Calculator inputs
Load and strain graph
For round ropes, the chart sweeps diameter around your chosen value. For flat or custom sections, it scales cross-sectional area.
Formula used
Cross-sectional area
Round rope: A = πd² / 4
Flat webbing: A = width × thickness
Custom section: A = entered area
Theoretical breaking load
Ftheoretical = σu × A
Effective breaking load
Feffective = Ftheoretical × ηconstruction × ηknot
Breaking stress
σbreak = σu × ηconstruction × ηknot
Breaking strain
εbreak = σbreak / E
Elongation at break
ΔL = εbreak × L
Safe working load
SWL = Feffective / Safety Factor
How to use this calculator
- Select metric or imperial units and choose the rope profile.
- Pick a material preset or use the custom option for your own strength and modulus values.
- Enter rope geometry, length, knot reduction, and safety factor.
- Submit the form to view breaking load, strain, elongation, and safe working load above the form.
- Review the graph to see how load changes with diameter or area scaling.
- Export the summary as CSV or PDF for reports, design notes, or field documentation.
Example data table
| Material | Profile | Diameter / Width × Thickness | Length | Eff. factor | Knot factor | Safety factor | Effective breaking load | Breaking strain |
|---|---|---|---|---|---|---|---|---|
| Nylon | Round | 18 mm | 12 m | 0.95 | 0.75 | 5 | 13.77 kN | 2.137% |
| Polyester | Round | 14 mm | 10 m | 0.95 | 0.80 | 5 | 10.54 kN | 0.855% |
| Steel Wire Rope | Round | 12 mm | 20 m | 0.97 | 0.90 | 5 | 176.66 kN | 1.072% |
| Polypropylene | Flat | 45 mm × 6 mm | 8 m | 0.98 | 0.78 | 6 | 7.22 kN | 1.784% |
FAQs
1. What does breaking strain mean for a rope?
Breaking strain is the fractional extension a rope reaches at failure. It equals breaking stress divided by Young’s modulus in this calculator.
2. Why is safe working load lower than breaking load?
Safe working load applies a safety factor to the effective breaking load. This protects against wear, shock loading, damage, and real-world uncertainty.
3. Why include knot reduction?
Knots reduce rope strength by creating stress concentrations and local bending. The knot factor models that reduction before calculating strain and safe load.
4. Should I trust preset material values?
Presets are planning values only. Manufacturer data, rope age, moisture, construction details, and test conditions can change actual performance significantly.
5. Can I use this for wire rope and fiber rope?
Yes. The calculator includes both fiber and steel-wire presets. Custom values are available when a manufacturer provides more specific material properties.
6. Why does diameter affect load so strongly?
For round ropes, area depends on diameter squared. Small diameter increases create much larger area, which raises breaking load noticeably.
7. Does longer rope change breaking strain?
Breaking strain itself stays the same for the same stress and modulus. However, longer rope produces greater total elongation at that strain.
8. Can this replace certified lifting calculations?
No. Use it for estimation, comparison, and education. Certified lifting plans should always follow manufacturer ratings, inspection rules, and project standards.