RigMaster 4-leg sling load calculation, and why a rigid load may hang on only 2 legs
For a rigid load, size each leg of a 3- or 4-leg bridle as if only two legs carry the load: tension = load ÷ 2 × 1/sin(angle). 4,000 lb at 60° gives 2,309 lb per leg, not 1,155 lb. RigMaster Pro handles 3 and 4 legs with a Load Type switch between Rigid and Flexible.
Why extra legs do not always share the load
A stiff load like a steel beam, a skid or a machine base can't bend to take up small differences in sling length. We've watched a four-leg pick where two of the legs went slack the moment the beam left the ground — the other two diagonal legs were doing all the work. That's why common rigging practice sizes a multi-leg bridle on a rigid load as if only two legs carry it.
A flexible load, such as a bundle that can sag, can settle until every leg is tight. Only then does the load spread over three or four legs.
The 4 leg sling formula
- Rigid: tension per leg = load ÷ 2 × 1/sin(θ).
- Flexible: tension per leg = load ÷ number of legs × 1/sin(θ).
- θ is the horizontal angle, the same for every leg.
Worked example: 4,000 lb at 60°
| Legs | Load Type | Legs Assumed Carrying | Tension per Leg |
|---|---|---|---|
| 4 | Rigid | 2 | 2,309 lb |
| 4 | Flexible | 4 | 1,155 lb |
| 3 | Rigid | 2 | 2,309 lb |
| 3 | Flexible | 3 | 1,540 lb |
In metric, 2,000 kg on 4 legs at 60° gives 1,155 kg per leg as Rigid and 577 kg as Flexible. The rigid figure is double, and that difference is exactly what catches people who assume four legs means a quarter each.
Steps in RigMaster
- Open Sling Tension and type the Total Load Weight.
- Under Sling Legs, pick 3 or 4. In the free version this opens the upgrade screen and the calculator stays on 2 legs.
- Choose the Load Type: Rigid (the default) or Flexible.
- Set the Horizontal Angle, or use Length & Height if you can measure a leg.
- Read Tension per Leg and check Legs Assumed Carrying: 2 for Rigid, 3 or 4 for Flexible.
- Tap Save, or Lift Plan to make a one-page PDF.
4 leg bridle sling capacity: checking the result
Compare the tension per leg with each leg's vertical rating. Multi-leg chain and wire rope assemblies usually carry a tag with the rating for the whole assembly at certain angles; that tag and the manufacturer's data govern, and the app does not read or replace them.
Limits of this calculation
- All legs use one angle, so the calculation assumes a symmetric bridle with the hook centred over the load.
- There is no per-leg analysis for unequal leg lengths or an off-center center of gravity on 3 or 4 legs. For a two-point off-center pick, use Unequal Legs.
- Choose Flexible only when you are sure the load can deflect enough to keep every leg loaded. When in doubt, keep Rigid.
Questions and answers
How do you calculate the load on a 4-leg sling?
When can I divide by 4?
Is the 4-leg sling tension calculator free?
Can I set a different angle for each leg?
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- How to determine the weight of a load before lifting it
- Rigging lift plan PDF: put the rigging numbers on one page for your supervisor
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