A sling leg can only pull along its own length. When the leg is vertical, all of its pull lifts the load. When it leans over, part of the pull is wasted squeezing the load sideways, so the leg has to pull harder to provide the same lift. That is the whole reason sling angles matter.
The basic relationship
For a symmetrical two-leg sling with the hook directly above the centre of gravity, each leg carries half the weight vertically. If β is the angle of each leg measured from the vertical, the tension in each leg is:
T = W ÷ (2 × cos β) — where W is the total weight lifted, including the rigging below the hook.
| Leg angle from vertical, β | Tension in each leg | Compared with a vertical leg |
|---|---|---|
| 0° | 0.50 W | ×1.00 |
| 30° | 0.58 W | ×1.15 |
| 45° | 0.71 W | ×1.41 |
| 60° | 1.00 W | ×2.00 |
At 60° from the vertical (an included angle of 120° between the legs) each leg carries the full weight of the load. Beyond that the tension climbs steeply, which is why 60° from vertical is the normal upper limit for multi-leg slings. Some references, mostly in North America, measure the angle from the horizontal instead; the same formula then uses sin in place of cos. Always check which convention a chart uses.
How multi-leg slings are rated: the uniform load method
Under BS EN 13414-1 and BS EN 818-4 (and LEEA guidance), multi-leg slings are commonly rated by the uniform load method. The marked working load limit of the assembly is a multiple of the single-leg WLL:
- Two-leg sling: 1.4 × single-leg WLL for 0–45° from vertical, 1.0 × for 45–60°.
- Three- and four-leg slings: 2.1 × single-leg WLL for 0–45°, 1.5 × for 45–60°.
The method assumes the load is shared evenly. A four-leg sling is rated as if only three legs carry the load, because on a rigid load one diagonal pair of legs often does most of the work.
Unequal legs and an offset centre of gravity
Real loads rarely have the centre of gravity in the middle of the lifting points. With the hook vertically above the COG, the vertical share taken by each attachment point is in inverse proportion to its horizontal distance from the COG: the point nearer the COG carries more. That leg is also usually shorter and steeper. Its tension is its vertical share divided by the cosine of its own angle. In an offset lift the most heavily loaded leg can carry far more than half the load even when both legs look similar, so calculate it rather than estimate it.
Practical points
- Include the weight of spreader beams, shackles and slings in W.
- A choke hitch reduces capacity; apply the mode factor from the sling standard or the manufacturer's table for the sling type and choke angle.
- Sharp edges, small bending diameters (low D/d ratios) and heat reduce capacity further.
- The weakest component in the load path governs: sling, shackle, eye bolt or padeye.
The sling tension calculator works through 17 arrangements, including unequal legs, offset COG, spreader beams and tailing lifts, and selects the sling and shackle sizes.
General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.