Lifting Sling Calculator: Leg Tension, Sling Angle Factor and Wire Rope Working Load Limit

Check the loads in lifting slings and wire ropes.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

Enter the load mass, the number of legs, the number you can rely on, and the leg angle from the vertical.

Enter the rope diameter, construction and grade, the end fitting efficiency and the factor of safety.

The calculator gives the leg tension, the rope breaking force and working load limit, and how much of the limit is used.

Formulas used

  • T = \frac{F}{n_e\cos\theta}
  • F_{min} = \frac{d^2R_rK}{1000}
  • WLL = \frac{F_{min}\,\eta}{N}

Worked example

With the default numbers in the calculator above:

  1. Weight of the load: 19.613 kN
  2. Angle factor (how much the tension rises as the legs spread): 1.155 (a leg angle of 30 degrees from vertical)
  3. Tension in each leg that carries the load: 11.324 kN (sharing between 2 legs)
  4. Minimum breaking force of the rope: 84.11 kN
  5. Working load limit of one leg (breaking force times the end fitting efficiency, divided by the factor of safety): 14.299 kN (about 1458.1 kg)
  6. How much of the leg's limit is used: 79.19 %

Change the numbers in the calculator to see the full working, every step, for your own case.

Common questions

Why does the sling angle matter?

The flatter the legs, the more of each leg's tension is pulling sideways, so the tension rises. At 60 degrees from vertical it is twice the load share.

How many legs of a 4 leg sling carry the load?

Assume two on a rigid load, because the load will not share evenly between all four.

What is a working load limit?

The largest load the sling is rated to lift in normal use, the breaking force divided by a factor of safety.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Wire rope breaking force by the usual rope factors. Lifting gear is covered by AS 4991 and AS 1666. This tool is not a lift plan.

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