Sling D/d Ratio Efficiency Calculator
Every time a wire-rope or synthetic sling wraps around a pin, shackle or sharp load edge it loses strength — the tighter the bend, the more it loses. This calculator turns the D/d ratio (bend diameter over sling diameter) into a termination efficiency and, if you know the sling's rated straight WLL, into the effective working load at that bend.
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Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the bend diameter D in millimetres — the diameter of the pin, shackle bow or radius the sling actually bends around (for a hard edge, twice the edge radius).
- Enter the sling body diameter d in millimetres.
- Optionally enter the sling's rated straight WLL in kilograms to get the derated effective WLL at that bend.
How it works
The D/d ratio is simply D divided by d. Strength efficiency is read from the standard wire-rope termination table (D/d 1 → 50%, 2 → 65%, 3 → 70%, 4 → 75%, 6 → 83%, 8 → 87%, 10 → 90%, 15 → 92%, 20 → 95%, 25 → 96%, 40+ → 100%), interpolating linearly between listed points and holding the end values beyond the table. The effective WLL is the rated straight WLL multiplied by that efficiency fraction.
Worked example
Worked example. A 25 mm wire-rope sling bent around a 100 mm shackle pin has a D/d of 4, giving 75% efficiency. A sling rated 2000 kg straight is therefore worth about 1500 kg over that pin.
Common mistakes
- Using the shackle's pin diameter for a synthetic sling that actually bears on the narrow bolt threads or a sharp bracket edge — measure the real bearing radius.
- Ignoring the derating entirely and loading a sharply bent sling to its full straight-line WLL.
- Forgetting that the choke or basket hitch, edge cutting and shock loading impose further reductions on top of the D/d loss.
Frequently asked questions
What is a safe minimum D/d ratio?
As a rule of thumb keep D/d at 4 or more for wire rope where practical, and higher for round-slings and grommets; below that the strength loss becomes severe. Always follow the sling manufacturer's own D/d chart, which governs.
Does this apply to chain slings?
No — this efficiency table is for wire-rope (and broadly synthetic) slings bent around a diameter. Grade 80/100 chain has its own fitting and edge rules; use the manufacturer's chain data and never load over an unprotected edge.
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