How to Calculate Wire Rope Breaking Load and Safety Factor
2 min read OSAK
To know how much load a steel wire rope can carry safely, two concepts must be separated: the minimum breaking load (MBL) is the force at which the rope breaks; the working load limit (WLL) is the load permitted in daily use, obtained by dividing by the safety factor.
Minimum breaking load formula
The approximate formula used in the standards is:
Fmin (kN) = d² × Rr × K ÷ 1000
- d: nominal rope diameter (mm)
- Rr: wire tensile grade (e.g. 1770 N/mm²)
- K: breaking load factor depending on the construction (for example about 0.330 for 6x19 fibre core ropes and about 0.356 for 6x36 steel core ropes)
Example: 10 mm, 6x19 + FC, grade 1770
Fmin = 10² × 1770 × 0.330 ÷ 1000 ≈ 58.4 kN, i.e. about 5.9 tonnes. With a safety factor of at least 5:1 for general lifting applications, the working load is about 1.2 tonnes.
Situations that affect the calculation
- End terminations: In eye connections made with clips, the termination efficiency is below the rope’s full capacity; calculate accordingly.
- Dynamic loads: Sudden pulls, swinging and shock loads can reach several times the static load.
- Sheave diameter and wear: Small sheaves and worn ropes reduce capacity; see our maintenance guide.
- Carrying people: Systems that carry people require much higher safety factors and certified products.
Important: The formula gives an approximate value. The actual breaking load is the value stated on the manufacturer’s certificate; always use certified rope and fittings for lifting applications. You can ask us to help you choose the right rope.
