FIGURE-EIGHT KNOT IN 8 MM NYLON: 33% strength loss
The figure-eight knot has a reputation as a safe knot — but measured in 8 mm nylon, one third of the breaking strength is lost. What remains, and is that sufficient?
A figure-eight knot in 8 mm braided nylon breaks at an average of 8.08 kN (824 kg) — that is 67% of the unknotted breaking strength of 12.07 kN (1,230 kg).
The figure-eight knot has a reputation as a reliable, easy-to-use knot. Simple to tie, simple to inspect, and — so the assumption goes — strong enough for most situations.
But when the actual breaking strength of a figure-eight knot in 8 mm braided nylon is measured, the loss is 33%. Not dramatic, but not negligible either. And the spread across the five measurements tells an equally important story.
Across five measurements, the breaking strength of the knotted rope ranged from 7.21 kN to 9.12 kN — a spread of 1.91 kN. In retention percentages: the lowest measurement retained only 60% of the unknotted strength, the highest 76%.
That lower bound of 60% is the figure that matters: a user cannot rely on the average. In three of the five tests, the rope broke completely within the knot. In the remaining two, the rope was cut through to the last strands without a complete break — the reported values for those measurements are the load at which the knot failed, not a complete break.
8 mm braided nylon rope — construction and material
This rope is a braided nylon (polyamide) construction with an 8 mm diameter, without core. The braided construction without a separate core means all fibres contribute to load absorption — there is no separate sheath providing protection.
Nylon has high elongation at break (20–35%) and absorbs shock better than polyester or HMPE. The drawback: nylon loses 10–15% of its tensile strength when wet, which is relevant for outdoor applications.
Unknotted breaking strength of this rope: 12.07 kN (1,230 kg) over five measurements (measured on 11 June 2026).
This rope is part of our Nylon rope collection.
How we test with a figure-eight knot
The breaking strength tests are carried out in-house by Otto Tromm on a universal testing machine with a capacity of 50 kN. Two sets of specially developed rope clamps are available for this machine, selected depending on the diameter and material of the rope. These clamps allow rope to be tested without requiring an eye splice beforehand.
All tests are conducted in accordance with the guidelines of ISO 2307:2019. This includes testing five times and using a test speed of 20 mm per minute. Each rope is secured in such a way that it is taut. No pre-tension is applied, as elongation is not measured in these tests.
Each test is filmed from two angles using a Sony camera on a tripod. The exact progression of the graph is also recorded separately on video.
This allows the stress profile and break behaviour to be matched precisely to the graph from the breaking strength test machine. For each of the five tests, observations are also recorded in writing by the tester, capturing details such as the degree of elongation, the manner in which the rope breaks, and the loudness of the snap at the point of failure.
For this test, the standard procedure was followed, with 2 deviations: a figure-eight knot was tied in the rope before it was placed in the clamps, and a soft shackle was used at the lower clamp for the connection to the clamp.
The machine, clamps, test speed, and the number of 5 measurements are identical to a test without a knot, so that the difference in measured breaking strength can be attributed to the figure-eight knot. The reference value without a knot comes from an earlier test of the same rope (5 measurements, 12.07 kN).
Test results: figure-eight knot in 8 mm nylon
Five measurements, test speed 20 mm/min. The spread of 1.91 kN between the lowest (7.21 kN) and highest (9.12 kN) measurement is large: 24% of the average. This indicates variation in how the knot sets under load and where the rope fails. Retention varies from 60% to 76% of the unknotted breaking strength.
The average of 8.08 kN (824 kg) with a retention of 67% is a usable reference value, but real-world conditions deliver the lower end. When dimensioning an application using a knotted 8 mm nylon rope, the lowest measured retention must be used: 60%, i.e. approximately 7.21 kN (735 kg).
Note: in two of the five measurements the rope did not break completely — the knot failed on the last fibres. The reported values are the load at knot failure, not at complete break.
Comparison: 8 mm nylon with and without figure-eight knot
The bars show the same rope in two states: unknotted (12.07 kN) and with figure-eight knot (8.08 kN). The difference makes the knot loss concretely visible.
The loss of 33% — from 12.07 kN to 8.08 kN — is a direct result of the knot geometry. The bend in the rope concentrates load at a small point, and the nylon material, which by nature offers low resistance to knot retention (qualitatively described as 'poor' for braided nylon rope in the Handbook of Fibre Rope Technology), yields relatively quickly.
The Handbook of Fibre Rope Technology cites a general reduction of 40–60% for knots; the measured loss of 33% for this rope therefore falls at the more favourable end of that range. No separate percentage is given for this specific knot type in that reference — a direct comparison per knot type is therefore not possible.
Alternatives to a knotted nylon rope
Conclusion: 33% loss is measurable — the spread is the real concern
A figure-eight knot in 8 mm braided nylon breaks at an average of 8.08 kN (824 kg), but the lower bound of 7.21 kN (60% retention) is the value to design to. The knot loss of 33% is smaller than the general range of 40–60% cited by the Handbook of Fibre Rope Technology for knots in general, but the spread between measurements is large.
This rope is best suited to temporary, shock-absorbing applications where the knot strength remains well above the working load. For permanent connections or safety-critical situations, an eye splice or thimble is the better choice: these restore strength to 90–100% of the unknotted value.
Otto Tromm tied the figure-eight knot, operated the testing machine, and noted that two of the five ropes chose not to break completely — on principle, or perhaps out of stubbornness. He held it against neither of them.