What is the D/d Ratio?

What You Should Know about the D/d Ratio


You’ve likely come across the term ‘D/d Ratio’ in previous research and real-world applications. It’s a fundamental concept in rigging, crucial for ensuring the safety and efficiency of your operations. Let’s demystify the D/d Ratio together and explore its vital role in rigging practices.


wire rope sling and sheave blocks from LGH on the jobsiteExploring the D/d Ratio Concept

Simply put, the D/d Ratio refers to the relationship between two diameters: the diameter (D) around which a sling is bent, and the sling’s own diameter (d).

Think of it as comparing the size of the load or rigging hardware to the sling’s thickness. All slings lose capacity when they are bent too much

For example, if the diameter around which a wire rope sling is bent (D) is 25″, and the wire rope sling’s diameter (d) is 1″, the D/d is 25. This gives us a 100% efficiency rating for this project.


illustrations explaining d/d ratio for wire rope and alloy chain slings


wire rope sling
A wire rope sling from LGH.

Why is the D/d Ratio So Important?

Every time a sling wraps around an object, it loses some of its capacity. The D/d Ratio helps you gauge this reduction in capacity. A lower ratio indicates a greater loss of efficiency.

A tight bend in the sling suggests a smaller-than-ideal D/d Ratio. This can cause accelerated wear, fatigue, and damage to your slings. It’s essential to assess this ratio correctly for every lift, adjusting for any capacity reductions.

Different types of slings, like wire rope and alloy chain slings, have varying strength efficiencies. The illustration above clarifies how the D/d Ratio applies to these slings.


Product picture of a poly round sling from LGH.
A poly round sling from LGH.

Further Insights for Polyester Round Slings

With polyester round slings, manufacturers typically specify minimum hardware diameters to safeguard the inner core yarns. It’s important to note that round slings can be more susceptible to damage if not handled properly, especially when used in a choker configuration. Even with the correct hardware diameter, using choker hitches reduces the sling’s capacity to 75% of its vertical rating. Overlooking this can lead to overloading and potential hazards.

Remember, all slings experience a reduction in capacity when bent excessively. For basket hitches, this capacity is based on a minimum diameter, and reductions must be made if the sling’s D/d is below this threshold.


Conclusion: The Significance of D/d Ratio in Rigging Safety

In our experience with rental equipment, we’ve seen how overlooking or miscalculating sling capacity reduction leads to damaged equipment, unsafe practices, and additional costs. Proper understanding and adherence to D/d Ratio specifications are key to safe and efficient rigging operations.

Stay informed and stay safe. Understanding the D/d Ratio is not just about compliance; it’s about ensuring every lift is executed with maximum safety and efficiency.


Looking for more info? Check out our handy rigging cards, sling cards, digital catalog, and more at rentlgh.com/resources or contact us today to learn more!



LGH is North America’s largest single organization devoted exclusively to the provision of lifting and moving equipment for rent. LGH holds the most comprehensive inventory for hoisting, rigging, jacking, pulling, material handling, and safety equipment. With over 90,000 pieces, discover your next project solution at RentLGH.com


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