The figure-eight family includes a stopper knot and doubled forms used to make a loop or tie around an attachment. These forms have different applications and must be identified correctly. The calculator shows the arithmetic effect of an assumed strength-retention factor; it cannot certify a knot, select a rescue system or establish a safe load.
Figure Eight Knot Interactive Calculator
Compare an entered rope strength, assumed retention factor and load. The diameter-based length is an arithmetic reference, not a universal tail requirement.
Equation Used
The strength output is the entered MBS multiplied by an assumed factor. It is not a tested minimum breaking strength or working-load limit. The 10-diameter length and 10:1 comparison are fixed arithmetic references, not acceptance criteria. Use the actual rope and equipment instructions, applicable procedures and qualified training for a life-safety system.
- Strength retention is an input assumption; rope-specific knot test data is needed to establish strength.
- The length output is ten times the entered diameter; it does not establish the required tail for an application.
- The ratio divides assumed strength by entered load; it does not evaluate the complete system.
- Dynamic effects, edge friction, shock loading, abrasion, and wet or icy rope effects are not included.
Stopper, loop and follow-through forms
The rope path in a figure-eight changes direction around itself. A single-strand stopper enlarges the rope end, while a doubled or retraced arrangement can form an attachment loop. Their similar names do not make them interchangeable.
The drawing is a visual introduction to the strand paths. Use the rope and equipment manufacturer's instructions for the specific knot, attachment points, dressing, tightening and tail length. A recognizable outline alone does not confirm correct routing or suitability.
Match the form to the task
Figure-eight forms appear in rope work, including stopper and attachment applications. The appropriate form depends on the rope, equipment and intended use.
For climbing, rescue or work at height, a numerical strength estimate is only one small part of a complete system assessment. This page is not a tying lesson or an instruction to rig, test or load such a system.
What the calculator actually compares
Let B be the entered rope minimum breaking strength, η an assumed retention factor and F the entered load, with B and F in kN. Assumed strength is S = Bη. The displayed strength/load ratio is S/F.
The 10:1 comparison is S − 10F in kN. A positive value only means that the assumed strength exceeds ten times the entered load; it does not approve the rope, knot or system. A negative value means it falls below that arithmetic reference.
The diameter output is 10d in millimetres. It is not a universal minimum tail. Strength retention and tail requirements depend on the actual rope, knot form, equipment instructions and conditions.
Arithmetic example with assumed inputs
Enter B = 30 kN, η = 0.73, F = 2.7 kN and d = 11 mm. Assumed strength is 21.9 kN, the strength/load ratio is 8.11, the 10:1 comparison is −5.1 kN and the 10d reference is 110 mm.
Changing η to 0.66 gives an assumed strength of 19.8 kN; changing it to 0.77 gives 23.1 kN. These differences come only from the chosen multiplier. They do not measure knot quality or demonstrate that tightening can produce a particular strength.
No result in this example establishes an acceptable rescue load, required tail or suitable rope.
Use test data for the actual rope and knot
Rope construction, condition, loading and knot configuration affect performance. A percentage from one test cannot automatically be applied to another rope or application. Manufacturer data may report strength with a particular knot as a separate tested value.
The tool does not model fall arrest, dynamic loading, multiple load paths, connectors, anchors, abrasion or environmental damage. Follow the product instructions and the procedures appropriate to the task rather than treating the displayed ratio as permission to use a system.
Frequently asked questions
Is Assumed strength a certified knotted MBS?
No. It is the entered rope MBS multiplied by a user-selected factor. It is not a test result or working-load limit.
Does 10d reference give the required tail length?
No. It only multiplies diameter by ten. Required tails and any backup knot must come from the instructions and procedures for the specific rope, knot and application.
Does a positive 10:1 comparison mean a rescue system is safe?
No. It compares assumed strength with ten times one entered load. It does not assess the rope system or establish an applicable acceptance criterion.
Are a stopper figure-eight and a follow-through tie-in interchangeable?
No. They have different strand arrangements and functions. Use the specific form required by the equipment instructions and your training.
Can the picture verify my knot?
No. The illustration does not verify routing through attachment points, dressing, tightening, tail length, rope condition or compatibility.
Can I infer rope condition from the efficiency slider?
No. The slider is an assumption, not an inspection or measurement. Condition must be assessed using the rope manufacturer’s instructions.
Manufacturer references
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