Why the angle matters
When a load hangs from a single vertical sling, the sling carries exactly the weight of the load. As soon as two or more legs spread out to reach attachment points, each leg also pulls sideways. The flatter the angle, the larger the tension, until a modest load can overload a sling that looked more than strong enough.
The formula
Tension per leg = load ÷ (number of legs × sin angle)
The angle is measured between the sling leg and the horizontal. The load angle factor is 1 ÷ sin angle:
| Angle from horizontal | Load angle factor |
|---|---|
| 90° (vertical) | 1.000 |
| 60° | 1.155 |
| 45° | 1.414 |
| 30° | 2.000 |
Example
A 2,000 lb load on a two-leg bridle at 60°: 2,000 ÷ (2 × 0.866) ≈ 1,155 lb in each leg. At 45° each leg sees 1,414 lb, and at 30° each leg carries the full 2,000 lb, as much as the whole load.
Three and four legs
With three legs evenly spaced, the load is shared by all three. With four legs, the rigging is almost never perfectly balanced, so standards such as ASME B30.9 treat a four-leg bridle as if only three legs carry the load. This calculator does the same. When the center of gravity is off-center, the legs closer to it carry more, sometimes much more.
Horizontal force on the load
Each leg also pulls the attachment points inward with a force of tension × cos angle. On a long, slender load, this compression can bend the part or the lifting lugs; a spreader bar removes it.
Sling length and headroom
For a given horizontal distance from the hook to each attachment point, the leg length is distance ÷ cos angle and the hook rises distance × tan angle above the attachments. Steeper angles need longer slings and more headroom, which is often the real constraint on site.
Good practice
- Aim for 60° or steeper; avoid anything below 30°.
- Compare the leg tension with the working load limit (WLL) printed on each sling’s tag, for the hitch you use (choker hitches reduce capacity).
- Protect slings from sharp edges with softeners.
- Include the weight of the rigging (shackles, spreader bars) in the load.
This calculator is for planning. Lifts must be planned and supervised by qualified people following the sling manufacturer’s data and local regulations.