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Crane Rigging Calculator - Sling Tension, Angle Factors & Working Load Limit for Lifts

Calculate sling loads, D/d ratios, and rigging hardware capacity for safe crane and hoist lifts

Calculate rigging sling tensions based on load weight, number of sling legs, sling angles, and hitch type. Enter the load weight and rigging configuration to see individual sling tensions, required sling capacity, and minimum hardware ratings. Supports single-leg, two-leg, three-leg, and four-leg bridle slings in vertical, choker, and basket hitches. Includes angle factor tables, D/d ratio derating for slings over pins and hooks, and wire rope, chain, synthetic web, and synthetic roundsling capacity charts per ASME B30.9.

Pro Tip: Never assume a four-leg bridle distributes load equally to all four legs. Unless the load is perfectly rigid and the sling attachment points are precisely symmetrical, only two or three legs carry the load at any given time. ASME B30.9 recommends designing four-leg bridles based on three legs carrying the full load (use a 3-leg calculation). The fourth leg provides stability, not additional capacity. This single misconception has caused more rigging failures than any other.

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Crane & Rigging Calculator
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How It Works

  1. Enter Load Weight

    Input the total weight of the load being lifted in pounds or kilograms. Include the weight of all rigging hardware (slings, shackles, spreader beams) below the crane hook. If the exact weight is unknown, use certified weighing equipment or calculate from material weights and dimensions.

  2. Select Sling Configuration

    Choose the number of sling legs (1, 2, 3, or 4) and the hitch type (vertical, choker, or basket). Each configuration has different capacity factors. A single vertical hitch uses 100% of sling capacity, a choker hitch uses 75%, and a basket hitch uses 200% (two legs sharing load).

  3. Enter Sling Angle

    Input the sling angle measured from horizontal (or vertical, as specified). Sling tension increases dramatically as the angle decreases. At 60 degrees from horizontal the tension is 1.15× the vertical load, at 45 degrees it is 1.41×, and at 30 degrees it is 2.0×. Never rig below 30 degrees from horizontal.

  4. Review Sling Tensions and Capacity

    See the calculated tension in each sling leg, the required minimum working load limit (WLL), and the design factor (typically 5:1 for wire rope slings). The calculator flags any configuration where sling tension exceeds the rated WLL for the selected sling size and type.

  5. Select Rigging Hardware

    Choose shackles, hooks, links, and other hardware from the capacity tables. All hardware in the rigging assembly must meet or exceed the maximum sling tension. The calculator checks D/d ratios for slings bent over hardware to apply derating factors per ASME B30.9.

Built For

  • Riggers calculating sling tensions and selecting wire rope slings for equipment lifts
  • Crane operators verifying load weight and rigging capacity before making critical picks
  • Lift planners preparing engineered lift plans for heavy or complex lifts requiring engineering review
  • Safety managers reviewing rigging configurations during pre-lift job safety analyses
  • Millwrights planning rigging for motor, pump, and compressor installations and removals
  • Construction superintendents evaluating sling angles for structural steel erection
  • Training coordinators teaching sling angle factors and capacity calculations to apprentice riggers

Features & Capabilities

Multi-Leg Sling Tension Calculator

Calculates individual sling leg tensions for 1, 2, 3, and 4-leg bridle configurations at any sling angle. Uses industry-standard angle factors and automatically flags configurations below the 30-degree minimum angle.

Sling Capacity Charts

Built-in capacity tables for wire rope slings (6x19 and 6x37 construction), alloy chain slings (Grade 80 and Grade 100), synthetic web slings (nylon and polyester), and synthetic roundslings per ASME B30.9 rated capacities.

D/d Ratio Derating

Calculates the sling efficiency reduction when bent around pins, hooks, or other hardware. A wire rope sling bent over a pin with a D/d ratio of 1 retains only 50% of its straight-pull capacity. The calculator applies the correct derating factor for the selected hardware geometry.

Design Factor Verification

Ensures the rigging configuration maintains the required design factor: 5:1 for wire rope slings, 4:1 for alloy chain, 5:1 for synthetic web and roundslings, and 6:1 for shackles and hardware per OSHA and ASME B30.9 requirements.

Hardware Capacity Check

Cross-references shackle, hook, link, and eyebolt working load limits against calculated sling tensions. All hardware in the load path must meet or exceed the maximum sling leg tension to maintain the safety factor chain.

Comparison

Sling Angle (from horizontal) Angle Factor Tension per Leg (2-leg, 10,000 lb load) Horizontal Force Recommendation
90° (vertical) 1.000 5,000 lbs 0 lbs Ideal - full capacity
60° 1.155 5,775 lbs 2,890 lbs Good - standard rigging
45° 1.414 7,070 lbs 5,000 lbs Acceptable - monitor angle
30° 2.000 10,000 lbs 8,660 lbs Minimum allowed angle
Below 30° > 2.000 > 10,000 lbs Excessive PROHIBITED - use spreader bar

Frequently Asked Questions

The sling angle factor is the multiplier applied to each sling leg's share of the load to account for the geometric effect of non-vertical sling angles. As sling angle decreases from vertical (90 degrees from horizontal), each sling must carry more tension to support the same vertical load. The angle factor equals 1 divided by the sine of the sling angle from horizontal. At 60 degrees the factor is 1.155, at 45 degrees it is 1.414, and at 30 degrees it is 2.0, meaning each sling carries double the load compared to a vertical hitch.
ASME B30.9 and OSHA standards effectively prohibit sling angles below 30 degrees from horizontal because the sling tension increases exponentially at shallow angles. At 30 degrees, each sling carries twice the vertical load share. At 20 degrees, the factor jumps to 2.92 times. Below 30 degrees, the horizontal component of force can pull attachment points together or slide the sling off the load. If geometric constraints prevent achieving 30 degrees, use a spreader beam or lifting beam to convert the sling geometry to a steeper angle.
Calculate load weight from dimensions and material density. Common weights: steel at 490 lbs per cubic foot, concrete at 150 lbs per cubic foot, water at 62.4 lbs per cubic foot, and aluminum at 169 lbs per cubic foot. For equipment, check manufacturer data plates, shipping documents, equipment drawings, or purchase specifications. When the weight is uncertain, ASME P30.1 recommends adding a minimum 10% contingency. For critical lifts, use certified load cells or dynamometers to verify weight before the lift.
Working Load Limit (WLL) is the maximum load that should be applied to rigging hardware under normal conditions, and is the current ASME B30.9 term. Safe Working Load (SWL) is an older, deprecated term that meant the same thing. Breaking strength (or minimum breaking force) is the load at which the sling or hardware will fail. The design factor is the ratio of breaking strength to WLL - typically 5:1 for wire rope slings and 4:1 for alloy chain. Never exceed the WLL of any component in the rigging assembly.
For a four-point lift to distribute load equally, all four attachment points must be in exactly the same plane, at precisely equal distances from the center of gravity, and the load must be perfectly rigid. In practice, manufacturing tolerances, rigging slack, and load flexibility mean that at any given instant, only two or three legs are taut and carrying load. ASME B30.9 recommends calculating four-leg bridle slings as if only three legs carry the total load. The fourth leg provides stability against tipping, not additional lifting capacity.
The D/d ratio compares the diameter of the pin, hook, or object the sling wraps around (D) to the diameter of the sling body (d). When a wire rope sling bends around a small radius, the outer wires are stressed more than the inner wires, reducing the sling's effective strength. At a D/d ratio of 1 (sling bent around a pin the same diameter as itself), capacity drops to about 50% of straight-pull rating. At D/d of 2, capacity is about 65%. At D/d of 5 or greater, the sling retains approximately 90% of rated capacity. Always use appropriately sized shackles and pins.
A choker hitch wraps the sling around the load and feeds the end back through itself, creating a self-tightening loop. It is rated at 75% of the sling's vertical hitch WLL because the sharp bend at the choke point reduces the sling's effective strength. Use choker hitches when the load could slide out of a basket hitch, when you need to grip irregularly shaped objects, or when attachment points are not available. Never use a choker hitch at an angle less than 120 degrees (measured at the choke point) as capacity drops further. Choker hitches should not be used with rigid slings.
ASME B30.9 requires visual inspection before each use. Remove a sling from service if you find: 10 randomly distributed broken wires in one rope lay, 5 broken wires in one strand, kinking, bird-caging, core protrusion, corrosion, heat damage (discoloration), end fitting damage, or any condition causing doubt about continued safe use. Additionally, monthly documented inspections should check for diameter reduction (more than 5% indicates internal wear or core degradation), abrasion, and fitting condition. Maintain inspection records as required by OSHA 1926.251.
Disclaimer: This calculator provides rigging load estimates for planning purposes. All critical lifts must be engineered by a qualified lift planner and reviewed by competent rigging personnel. Rigging errors can result in dropped loads, equipment damage, serious injury, or death. Always follow ASME B30.9, OSHA regulations, and your company's lift planning procedures. ToolGrit is not responsible for rigging design, lift planning, or safety outcomes.

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