Rope Strength Converter
Look up rope breaking strength and working load limit by diameter and material.
Covers nylon, polyester, polypropylene, manila, plus knot and splice losses.
Select a rope material and enter a diameter to see breaking strength, working load limits, and what a knot or splice costs you.
Rope breaking strength depends on both the diameter and the material of the rope. The breaking strength is the maximum load a new, undamaged rope can hold before it breaks.
Important: Working Load Limit (WLL) Never load a rope to its breaking strength. The Working Load Limit is typically:
- Static loads (no shock): 1/5 of breaking strength (safety factor of 5:1)
- Overhead lifting (people): 1/10 of breaking strength (safety factor of 10:1)
- Dynamic/shock loads: 1/10 or more depending on the application
Approximate breaking strength by material and diameter:
| Diameter | Nylon (3-strand) | Polyester (3-strand) | Polypropylene | Manila |
|---|---|---|---|---|
| 1/4" (6mm) | 1,490 lbs | 1,250 lbs | 1,125 lbs | 540 lbs |
| 3/8" (10mm) | 3,340 lbs | 2,800 lbs | 2,440 lbs | 1,220 lbs |
| 1/2" (13mm) | 5,750 lbs | 4,830 lbs | 3,780 lbs | 2,380 lbs |
| 5/8" (16mm) | 8,600 lbs | 7,200 lbs | 5,600 lbs | 3,960 lbs |
| 3/4" (19mm) | 12,800 lbs | 10,400 lbs | 7,650 lbs | 5,400 lbs |
| 1" (25mm) | 22,200 lbs | 18,000 lbs | 13,200 lbs | 9,000 lbs |
| 1-1/4" (32mm) | 33,800 lbs | 27,500 lbs | 19,800 lbs | 13,500 lbs |
| 1-1/2" (38mm) | 47,800 lbs | 38,800 lbs | 27,000 lbs | 18,500 lbs |
The calculator above reads straight off this table and interpolates between the rows, so the two can never drift apart. Strength does not follow a clean square law, which is worth knowing if you are tempted to scale a figure yourself: doubling the diameter of nylon from 1/4 to 1/2 inch multiplies the strength by 3.9, not 4, and going from 3/4 to 1-1/2 inch multiplies it by 3.7. Larger ropes are slightly weaker than the geometry alone suggests, because the outer fibres in a big lay take more of the load than their share.
Material characteristics:
- Nylon: Strongest, excellent shock absorption due to stretch (15-30%). Good UV and abrasion resistance. Best for anchor lines, towing, and dynamic loads.
- Polyester (Dacron): Strong with minimal stretch (3-5%). Excellent UV and chemical resistance. Best for halyards, sheets, and fixed-length applications.
- Polypropylene: Lightweight and floats on water. Moderate strength. Poor UV resistance (degrades in sunlight). Good for water ski tow ropes and temporary use.
- Manila (natural fiber): Traditional natural rope. Lower strength and degrades when wet. Good grip and appearance. Used for decorative applications and traditional rigging.
Factors that reduce rope strength:
| Condition | Strength Reduction |
|---|---|
| Knots (any type) | 40-60% reduction |
| Spliced eye | 10-15% reduction |
| UV exposure (aged rope) | 10-50% depending on material and time |
| Wet manila rope | 10-15% reduction |
| Sharp bends over edges | Up to 50% reduction |
| Heat damage | Variable, potentially catastrophic |
Practical tips:
- Inspect rope regularly for fraying, glazing, stiffness, or discoloration. Replace damaged rope immediately.
- Store rope in a cool, dry, dark location to maximize lifespan.
- Never exceed the Working Load Limit, especially for overhead or life-safety applications.
- When in doubt, use a thicker rope or a stronger material.
How we build and check this converter
This converter runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.
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