The most common question we get about slings is "how much can this lift?". The answer depends on three things: the rope's breaking strength, the safety factor, and how the sling is rigged. Here is how to work it out.
SWL, WLL and MBL
- MBL (minimum breaking load): the load at which the rope is guaranteed not to break, from the rope tables or test certificate.
- SWL or WLL (safe working load / working load limit): the maximum load the sling may lift in normal service.
- Safety factor: the ratio between them. Wire rope slings are normally rated at 5:1.
The wire rope sling capacity formula
SWL = MBL ÷ safety factor
For example, if a rope has an MBL of 100 kN and the safety factor is 5, the SWL is 20 kN, which is about 2,040 kg (1 kN ≈ 102 kg).
The sling's rating also depends on the eye termination. A ferrule-secured or spliced eye does not develop the full strength of the rope, which is why published sling charts are lower than "rope MBL ÷ 5".
A quick rule of thumb
For single-leg mechanical slings in a straight vertical lift, our chart works out close to SWL (kg) ≈ 8 × d², where d is the rope diameter in mm. A 10 mm sling gives 8 × 100 = 800 kg, which matches the chart. Use this only for a quick estimate; always check the sling's tag and certificate before lifting.
Wire rope sling capacity chart
Maximum safe working load in kg for single-part slings, safety factor 5:1, from the R.B. Traders catalogue:
| Rope Dia | Single Vertical SWL | Basket Hitch SWL | Choker Hitch SWL |
|---|---|---|---|
| 3 mm | 70 kg | 140 kg | 50 kg |
| 4 mm | 130 kg | 260 kg | 100 kg |
| 5 mm | 200 kg | 400 kg | 190 kg |
| 6 mm | 250 kg | 500 kg | 200 kg |
| 8 mm | 550 kg | 1,100 kg | 210 kg |
| 9 mm | 670 kg | 1,340 kg | 500 kg |
| 10 mm | 800 kg | 1,600 kg | 600 kg |
| 11 mm | 1,000 kg | 2,000 kg | 750 kg |
| 12 mm | 1,200 kg | 2,400 kg | 900 kg |
| 13 mm | 1,400 kg | 2,800 kg | 1,050 kg |
| 14 mm | 1,750 kg | 3,500 kg | 1,300 kg |
| 15 mm | 2,000 kg | 4,000 kg | 1,500 kg |
| 18 mm | 2,500 kg | 5,000 kg | 1,875 kg |
| 19 mm | 3,000 kg | 6,000 kg | 2,260 kg |
| 20 mm | 3,300 kg | 6,600 kg | 2,475 kg |
| 22 mm | 4,000 kg | 8,000 kg | 3,000 kg |
| 24 mm | 4,600 kg | 9,200 kg | 3,450 kg |
| 25 mm | 5,000 kg | 10,000 kg | 3,750 kg |
| 28 mm | 6,500 kg | 13,000 kg | 4,875 kg |
| 32 mm | 8,000 kg | 16,000 kg | 6,000 kg |
| 35 mm | 9,200 kg | 18,400 kg | 6,900 kg |
| 36 mm | 10,400 kg | 20,800 kg | 7,800 kg |
| 38 mm | 12,000 kg | 24,000 kg | 9,000 kg |
| 40 mm | 12,700 kg | 25,400 kg | 9,626 kg |
| 44 mm | 16,700 kg | 33,400 kg | 12,625 kg |
| 48 mm | 20,000 kg | 40,000 kg | 15,000 kg |
| 52 mm | 23,000 kg | 46,000 kg | 17,250 kg |
Hitch factors: vertical, choker and basket
- Vertical (straight): 100% of the rated SWL.
- Choker: the sling bends sharply around itself, so capacity drops to about 75% of vertical (compare the two columns in the chart above).
- Basket: the load is carried by two legs, so with both legs vertical the capacity is 2× the vertical SWL. As soon as the legs spread out at an angle, that drops.
How sling angle reduces capacity
When two legs (or a basket) spread apart, each leg carries more than half the load. The capacity of two legs is 2 × SWL × cos(half the included angle):
| Included angle between legs | Angle from vertical | Two-leg capacity |
|---|---|---|
| 0° | 0° | 2.00 × SWL |
| 60° | 30° | 1.73 × SWL |
| 90° | 45° | 1.41 × SWL |
| 120° | 60° | 1.00 × SWL |
Example: a 12 mm sling rated 1,200 kg vertical, used in a basket with a 90° included angle, can carry about 2 × 1,200 × 0.707 ≈ 1,700 kg, not 2,400 kg.
Never rig with legs more than 60° from vertical (120° included). European practice under EN 13414-1 simplifies this with a mode factor of 1.4 for two-leg slings up to 45° from vertical and 1.0 from 45° to 60°.
Other things that reduce capacity
- Bending the sling over a sharp edge or a small pin: use packing or a larger shackle.
- Damage: broken wires, kinks, crushing, heat or corrosion. Take a damaged sling out of service.
- Uneven loads on multi-leg slings: if the centre of gravity is off, one leg may carry most of the load.
Need a sling made to a specific length and capacity? We make wire rope slings and assemblies with test certificates.
Frequently asked questions
How do you calculate wire rope sling capacity?
Divide the minimum breaking load by the safety factor (normally 5 for wire rope slings), then apply the hitch factor and the angle factor for how the sling is rigged.
What is the SWL of a 12 mm wire rope sling?
In our catalogue chart a 12 mm single-part mechanical sling is rated 1,200 kg vertical, about 900 kg in choker and 2,400 kg in a basket with vertical legs.
Does a basket hitch double the capacity?
Only when both legs hang vertically. At a 90° included angle a basket carries about 1.41 times the vertical rating, and at 120° it is no more than a single vertical leg.