About Rigging Sling Load Calculator (Tension & WLL)
The rigging sling load calculator returns the tension in each leg of a lifting sling from the load weight, the number of legs, and the sling angle measured from horizontal. As the angle flattens, the tension in every leg grows by the angle factor 1/sin(theta): at 30 degrees each leg of a two-leg bridle carries the entire load weight, which is why OSHA 1926.251 and ASME B30.9 practice treat 30 degrees as the minimum permissible angle.
From the leg tension the tool derives the required vertical-rated working load limit (WLL) per leg for vertical, choker, and basket hitches, the minimum sling breaking strength for the chosen design factor, and the horizontal compression the sling legs squeeze into the load, which drives spreader-beam decisions. For three- and four-leg bridles it defaults to the conservative standard assumption that only two legs carry the load, since equal sharing requires a rigid load and perfectly matched leg lengths.
How It Works
- Choose metric (kg) or imperial (lb) units and enter the load weight, the number of sling legs (1 to 4), and the sling angle from horizontal (15 to 90 degrees).
- Pick the load-sharing assumption for 3-4 leg bridles: conservative (only two legs carry, per standard rigging practice) or ideal equal sharing, and the hitch type (vertical, choker, or basket).
- The calculator evaluates the tension per leg T = W / (n_eff * sin(theta)) in kN, kgf, and lbf, the angle factor, the horizontal compression per leg H = T cos(theta), and the required WLL per leg = T / hitch factor for each hitch.
- Sling angles below 30 degrees are flagged as not permitted per OSHA / ASME B30.9, angles below 45 degrees get a caution, and the vertical-equilibrium identity n_eff * T * sin(theta) = W is reported as a check.
Worked Example
A 1000 kg load hangs from a two-leg bridle with the legs at 60 degrees to horizontal. Each leg carries T = 1000 / (2 x sin 60) = 577 kg (5.66 kN), an angle factor of 1.15. If the same lift is re-rigged with shorter slings at 30 degrees, each leg carries T = 1000 / (2 x sin 30) = 1000 kg, the full load weight in every leg, and the horizontal compression rises to 866 kg per leg. With a choker hitch at 60 degrees the required vertical-rated WLL per sling is 577 / 0.75 = 770 kg, and with a 5:1 design factor the sling needs a breaking strength of at least 28.3 kN.
Formulas
- Tension per sling leg
T = W / (n_eff * sin(theta))- Angle factor (tension multiplier)
AF = 1 / sin(theta)- Required WLL per leg
WLL_req = T / hitch factor- Horizontal compression on the load
H = T * cos(theta)- Vertical equilibrium check and breaking strength
n_eff * T * sin(theta) = W; MBS >= WLL_req * DF
Standards & References
- ASME B30.9 Slings
- OSHA 29 CFR 1926.251 rigging equipment for material handling
- EN 13414-1 wire rope sling safety
Frequently Asked Questions
Why does sling tension increase as the sling angle decreases?
Only the vertical component of the leg tension supports the load, so at an angle theta from horizontal each leg must carry 1/sin(theta) times its vertical share. At 60 degrees the multiplier is a modest 1.15, at 45 degrees it is 1.41, and at 30 degrees it reaches 2.0, meaning each leg of a two-leg bridle carries the entire load weight. Below 30 degrees the tension grows so quickly that the angle is not permitted in normal rigging practice.
Is the sling angle measured from horizontal or from vertical?
This calculator uses the horizontal sling angle, the convention in ASME B30.9 and on US sling capacity charts: 90 degrees means a vertical leg and smaller angles mean flatter slings. Some European charts use the included angle from vertical instead, so a "60 degree" sling in one convention is a "30 degree" sling in the other. Always confirm which convention a capacity chart uses before applying it.
Why assume only two legs carry the load in a 3- or 4-leg bridle?
Unless the load is rigid and the legs are perfectly matched in length, small differences in geometry let one pair of diagonal legs take almost all of the load while the others merely balance it. Standard rigging practice per ASME B30.9 commentary therefore rates 3- and 4-leg bridles as if only two legs carry the full load. The calculator defaults to this conservative assumption and offers ideal equal sharing for comparison.
What do the hitch factors for choker and basket hitches mean?
A sling rated at some vertical WLL has different capacities depending on how it is hitched. A choker hitch bends the sling around itself and reduces capacity to about 75% of vertical, while a true basket hitch supports the load on two parts of the sling and doubles capacity, provided both legs are vertical. The calculator divides the computed leg tension by the hitch factor to give the vertical-rated WLL you must look for on the sling tag.
What is the horizontal force output used for?
The inward component T cos(theta) of each inclined leg squeezes the load (or the lifting beam) horizontally. At 30 degrees this compression is 1.73 times the vertical share of each leg and can buckle slender loads, crush edges, or overload lifting lugs designed for vertical pull. When the horizontal force is significant, riggers use a spreader beam so the slings to the load hang vertically.
What design factor do slings require?
ASME B30.9 requires a design factor of 5:1 for most synthetic and chain sling types, and wire rope slings are also commonly rated at 5:1. The working load limit on the sling tag already includes this factor, so you select slings by comparing the calculated required WLL to the tag rating. The calculator additionally reports the implied minimum breaking strength (required WLL times the design factor) for specification checks.