OFFSHORE TOOLKIT · CALCULATOR 03

Wire Rope Discard Calculator

Diametric reduction, broken-wire counts, and physical damage — the three main ISO 4309 discard checks for a wire rope inspection, in one place.

Nominal diameter (mm)

Reference diameter (mm) — actual as-manufactured size, usually slightly over nominal

Measured diameter (mm) — average of two measurements at right angles

Rope construction

Severity bands and discard thresholds are matched to the official three-category table used in Carl Stahl's ISO 4309 diametric reduction calculator.

DIAMETER LOSS

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% of nominal

Severity—

Rating—

Rope diameter (mm)

Broken wires counted over 6 × diameter length ("6d rule")

Broken wires in a single strand within one lay length

6d rule: discard a running rope at 6 or more broken wires visible over a length of 6 × the rope diameter.
Single-strand rule: discard immediately at 3 or more broken wires in one strand within a single lay length, regardless of the 6d count.

BROKEN WIRE CHECK

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6d length—

Strand rule—

Check any that apply — any single item is an automatic discard Kinking Crushing Birdcaging Core collapse / core protrusion Heat or electrical arc damage Severe corrosion or pitting Waviness or strand distortion

This is a checklist for the visible physical-damage discard criteria, alongside diameter and broken-wire checks — it doesn't replace a full inspection against the standard.

PHYSICAL DAMAGE CHECK

PASS

Issues found0

Quick reference — ISO 4309 discard criteria

Diameter loss

Discard at ≥10% for single-layer, fibre core
Discard at ≥7.5% for single-layer steel core / parallel-closed
Discard at ≥5% for rotation-resistant rope
Measure at two points 90° apart and average, away from terminations

Broken wires

6d rule: 6 or more breaks in 6 × diameter length — discard (running rope)
Single strand: 3 or more breaks in one strand within a lay length — discard immediately
Standing (non-running) ropes generally tolerate more breaks before discard — check the applicable standard

Physical damage

Kinking, crushing, or birdcaging — discard, these are structural, not wear-related
Core collapse or protrusion — discard
Heat or arc damage — discard, strength cannot be reliably assessed visually

Best practice

Inspect the full length, not just accessible sections
Record findings and trend diameter loss over time
When any single criterion says discard, the rope is discarded — criteria aren't averaged