Four-Leg Sling Load Share Calculator
Size the legs of a four-leg lift the way experienced riggers actually do it — on 3 sharing legs (or 2 for the worst case), not on the flattering assumption that all four share evenly. Enter the load, the sling angle and your design basis and the tool returns the vertical share and tension per leg.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the total load W in kilograms and the sling angle θ from the horizontal.
- Choose the design basis: 3 sharing legs for common practice, or 2 for a conservative worst case on a rigid or uncertain load (default 3).
- Read the tension per leg and check it against the WLL stamped on the sling at that angle before lifting.
How it works
Because a rigid load slung from four legs is statically indeterminate, the true leg forces can't be solved from statics alone — small differences in leg length or load flatness throw most of the weight onto two diagonal legs. Standard practice therefore rates the lift on an assumed number of sharing legs: vertical share per leg = W / (sharing legs), and leg tension = that share / sin(θ). Rating on 3 (or 2) sharing legs builds in the margin this uncertainty demands.
Worked example
Worked example. A 4000 kg load on a four-leg sling at 60°, rated on 3 sharing legs: share = 4000/3 = 1333.33 kg, tension = 1333.33/sin(60°) = 1539.6 kg per leg. Rate the same lift on the conservative 2-leg basis and each leg is designed for 2000/sin(60°) = 2309.4 kg.
Common mistakes
- Dividing the load by 4 and assuming all legs share equally — on a rigid load they don't, and two legs can end up carrying nearly the whole weight.
- Ignoring the sling angle: shallow angles multiply the already-conservative share, so a low-angle four-leg lift can overload a leg quickly.
- Only assuming 4-leg sharing when it isn't justified — that is safe only with a flexible load or a properly equalising spreader/beam.
Frequently asked questions
Why not just divide the load by four?
Because a four-leg lift on a rigid load is statically indeterminate: the legs are never identical in length and the load is never perfectly level, so at any moment two diagonally-opposite legs can carry most of the weight while the other two just steady it. Dividing by four assumes a perfection that doesn't exist and can badly under-rate the legs, which is why practice rates on 3 (or 2) sharing legs.
When can I assume all four legs share equally?
Only when the load is flexible enough to settle onto all four picks, or the slings run over an equalising beam or spreader that genuinely balances the legs. For a rigid load, stay on the 3-leg (or worst-case 2-leg) basis. This is a planning estimate only — the sling tag WLL/SWL and a licensed rigger/dogger to AS 1418, AS 2550 and AS 4991 govern the lift, and you must never lift over people.
Related tools
- Sling Angle Load Factor Calculator
- Two-Leg Sling Load Share Calculator
- Wire Rope SWL / Load Checker
- WLL from Breaking Load & Safety Factor Calculator
- Sling D/d Ratio Efficiency Calculator
- Sling Length for Angle Calculator
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