Sling tension is the single most important calculation in rigging. Every time a load is lifted, the tension in each sling must remain below the sling's working load limit (WLL) or the lift is unsafe. The complicating factor is that sling tension increases dramatically as the sling angle decreases from vertical. A pair of slings at 30 degrees from horizontal carries twice the load per sling compared to vertical. Riggers who do not understand this relationship overload slings on every angled lift.
This guide covers the sling tension formula, how sling angle affects loading, ASME B30.9 working load limits for different sling types and hitch configurations, inspection criteria that determine when a sling must be removed from service, and the basics of a rigging plan. The sling tension calculator handles the trigonometry, but every rigger needs to understand why a shallow sling angle is dangerous and how to recognize an overloaded sling before it fails.
Sling Tension Formula and Angle Effects
The basic sling tension formula for a symmetrically loaded two-sling lift is: T = W / (n × cos(θ)), where T is the tension in each sling, W is the total load weight, n is the number of slings sharing the load, and θ is the angle measured from horizontal to the sling (the sling angle).
At a 90° sling angle (vertical), cos(90°) = 0. But wait, that is the angle from horizontal, so cos(90°) is actually 0; we measure from horizontal, so a vertical sling has θ = 90° and T = W/n. More practically: at 60° from horizontal, T = W / (n × 0.866) = 1.15 × W/n. At 45°, T = W / (n × 0.707) = 1.41 × W/n. At 30°, T = W / (n × 0.5) = 2.0 × W/n.
The critical lesson: at 30 degrees from horizontal, each sling carries twice the load it would carry vertically. At 15 degrees, each sling carries nearly 4 times the vertical load. This is why OSHA and ASME B30.9 strongly discourage sling angles below 30 degrees from horizontal, because the tension multiplier makes it nearly impossible to stay within the sling's WLL for heavy loads.
For asymmetric loads where the center of gravity is not centered between the sling attachment points, the slings carry unequal portions of the load. The sling closer to the center of gravity carries more than its proportional share. Unequal sling lengths compound this problem by creating different sling angles on each side.
T = W / (n × cos(θ))
Where: T = tension per sling, W = load weight, n = number of slings, θ = angle from horizontal
Tension multipliers by angle:
90° (vertical): 1.00 × W/n
60°: 1.15 × W/n
45°: 1.41 × W/n
30°: 2.00 × W/n ← minimum recommended angle
Multi-Leg Sling Tension Calculator
Calculate sling leg tension for multi-leg rigging configurations with ASME B30.9 WLL reductions.
Sling Types and Working Load Limits
Wire rope slings are the workhorse of heavy rigging. They handle high temperatures (up to 400°F for standard wire rope), resist abrasion, and are available in capacities from a few hundred pounds to over 100 tons. Wire rope WLL depends on the rope diameter, construction (6x19, 6x37, etc.), and the core type (fiber or independent wire rope core). IWRC ropes have approximately 7.5% higher capacity than fiber core.
Chain slings (alloy steel) are the most durable option, resistant to cutting, abrasion, and high temperatures (up to 600°F for Grade 80 and 100 alloy chain). They are heavier than wire rope for equivalent capacity but easier to shorten with grab hooks. Chain slings must be Grade 80 or 100 alloy steel for overhead lifting. Proof coil (Grade 30) and high-test (Grade 43) chain are NOT rated for overhead lifting.
Synthetic slings (nylon and polyester web slings, roundslings) are lightweight, non-marring, and conform to load shapes. They are ideal for finished surfaces and applications where wire rope or chain would damage the load. However, synthetic slings are vulnerable to cuts, abrasion, UV degradation, and chemical exposure. Temperature limits are approximately 180°F for nylon and 194°F for polyester.
The hitch type significantly affects the sling's rated capacity. A vertical hitch (straight pull) uses the full WLL. A choker hitch reduces capacity to approximately 75% of the vertical WLL. A basket hitch (both eyes on the hook) provides approximately 200% of the vertical WLL when the sling angle is 90 degrees, but decreases with the sling angle.
• Vertical hitch: 100% of rated WLL
• Choker hitch: ~75% of rated WLL (varies with D:d ratio)
• Basket hitch: up to 200% of rated WLL (at 90° angle)
Never use for overhead lifting:
• Grade 30 (proof coil) chain
• Grade 43 (high-test) chain
• Log chain or binding chain
Only Grade 80 and Grade 100 alloy chain are rated for overhead lifting.
Multi-Leg Sling Tension Calculator
Calculate sling leg tension for multi-leg rigging configurations with ASME B30.9 WLL reductions.
Sling Inspection and Removal Criteria
ASME B30.9 and OSHA require sling inspection before each use (by the user) and periodic thorough inspections (by a designated competent person). Slings that fail any removal criterion must be taken out of service immediately. They cannot be downrated or restricted to lighter loads.
Wire rope sling removal criteria: 10 or more randomly distributed broken wires in one rope lay (strand pitch length), or 5 or more broken wires in one strand in one lay. Severe corrosion, kinking, crushing, birdcaging, or core protrusion. Any visible evidence of heat damage (discoloration, loss of lubricant). End fitting damage including cracked or deformed fittings, loose splices, or damaged swage sleeves.
Chain sling removal criteria: Stretched (elongated) links exceeding the manufacturer's allowance (typically 5% stretch = removal). Bent, twisted, or deformed links. Nicks, gouges, or corrosion beyond allowable limits. Cracks in any link or fitting. Missing or illegible identification tags.
Synthetic sling removal criteria: Acid or caustic burns, melting or charring, snags, punctures, or cuts. Broken or damaged stitching. Crushed or distorted fittings. Knots in the sling body (knots reduce capacity by up to 50%). Red core yarn visible on roundslings (indicates the load-bearing core is exposed and damaged).
• Wire rope: ≥10 broken wires in one lay, kinks, core protrusion, heat damage
• Chain: ≥5% stretch, bent/cracked links, corrosion, missing tags
• Synthetic: cuts, burns, UV degradation, exposed core yarn, knots
Damaged slings cannot be downrated. They must be destroyed to prevent reuse.
Multi-Leg Sling Tension Calculator
Calculate sling leg tension for multi-leg rigging configurations with ASME B30.9 WLL reductions.