SWL, MBS, SF Explained: Rigging Safety Made Simple

Quick answer: Minimum Breaking Strength (MBS) is the force at which equipment fails. Safety Factor (SF) is how many times stronger the gear is than the load you put on it. Safe Work Load (SWL) is the maximum load you may apply, calculated as SWL = MBS ÷ SF. Most aerialists work to a 10:1 safety factor, because a drop can multiply your body weight several times over.

When you trust your body weight to a single point of hardware, the numbers stamped on that hardware matter more than anything else in the room. Aerial silks, hoops and trapezes all depend on rigging that can take far more than your weight — and the way to know that is to understand three terms that come straight from the lifting industry, where lives depend on getting the arithmetic right.

The three numbers at a glance

What is Minimum Breaking Strength (MBS)?

Minimum Breaking Strength, also called Minimum Breaking Load (MBL), is the force required to make a piece of equipment break. It is the point of no return — the load at which a carabiner, swivel or span set gives out.

Manufacturers establish MBS by destructive testing: they pull the item until it fails. The value is usually given in kilonewtons (kN) or in kilograms. A carabiner rated to 25 kN takes roughly 2,500 kg of force to break, since 1 kN ≈ 102 kg ≈ 225 lb.

The thing to remember: never load equipment anywhere near its MBS. Breaking strength exists to give you a baseline from which to calculate a much safer working limit.

What is Safety Factor (SF)?

Safety Factor, sometimes called the design factor, is the ratio between breaking strength and safe working load. It tells you how much stronger the equipment is than the load you plan to put on it, and it is written as a ratio: 5:1, 7:1, 10:1.

That margin is not padding. It absorbs the things you cannot predict:

• Shock loading — a drop or a hard catch can multiply your body weight several times over
• Wear and age — textiles lose strength long before they look worn
• Human error — a choke hitch, a cross-loaded carabiner, a twisted sling
• Material variation between individual items from the same batch

Safety factors in practice

Different materials and disciplines work to different ratios. These are the figures commonly used across rigging and aerial practice:

All Aerial Obsessed equipment is rated at 10:1, the ratio most aerialists and riggers work to.

What is Safe Work Load (SWL)?

Safe Work Load, also known as Working Load Limit (WLL), is the maximum load you may apply under normal conditions. It is the number you actually work to.

There is a small but useful distinction. WLL is set by the manufacturer and printed on the product. SWL can also mean a reduced limit set by a competent person after assessing the specific conditions of use — a worn anchor, an awkward angle, a hot studio. Some manufacturers also publish a ULL (Ultimate Load Limit) or a separate user weight limit, which is not the same thing as the equipment's SWL.

Either way the principle is identical: the SWL is the line you do not cross.

How the three connect

One formula ties them together:

SWL = MBS ÷ SF

Rearranged for whatever you need to find:

That last one is the one to use when you are comparing products. Two brands can quote very different-looking numbers simply because they apply different safety factors. Divide breaking strength by working load and you get a figure you can compare honestly.

Worked examples

Notice rows two and three: the same fabric, rated at two different safety factors, produces working loads that differ by 64 kg. Neither number is wrong — but only one of them tells you the margin you have left when you drop.

Published ratings: Aerial Obsessed equipment

Every figure below is published rather than implied, and every one uses a 10:1 safety factor. Where a rating is per item rather than per pair, we say so, because that distinction is where most miscalculations start.

Per strap and per chain matter. A pair of straps used together does not give you a single 160 kg limit in every configuration — loading is rarely shared evenly, and a choke or a single-side hang puts the whole load on one item. When in doubt, rig to the single-item figure.

The weakest link sets the limit

A single component never tells the whole story. When you build a rigging system, the lowest SWL in the chain is the SWL of the entire setup. If your carabiner is rated higher than your swivel, the swivel is your limit.

Our rigging and hardware collection is built around this principle: the Universal Hanging Kit is rated to 360 kg, comfortably above every apparatus we make, so the fabric or rope is always the limiting component rather than the hardware holding it.

For the components themselves — which carabiners, which swivel, what order they go in and what an anchor point actually needs to be — see Aerial Rigging Hardware: The Essential Safety Guide.

Frequently asked questions

What safety factor should aerialists use?

10:1 is the working standard for silks, straps, rope and trapeze, because dynamic drops generate shock loads several times body weight. Aerial yoga and hammock work is commonly rated at 5:1 minimum since the movement is mostly static. General lifting sits between 4:1 and 7:1.

Are SWL and WLL the same thing?

Closely related and often used interchangeably. WLL is set by the manufacturer and marked on the product. SWL can also mean a reduced limit set by a competent person for specific conditions. Both are limits you never exceed.

Can I ever exceed the Safe Work Load?

No. Exceeding the SWL removes the margin that absorbs shock loading, wear and error. The gap between SWL and MBS is not spare capacity — it is the reason the equipment survives a drop.

How do I convert kilonewtons to kilograms?

1 kN ≈ 102 kg ≈ 225 lb. A 25 kN carabiner therefore breaks at roughly 2,500 kg. Climbing hardware is usually marked in kN, while industrial rigging hardware is usually marked in kilograms or pounds — which is exactly why comparing two products by their headline number alone is unreliable.

How much force does a drop actually generate?

Far more than your body weight. Dynamic aerial movement can multiply static load several times over, and the shorter and harder the stop, the higher the peak. This is the entire reason aerial work uses a 10:1 factor while static lifting does not.

Does a choke or a knot reduce strength?

Yes, substantially. A choke hitch can cut the strength of a sling or strap dramatically, and knots create stress concentrations that reduce breaking strength. Rig with rated connectors rather than knots wherever you can, and if you must choke, derate accordingly.

How often should I inspect my equipment?

Visually before every session, and in detail on a schedule. Retire anything with cuts, burns, fraying, thinning, corrosion or permanent deformation — textiles lose strength well before the damage looks alarming.

Rig with confidence

Understanding these three numbers turns rigging from guesswork into arithmetic. MBS is where the gear breaks. SF is your built-in margin. SWL is the limit you work to, and it is MBS divided by SF.

Before your next session: check the rating on every component, find the lowest one, and confirm that the safety factor matches what your discipline actually demands. When the rigging is sound, the only thing left to think about is the performance.

Published by Aerial Obsessed, Alicante, Spain. All equipment ratings in this guide are the figures we publish on our own product pages. This article is general guidance and does not replace assessment of your specific rig by a qualified rigger.