What is the safe working load of this wire rope?

Formula, assumptions, and limits

A wire rope’s safe working load is its certified minimum breaking load divided by a factor of safety, and the ILO code of practice Safety and health in ports (Revised 2016) sets that factor for wire rope in a sling by the sling’s own SWL. Paragraph 188 of the code states it the other way round: the guaranteed minimum breaking load should not be less than the safe working load times a factor of safety from Appendix E. The definitions chapter calls the factor the number got by dividing the minimum breaking load of an item by its certificated safe working load, so the two statements are one equation.

Formula

For wire rope forming part of a sling, ILO Appendix E.1.1 gives the factor by the SWL of the sling in tonnes:

for an SWL over 10 t up to and including 160 t, with $F_S = 5$ up to and including 10 t and $F_S = 3$ over 160 t.

• $\mathrm{SWL}$: safe working load of the sling, in tonnes. For a multi-leg sling, Appendix E footnote 2 says this is the SWL of the complete sling.
• $\mathrm{MBL}$: the rope’s minimum breaking load from the maker’s certificate, in tonnes. A figure in kN is divided by 9.80665, the kilogram-force factor in NIST SP 811 Appendix B.9.
• $F_S$: factor of safety, also called the safety factor, design factor or coefficient of utilization. It has no unit.

Derivation

Going from the SWL to the breaking load is direct: work out $F_S$ at that SWL and multiply. A 25 t sling needs $F_S = 10\,000 / (8.85 \times 25 + 1\,910) = 4.692$, so an MBL of at least 117.30 t, or 1,150.3 kN.

Going from the breaking load to the SWL is circular, since the factor depends on the SWL being solved for. In the middle band, put $\mathrm{SWL} = \mathrm{MBL}/F_S$ into the formula and solve:

That holds for an MBL from 50.04 t to 481.06 t. Below 50 t the SWL is MBL ÷ 5, and above 481.06 t it is MBL ÷ 3.

Assumptions

• One leg in a straight pull. The calculator takes the rope’s full MBL as the sling’s strength. The table under the result multiplies that SWL by the ILO mode factors, paragraphs 125 and 127 of the ports code: 0.8 for a choke hitch, 2.0 for a basket with vertical legs, 1.4 for a basket or two legs up to 45° from the vertical and 2.1 for three or four legs. Paragraph 128 rates a four-leg sling as a three-leg one, since most loads do not share evenly, and paragraph 129 says a leg should never pass 60° from the vertical, where each leg of a two-leg sling carries the whole load. Appendix E footnote 2 picks a multi-leg sling’s factor by the SWL of the complete sling, which in the formula band is a lower factor, so the table’s basket and multi-leg rows are on the safe side: 29.97 t for two legs at the default, where that reading gives 31.21 t. The SWL marked on a multi-leg sling is the one its certificate gives.
• The certificate MBL is for the rope before the eyes were made, which is the figure ILO paragraph 65 asks the maker to certify.
• The eye or fitting is at least as strong as the code requires. ILO paragraph 199 asks terminal fittings to hold 95% of the rope’s MBL up to 50 mm rope diameter and 90% above.
• A rope in new condition. Wear, corrosion, kinks and broken wires reduce the strength, and the examination of the rope decides whether it stays in use.
• The rule picked fits the gear. ILO Appendix E covers ports and ships' gear; other codes and class rules set their own factors.

Worked example

A single-leg sling is made from wire rope whose certificate gives a minimum breaking load of 1,000 kN.

1. MBL in tonnes: 1,000 ÷ 9.80665 = 101.97 t.
2. 101.97 t is between 50.04 t and 481.06 t, so the sling falls in the middle band.
3. SWL = 1,910 × 101.97 ÷ (10,000 − 8.85 × 101.97) = 194,763 ÷ 9,097.6 = 21.41 t, or 210 kN.
4. Check: $F_S$ at 21.41 t is 10,000 ÷ (8.85 × 21.41 + 1,910) = 4.763, and 101.97 ÷ 21.41 = 4.763.
5. A flat factor of 5 would give 101.97 ÷ 5 = 20.39 t, 4.7% less.

With ferrule-secured eyes, ILO paragraph 65 puts the proof test at no more than 2 × 21.41 = 42.82 t.

Edge cases and limits

• The bands do not meet exactly. The formula gives 5.004 at 10 t and 3.007 at 160 t, not 5 and 3. For an MBL from 50.00 t to 50.04 t, MBL ÷ 5 is just over 10 t and the formula gives just under, so neither band fits; the calculator holds the SWL at 10 t, which keeps the factor above 5. From 480.0 t to 481.06 t both bands fit, and the calculator takes the lower SWL, the formula’s.
• Derricks differ below 10 t. For wire rope forming part of a derrick, the ILO table applies the formula from zero to 160 t with no flat 5, so the factor is 5.21 at 1 t and 5.12 at 5 t. That table selects the factor by the derrick’s SWL, not the rope’s, and the calculator does not cover it.
• Splices. ILO paragraph 195 says a splice at the largest rope sizes may be only 70 to 75% as strong as the rope, and that the loss should be taken into account when the factor is decided.
• Fiber rope and webbing take a factor of at least 6 under ILO Appendix E.2 and E.3, with 8 recommended for most cargo work in fiber rope. The calculator is for steel wire rope only.

Regulatory basis

The ILO code of practice Safety and health in ports (Revised 2016, first published 2018) is the source of the factor table, in Appendix E.1.1, and of the proof test limits for slings in paragraphs 65 and 66. Appendix E.4 says the code’s factors apply unless a recognized national or international standard sets other requirements.

Lloyd’s Register’s Code for Lifting Appliances in a Marine Environment (July 2021 edition) uses the same formula for crane ropes in Chapter 4, 2.26.1: 5.0 at an SWL of 10 t or less, 3.0 at 160 t or more. Its derrick table, Chapter 2 Table 2.2.2, caps running rigging at 5 and floors it at 3, and lets standing rigging stop at 3.5.

SOLAS regulation II-1/3-13, adopted by resolution MSC.532(107) on 8 June 2023 and in force from 1 January 2026, sets no factor. It requires lifting appliances installed from that date to be built to a recognized classification society’s rules or an equivalent standard and marked with their SWL; MSC.1/Circ.1663 gives the test loads. In the United States, 29 CFR 1917.42(b)(1) and 1918.62(b)(1) require a design safety factor of at least 5 for running wire used as falls and for light load slings.

Common errors

1. Mixing kN and tonnes. The ILO and LR tables band the factor by SWL in tonnes. Look up the factor with a kN figure and a 210 kN sling reads as 210 t, in the 3.0 band.
2. Using 5 at every SWL. Above 10 t the ILO factor is lower, so a flat 5 understates the SWL of a heavy sling: a rope of 300 t MBL is a 78.0 t sling under ILO Appendix E and a 60.0 t sling at a flat 5.
3. Treating a crane’s SWL as the rope’s load. LR CLAME Chapter 4, 2.26.1 and 2.26.4 apply the factor to the rope’s own load: the live load shared over the parts of the reeving, plus rope and jib weight, with 2% friction per ball or roller bearing sheave and 5% per plain or bushed sheave.
4. Using the broken-sample figure. The rope certificate in COSWP Annex 19.1 records the load at which a test sample broke as well as the rope’s SWL; the calculation uses the certified minimum breaking load, the guaranteed lower figure.
5. Leaving out the eye. A hand splice, a wedge socket or a lengthening joint holds less than the rope. ILO paragraph 189 says a hoisting rope that has to be lengthened gets a reduced SWL.

Safety factor by SWL

The ILO factor stays at 5 to a 10 t sling and then falls, fast at first and slower as the sling gets heavier: 4.79 at 20 t, 3.58 at 100 t, 3.01 at 160 t. The minimum breaking load a sling needs still rises with its SWL, only more slowly than five times.

SWL from MBL: common breaking loads

Read across from the certificate figure in kN. The ILO column is the sling’s SWL under Appendix E; the last column is what a flat factor of 5 gives the same rope, as US marine terminal and longshoring rules apply it.

Proof test loads for slings and loose gear

A proof test checks the made-up gear, not the rope’s breaking strength, and the ILO code limits it for wire rope slings rather than setting a minimum. A sling with ferrule-secured eyes is tested at no more than twice its SWL in a straight pull (paragraph 65). A hand-spliced sling is never proof loaded above its SWL (paragraph 66), and the LR code tests a terminal fitting at no more than half the rope’s MBL where the rope’s factor is between 3 and 4.

For other loose gear, MSC.1/Circ.1663 Table 2, as corrected by Corr.1 in January 2026, sets these minimum test loads; COSWP Table 19.2 gives the same figures.

Lifting appliances themselves are tested at 1.25 × SWL up to 20 t, SWL + 5 t from 20 t to 50 t and 1.1 × SWL above 50 t, in MSC.1/Circ.1663 Table 1.

Where to find each input

• Minimum breaking load. The rope maker’s certificate. ISO 2408:2017 certificates give the minimum breaking force in kN; the certificate form in COSWP Annex 19.1, the ILO Convention No. 152 form, gives tonnes. Use the certified minimum, not the load at which a sample broke.
• The SWL needed. The heaviest load the sling will lift, with the hook block, spreader or other gear hanging below the sling’s eye.
• The factor rule. The code the port, the flag or the gear’s certification applies. A sling in a port that follows the ILO code takes Appendix E; a ship’s crane takes its class rules; a US marine terminal takes 29 CFR 1917.42.
• The eye. The sling’s marking, its certificate or a look at the eye: a pressed metal ferrule or a tucked splice.