Rigging Sling Calculator
Sling capacity from load weight and sling angle.
About this calculator
This calculator translates a load weight into the tension each sling leg must actually carry, which is the number that determines what capacity of sling you need to buy — not the load weight itself. The core physics is the sling-angle factor: as the angle from horizontal drops, each leg has to carry more of the vertical load, following 1/sin(angle), so a 30° sling angle roughly doubles leg tension compared to a 90° vertical hitch on the same load. For multi-leg configurations, the calculator conservatively caps the load-sharing legs at two, per ASME B30.9 practice — a 3- or 4-leg basket hitch is assumed to load only two legs fully, since rigging geometry rarely guarantees perfectly even distribution across more than two legs, and designing to that assumption keeps the sling from being undersized if the load shifts. Required Working Load Limit is simply the computed per-leg tension; from there, required breaking strength multiplies WLL by your chosen safety factor (5:1 is the wire-rope default).
The wire rope diameter is a rough field estimate — approximating roughly 40,000 lb of breaking strength per square inch of cross-section for 6x19 IWRC construction — good for a ballpark sizing check, not a substitute for a manufacturer's rated chart. The minimum sheave diameter enforces the standard 20:1 D/d ratio (sheave or drum diameter to rope diameter) used for choker-hitch bending to avoid excessive wire fatigue. Always cross-check final selections against the actual sling and hardware manufacturer's rated capacity tables before a lift.
Inputs
Results
Load per leg (lb)
2,887
Figures current as of 2025. Source: American Society of Mechanical Engineers, ASME B30.9-2025, Slings
How to Use This Calculator
- Enter the total load weight (lb) to be lifted.
- Select the number of sling legs (1 = vertical, 2 = bridle, 3–4 = basket configurations).
- Set the sling angle (deg) — the angle from horizontal; avoid angles below 30° which dramatically increase leg tension.
- Enter the safety factor (5:1 is standard for wire rope per ASME B30.9).
- Review Load per leg (lb) and Required WLL (lb) to select the correct sling, and use Wire rope dia (in) as a sizing reference.
How the result changes with Load weight (lb)
| Load weight (lb) | Load per leg (lb) |
|---|---|
| 2,500 | 1,443 |
| 3,750 | 2,165 |
| 7,500 | 4,330 |
| 12,500 | 7,217 |
What each input means
- Load weight (lb)
- Total weight of the load being lifted.
- Number of legs
- Number of sling legs (1=vertical, 2=bridle, etc.).
- Sling angle (deg)
- Angle of sling from horizontal (60 degree min recommended).
- Safety factor
- Design safety factor (5:1 standard for wire rope).
What each result means
- Load per leg (lb)
- Actual tension in each sling leg.
- Required WLL (lb)
- Working Load Limit needed per sling leg.
- Breaking strength (lb)
- Required minimum breaking strength.
- Wire rope dia (in)
- Approximate wire rope diameter needed.
- Angle factor
- Load increase factor from sling angle.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersLoad weight (lb) = 5000, Number of legs = 2, Sling angle (deg) = 60, Safety factor = 5 = 4 input(s) provided
- Calculate Load per legLoad per leg = (loadWeightLb / effectiveLegs) * angleFactor2887 = 2887
- Calculate Required WLLRequired WLL = round(loadPerLeg)2887 = 2887
- Calculate Breaking strengthBreaking strength = round(requiredWLL * safetyFactor)14435 = 14435
Figures and sources
- Two-leg load-sharing convention for multi-leg sling baskets, and the 5:1 wire-rope sling design factor (2025) — American Society of Mechanical Engineers, ASME B30.9-2025, Slings
Engine last updated . Checked against 3 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does entering 4 legs give the same load-per-leg result as entering 2 legs?
The engine caps effectiveLegs at Math.min(numLegs, 2) before dividing the load, so any input of 2, 3, or 4 legs produces an identical per-leg tension — only 1 leg is treated differently. This reflects ASME B30.9 practice of not counting on more than two legs sharing a basket-hitch load evenly, since rigging geometry can't guarantee all four legs go taut at exactly the same tension.
Why does dropping the sling angle from 60° to 30° increase the required breaking strength so much?
The angle factor is 1/sin(angle), and sin(30°) is 0.5 versus sin(60°) at about 0.866, so lowering the angle to 30° roughly doubles the angle factor compared to 60° — and that factor multiplies straight through to load per leg, required WLL, and required breaking strength. This is why rigging guidance generally discourages sling angles below 30°: leg tension rises sharply as the sling flattens out, even though the load itself hasn't changed.
How is the recommended wire rope diameter actually calculated?
The calculator assumes roughly 40,000 lb of breaking strength per square inch of wire rope cross-section for 6x19 IWRC construction, so it solves diameter as the square root of (required breaking strength divided by 40,000), rounded to two decimal places. That's a field approximation for a quick sizing check, not a substitute for the manufacturer's actual rated-capacity table for the specific rope construction you're buying.
What does the minimum sheave diameter output actually protect against?
It's the wire rope diameter multiplied by 20, enforcing the standard 20:1 D/d ratio between sheave or drum diameter and rope diameter used for choker-hitch bending. Bending a wire rope around too small a radius accelerates wire fatigue and can shorten sling life dramatically, so this figure sets a floor for any sheave or bend the rope will pass around during the lift.
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