Ultimate Guide to Rigging & Sling Tension
Rigging a load safely is a discipline of small numbers that compound. The load weighs what it weighs, but the tension in each sling leg depends on how the load is shared and on the angle at which the slings pull. Get the angle wrong and a leg that was comfortably within its rated capacity is suddenly overloaded — the same load, the same slings, a dangerously different result. This guide covers the trigonometry behind sling tension and the capacity math that keeps rigging within safe limits.
The starting point is load sharing. In a symmetric two-leg bridle, each leg carries half the load in the vertical direction; a three-leg bridle splits it three ways. Four legs, however, are never credited with four equal shares. Because real slings stretch differently and real attachment points are never perfectly level, one leg will carry more than a quarter of the load, and the accepted rigging rule treats a four-leg bridle as a three-leg share. This conservative assumption is baked into the solver: select four legs and it computes with three effective legs, which is the honest number for selecting sling capacity.
The angle is where the tension multiplies. A sling leg carrying a vertical share of W/n pounds pulls along its own axis, which is inclined from horizontal. Resolving the force triangle, the tension in that leg is T = (W/n) / sin(θ), where θ is the angle between the sling and horizontal. At 90° (vertical) the tension equals the share. At 60° the tension is 1.15 times the share; at 45°, 1.41 times; at 30°, 2.0 times; and at 15°, nearly 4 times. The shallow angles are the dangerous ones: a small mistake in sling angle produces a large change in tension.
The same angle that multiplies tension also derates the sling's capacity. A sling rated with a working load limit (WLL) for vertical lifting can carry only WLL × sin(θ) at a given angle from horizontal. This is the standard rigging angle-factor table: 100 percent at 90°, 86.6 percent at 60°, 70.7 percent at 45°, 50 percent at 30°, and so on. Riggers never compare the sling's vertical rating to the actual tension — they compare the tension to the derated capacity at the actual angle. The two effects move together, which is why angle discipline is so important.
There is also a horizontal force to contend with. Each sling leg pushes outward on its anchor point with H = (W/n) / tan(θ). At steep angles this horizontal force is modest, but at shallow angles it grows rapidly — at 30° the horizontal force per leg is already 1.73 times the load share, and at 15° it is 3.7 times. These outward forces load the crane hook, the shackles, and the anchor beams, and they are frequently the overlooked failure point in a rigging plan. The solver reports the horizontal force per anchor so the whole assembly is checked, not just the slings.
Capacity is not a mystery number — it is a documented working load limit. The WLL on a sling tag already includes the design safety factor for the sling type: wire rope slings are typically rated at a 5:1 minimum design factor, synthetic web slings around 5:1 to 7:1, and alloy chain slings at 4:1. The rigging engineer must never add load beyond the tag rating, and must apply the angle derating on top of it. Comparing the computed tension against the derated WLL is the check that converts the math into a go/no-go decision.
Recommended practice keeps the sling angle at 60° or steeper whenever possible. At 60°, tension is only 15 percent above the per-leg share and the horizontal force is manageable. Below 30°, the tension and horizontal forces explode, and most codes and best practice treat 30° as the absolute minimum for any rigged lift. The solver flags any configuration below 30° with an explicit warning, and the angle-factor table in the results makes the consequence of every angle visible at a glance.
The model assumes a symmetric bridle with equal-length legs and evenly distributed attachment points. Real lifts are rarely perfect: an off-center load, an uneven pad eye, or a shock load from a snagged lift can concentrate tension in one leg. The equal-share calculation is therefore a planning floor, not a guarantee. Riggers compensate by using the rated capacities conservatively, verifying even loading before the lift leaves the ground, and applying the standard 4-leg to 3-leg rule even on apparently balanced rigs.
Use the Rigging & Bridle Sling Tension Solver as the planning layer of every lift: enter the load, choose the legs, set the angle, and compare the per-leg tension and horizontal force against the derated capacity. The tool produces a complete, copyable lift plan, and the safety check turns the trigonometry into a clear pass or fail. In rigging, the calculation is never the substitute for a qualified rigger's judgment — but it is the discipline that keeps the judgment honest.