Sling Angle & Leg Load Calculator
Work out the angle of the sling legs and the minimum working load limit each leg needs, for 1, 2, 3 and 4-leg slings. It uses the uniform load method that UK slings are rated and tagged by, from the LEEA Code of Practice, and was built by a CPCS A61 Appointed Person.
Sling angle calculator
Include anything hung below the sling, such as a spreader beam or shackles at the load
Number of sling legs
Bearing point of the master link to the bearing point at the load
A preliminary check for slings with equal legs and the hook over the centre of gravity. Unequal legs, an offset centre of gravity, sharp edges, the master link, shackles and the load's lifting points all need checking too. Need the rigging designed in a lift plan?
How the calculation works
The leg angle
The angle that matters is between each leg and the vertical, which is half the included angle between two opposite legs. From the leg length and the distance in plan from the hook to each lifting point:
sin (angle to vertical) = plan distance from hook to lifting point ÷ leg length
For a 2-leg sling the plan distance is half the spacing between the two lifting points. For a 4-leg sling on a rectangle it is half the diagonal.
The rating: uniform load method
| Sling | 0° to 45° to vertical | 45° to 60° to vertical |
|---|---|---|
| 1 leg | 1.0 × WLL of the leg (straight lift) | |
| 2 legs | 1.4 × WLL of one leg | 1.0 × WLL of one leg |
| 3 and 4 legs | 2.1 × WLL of one leg | 1.5 × WLL of one leg |
So the minimum WLL each leg needs is the load divided by the factor, and by a further 0.8 in a choke hitch. Above 60° to the vertical there is no rating at all.
The actual pull in each leg
Pull in each leg = load ÷ (number of legs carrying it × cos angle to vertical)
The calculator shows this for information, and for 3 and 4-leg slings also the pull if only two legs end up carrying the load, which is what happens with a rigid load or legs of slightly different lengths.
Worked example
A 3,000 kg load is lifted on a 2-leg sling with 2.5 m legs to lifting points 3.0 m apart.
- Angle to vertical: sin⁻¹ (1.50 ÷ 2.5) = 36.9°, an included angle of 73.7°
- Within 0° to 45°, so the mode factor is 1.4
- Minimum WLL of each leg: 3,000 ÷ 1.4 = 2,143 kg (2.14 t)
- Actual pull in each leg: 3,000 ÷ (2 × cos 36.9°) = 1,875 kg
Illustrative figures only. Use the weights and dimensions for your own lift.
Questions
What is the uniform load method?
It is how multipurpose slings sold in the UK and Europe are rated and tagged. Instead of working out the load at every angle, the sling gets one rating for legs between 0° and 45° to the vertical and a lower one between 45° and 60°. A 2-leg sling is rated at 1.4 times a single leg up to 45° and 1.0 times from 45° to 60°; a 3- or 4-leg sling at 2.1 and 1.5 times.
Why does the sling rating drop as the angle increases?
Each leg pulls along its own line, so the wider the legs spread, the harder each one has to pull to hold up the same weight. At 60° to the vertical each leg of a 2-leg sling carries the whole weight of the load, even though there are two of them.
Can a sling be used above 60° to the vertical?
No. Slings have no rating above 60° to the vertical (120° included angle). Use longer legs, or a spreader or lifting beam, to bring the angle down.
Why is a 4-leg sling rated the same as a 3-leg sling?
Because the four legs rarely share the load equally. Under the uniform load method a 4-leg sling is rated as if only three legs carry the load, and some standards assume only two. A rigid load, legs of slightly different lengths or an offset centre of gravity can leave it hanging on two legs.
What about a choke hitch?
Choking a sling around the load reduces what it can lift. LEEA gives a mode factor of 0.8, so the sling can lift 80% of its straight-pull rating, and less again if the choke angle is tight.
Source: LEEA, Code of Practice for the Safe Use of Lifting Equipment (COPSULE), Edition 10, section 1.A5.4 Principles for the Selection of Slings.
Want the rigging drawn for you?
Every RMT lift plan includes a rigging drawing: the master link, each leg at its real angle, the shackles and the lifting points, with each item's rating and the gross load the duty is checked against.