Sling D/d Ratio Calculator
Enter the diameter of the load, pin or hook the sling bends around and the diameter of the sling itself. The calculator returns the D/d ratio, checks it against the minimum diameter of curvature required by 29 CFR 1910.184 Fig. N-184-4, and tells you whether you are still at the ratio your sling's rated-load table assumes.
Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.
What D and d actually are
D is the diameter of curvature the sling has to follow: the round load in a basket hitch, the pin it is seated on, the bail of the hook, the bolt of a shackle. d is the diameter of the sling — the nominal rope size printed on the tag, and on a cable-laid or braided sling the diameter of one component rope, not of the finished body.
Both are in the same unit, so the ratio has none. OSHA states it in exactly this form in Fig. 4 of its sling guidance: when D is 25 times the component rope diameter, the D/d ratio is expressed as 25/1. The tighter the bend, the more the outer wires of the rope are stretched while the inner ones are compressed, and the less of the rope's strength reaches the load.
The minimums that are in the regulation
Figure N-184-4 of 29 CFR 1910.184 carries a legend headed Explanation of symbols: minimum diameter of curvature, and it puts a different symbol at each contact surface of each hitch. These are the only D/d figures that are law rather than guidance, and they are floors — being above them says nothing about whether the sling's rated load still applies.
| Where the sling bends | What the figure shows | Minimum D |
|---|---|---|
| Sling body around the load | Basket hitch — the rope itself curves under the load | 8 × d |
| Sling body over the hook or pin | Endless sling or grommet — the body, not an eye, sits in the hook | 8 × d |
| Sling eye on a hook, pin or shackle bolt | The eye is the contact surface, not the rope body | 2 × d |
| Load surface inside a choker hitch | Where the rope grips and turns on the load | 2 × d |
The 8× surfaces are the ones where the rope body does the bending; the 2× surfaces are an eye seated in a fitting and the load surface a choker grips. OSHA repeats the same two contact symbols for synthetic fiber rope slings in Fig. 5 of its sling guidance.
The ratio your rated-load table assumes
Every published rated load already has a D/d ratio baked into it. Go below that ratio and the number on the tag is no longer the number that applies — OSHA's own instruction on its wire rope sling page is that the rated load of the sling is decreased, and that you consult the sling manufacturer or the Wire Rope Technical Board's Wire Rope Sling Users Manual for the specific data.
| Sling type | OSHA tables | Rated loads based on |
|---|---|---|
| Wire rope, single-part 6×19 or 6×36 (EIPS, EEIPS) | Tables 7 to 12 | 25/1 |
| Cable-laid wire rope, mechanical splice | Table 13 | 10/1 |
| Braided wire rope, six-part and eight-part | Tables 14 and 15 | 25 × the component rope diameter |
| Synthetic fiber rope (nylon, polyester, polypropylene) | Tables 18 to 20 | the 2× and 8× contact surfaces of Fig. 5 |
| Synthetic web slings and roundslings | Tables 21 to 26 | no numeric ratio published — manufacturer only |
What the ratio does not tell you
- The pin can also be too big. OSHA's rated-load tables assume a pin diameter no larger than natural eye width or less than the nominal sling diameter. A wide pin spreads the eye, a narrow one pinches it; both put you outside the table even when D/d looks healthy.
- A sharp corner is not a small diameter. There is no D to divide by. Slings must be padded or protected from the sharp edges of their loads — 1926.251(c)(9) and 1910.184(c)(7) — and the value for a given edge radius comes from the manufacturer.
- The choker angle is a separate reduction. A choker rating in OSHA's tables holds only where the angle of choke is 120° or more. Below that the rating falls again, on top of anything D/d costs you.
- The sling angle is a separate reduction too. D/d is about the bend; the leg angle is about the tension. Both apply at once, and they multiply — that is what the sling tension calculator covers.
- Condition beats geometry. A perfect D/d ratio on a sling with broken wires, a kink or an illegible tag is worth nothing: rigging is inspected prior to use on each shift, and a sling without legible identification markings may not be used at all.
- This is not a lift plan. Nothing here addresses the weight of the load, the center of gravity, the crane's chart or a critical lift.
How it's calculated
- D/d ratio
D/d = D ÷ dD is the diameter of curvature the sling bends around; d is the sling's nominal component rope diameter. Both in the same unit, so the ratio has none. OSHA's Fig. 4 states it this way: when D is 25 times the component rope diameter, the ratio is expressed as 25/1.- Minimum diameter of curvature — sling body
D ≥ 8 × dWhere the sling body itself bends: under the load in a basket hitch, or over the hook on an endless sling. 29 CFR 1910.184 Fig. N-184-4 requires a contact surface with a diameter of curvature at least 8 times the diameter of the rope.- Minimum diameter of curvature — eye or choker
D ≥ 2 × dAn eye seated on a hook, pin or shackle bolt, and the load surface inside a choker hitch: Fig. N-184-4 requires a diameter of curvature at least double the diameter of the rope.- Bend diameter needed for a target ratio
D needed = target ratio × dThe smallest D that keeps you at the ratio your rated-load table is based on — 25/1 for single-part and braided wire rope slings, 10/1 for cable-laid, 8/1 for synthetic fiber rope.- Largest sling that still reaches the ratio
d max = D ÷ target ratioOn a fixed pin or a fixed load radius, a smaller sling reaches a higher D/d than a bigger one. This is the counter-intuitive half of the problem: upsizing the sling can lose you capacity.
Frequently asked questions
Sources & references
- 29 CFR 1910.184 — Slings, Figure N-184-4 (Basic Sling Configurations with Vertical Legs): minimum diameter of curvature, at least double and at least 8 times the diameter of the rope
- OSHA — Guidance on Safe Sling Use, Tables and Figures: Fig. 4 and Fig. 6 (D/d ratio) and the general notes of Tables 7 to 20 (rated loads based on a minimum D/d ratio of 25/1, 10/1 or 8/1)
- OSHA — Guidance on Safe Sling Use, Wire Rope Slings: "When D/d ratios are smaller than those listed in the tables, consult the sling manufacturer" and the referral to the WRTB Wire Rope Sling Users Manual
- 29 CFR 1926.251 — Rigging equipment for material handling: inspection prior to use on each shift, legible identification markings, never loaded above the marked safe working load, padded or protected from sharp edges
- ASME B30.9 — Slings (paid consensus standard; named here, never reproduced). The capacity-reduction percentages for a low D/d ratio live here and in each manufacturer's chart, which is why this page does not print any.
Content checked against these sources — last reviewed August 28, 2026.