that matches your rope: polyester double braid is the common cruising sheet and halyard rope, nylon three-strand is the classic stretchy anchor and mooring rope, polypropylene is the light floating line, Dyneema braid is the very strong low-stretch racing line, and stainless wire rope is the standing rigging and guardrail material. Then pick a safety factor. Five to one is the usual general marine choice. Twelve to one is the cautious end used where a failure could hurt someone.
Press the button and read the two numbers. The breaking strength tells you what a new rope of that size and material typically withstands. The working load is the number to plan around. If your job needs more than the working load, go up a size, choose a stronger material, or split the load across more lines. Never plan around the breaking strength itself, because that leaves no margin for wear, knots, sunlight, or shock loads.
Why the safety factor matters so much
A rope in service is never as strong as it was on the test bench. Knots can cut strength by a third or more, depending on the knot. Sunlight, chafe, salt, and repeated loading all wear fibres down over time. A sudden snatch load, like a boat jerking against a mooring line in a gust, can briefly reach several times the steady load. The safety factor is the margin that absorbs all of that. A factor of five means you are using only a fifth of the new-rope strength, which leaves room for the rope to age and still hold.
Different jobs deserve different factors. A fender line or a clothesline can live with a modest factor. A mooring line that holds your boat in a storm, a tow line, or anything above people deserves a bigger one. When in doubt, choose the larger factor and the larger rope. Rope is cheap compared with a boat, and a failure at the wrong moment is expensive in every sense.
A worked example
Take a 12 mm polyester double braid rope with a safety factor of five. The tool estimates a breaking strength of about 3,456 kg, which is roughly 3.46 tonnes. Divide by five and the safe working load is about 691 kg. That means a steady load of up to roughly 690 kg is a sensible plan for that line, while a load approaching 3.5 tonnes would be right at the edge of what a new rope survives. If you need a working load of a full tonne, the same rope is not enough, and you should step up to 14 mm or switch to a stronger material.
Try the same size in Dyneema and the picture changes sharply. The estimate jumps to about 10,080 kg breaking strength, giving a working load of about 2,016 kg at five to one. That is why racing boats can use much thinner lines for the same job. The trade-off is that Dyneema stretches very little, so shock loads pass straight through instead of being absorbed, and it needs careful handling at knots and splices.
Limits and safety
These are typical-value estimates, not the rated values for your exact rope. Makers publish breaking loads for each product, and those numbers vary with construction, coating, and quality. Always check the maker’s data for the rope you actually own before trusting any estimate for a serious job. Old, sun-damaged, chafed, or stiff rope should be retired no matter what the calculator says, because the tool cannot see wear.
This calculator is an educational estimator only. It does not replace the rope maker’s guidance, rigging standards, or professional advice for lifting, towing, or mooring. If a failure could injure someone or lose a boat, get the rope specified by someone qualified for that job. Use the calculator to learn, compare sizes, and sanity-check a plan, not as the sole basis for a safety-critical decision.
Common questions
Does a knot change the answer? Yes, in real life. A knot weakens a rope, sometimes a lot, and splices are much better than knots for permanent eyes. The calculator assumes a straight pull on a new rope, so treat its working load as an upper guide when knots are involved. Should I use the same factor for wire rope? Wire behaves differently from fibre rope: it is stiff, it kinks, and broken strands are a warning sign. Use the maker’s ratings for wire, and inspect it often.
Versión en español: Calculadora de Carga de Trabajo de Cabo