Sling angle calculator
Enter the load, the number of legs and the angle from horizontal. This returns the tension in each leg and the minimum working load limit the sling needs.
The angle is measured from the horizontal, not from the vertical. A steep, near-vertical sling is a large angle and a low factor; a wide, flat sling is a small angle and a high factor.
- Tension in each leg
- 578 kg
Load angle factor 1.155 at 60°. Each leg needs a working load limit of at least this figure, in the hitch you are using.
Assumes the load shares evenly between legs, which only holds on a symmetrical lift. With three or four legs on a rigid load, plan on two legs taking the weight. Never rig below 30 degrees.
Sling angle factors
The load angle factor is 1 divided by the sine of the angle from horizontal. Multiply each leg share by it to get real tension.
The jump between 45 and 30 degrees is what catches people out. Dropping 15 degrees adds 41 percent to the tension in every leg, and it also adds a large horizontal crushing force on the load itself.
| Angle from horizontal | Load angle factor | Tension on a 1,000 kg two-leg lift |
|---|---|---|
| 90° | 1.000 | 500 kg per leg |
| 75° | 1.035 | 518 kg per leg |
| 60° | 1.155 | 578 kg per leg |
| 45° | 1.414 | 707 kg per leg |
| 40° | 1.556 | 778 kg per leg |
| 35° | 1.743 | 872 kg per leg |
| 30° | 2.000 | 1,000 kg per leg |
| Below 30° | Not permitted | Do not rig |
Hitch capacity
A sling is rated for a vertical hitch. Change the hitch and the capacity changes with it.
A choker hitch grips the load by pulling back on itself, and that bend costs capacity: ASME B30.9 rates a choker at 75 percent of vertical for choke angles of 120 degrees or more, and less below that.
A basket hitch doubles capacity when both legs are vertical, but only when the D over d ratio at the bend is adequate. Once the legs angle in, apply the load angle factor to the doubled figure.
| Hitch | Capacity | Condition |
|---|---|---|
| Vertical | 100% | Straight pull, single leg |
| Choker | 75% | Choke angle 120° or greater |
| Choker | Less than 75% | Choke angle under 120° |
| Basket, legs vertical | 200% | Adequate D/d ratio at the bend |
| Basket, legs angled | 200% ÷ angle factor | Apply the load angle factor |
Working load limit versus breaking strength
Breaking strength is the load at which a new component fails in a test. Working load limit is the maximum you are permitted to apply in service, and it is the breaking strength divided by a design factor.
The design factor is not a safety margin you can spend. It covers shock loading, wear, corrosion, manufacturing variation and the fact that the sling in your hand is not new. Synthetic web slings and chain slings normally use a factor of 5 to 1, wire rope slings 5 to 1, and rigging hardware such as shackles 5 or 6 to 1.
Rate every lift against working load limit. If a tag shows only a breaking strength, the item is not rated for lifting.
