SWL, MBS & SF: The Rigging Numbers That Keep You Safe

Quick answer: Minimum Breaking Strength (MBS) is the force at which equipment fails. Safety Factor (SF) is how many times stronger your gear is than its working load. Safe Work Load (SWL) is the maximum load you can safely apply, calculated as SWL = MBS ÷ SF. Together, these three numbers protect you every time you rig.

When you trust your body weight to a single point of hardware, the numbers stamped on that hardware matter more than anything. 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 simple terms: Safe Work Load, Minimum Breaking Strength, and Safety Factor.

These terms come straight from the lifting and rigging industry, where lives depend on getting the math right. They apply just as much to aerialists hanging from a rig point. This guide breaks down what each one means, how they connect, and how to use them with confidence before your next climb.

What is Minimum Breaking Strength (MBS)?

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

Manufacturers determine MBS through destructive testing, where they apply force to the equipment until it breaks. The value is usually given in kilonewtons (kN) or kilograms/pounds. For example, a carabiner might be rated to a MBS of 25 kN, meaning it takes roughly 2,500 kg of force to break it.

Here's the key thing to remember: you should never load equipment anywhere near its MBS. Breaking strength is the absolute ceiling, not a target. It exists to give you a baseline from which to calculate a much safer working limit.

What is Safety Factor (SF)?

Safety Factor (SF), sometimes called the design factor, is the ratio between an item's breaking strength and its safe working load. In plain terms, it tells you how much stronger your equipment is than the load you plan to put on it.

Safety factors are usually written as a ratio, like 5:1 or 10:1. According to lifting industry standards, safety factors typically range between 4:1 and 7:1 for general rigging equipment. A 5:1 safety factor means the gear is five times stronger than its rated working load.

So why build in all that extra strength? Because real-world conditions are unpredictable. A safety factor accounts for:

• Shock loading, like a sudden drop or a dynamic move that multiplies force
• Wear and age that weaken equipment over time
• Human error and small mistakes in setup
• Material variation between individual products

For aerial arts, where dynamic drops and sudden stops are part of the performance, a higher safety factor isn't a luxury. It's essential. Many aerialists and riggers work to a 10:1 safety factor precisely because of the shock loads involved in dynamic movement.

What is Safe Work Load (SWL)?

Safe Work Load (SWL), also known as Working Load Limit (WLL), is the maximum load you can safely apply to a piece of equipment under normal conditions. It's the number you actually work to—the limit that should never be exceeded during use.

There's a small but useful distinction between the two terms. Working Load Limit (WLL) is set by the manufacturer and printed on the product. Safe Work Load (SWL) can also refer to a reduced limit set by a competent person after assessing the specific conditions of use. In practice, the offshore and entertainment rigging industries still use SWL widely, while many lifting standards have shifted toward WLL.

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

How do SWL, MBS, and SF connect?

These three values are tied together by one simple formula:

SWL = MBS ÷ SF

Let's walk through a real example. Say you have a span set with a Minimum Breaking Strength of 5,000 kg, and you're applying a 10:1 safety factor for aerial use:

• SWL = 5,000 kg ÷ 10
• SWL = 500 kg

That means your span set is safe to work to 500 kg, even though it won't physically break until 5,000 kg. The 4,500 kg gap is your margin of safety—your protection against shock loads, wear, and the unexpected.

You can rearrange the formula depending on what you need to find:

• To find SWL: SWL = MBS ÷ SF
• To find MBS you need: MBS = SWL × SF
• To find the safety factor: SF = MBS ÷ SWL

This last one is handy when you're choosing gear. If you know your maximum load and the breaking strength of a product, you can quickly check whether it gives you the safety factor your discipline demands.

Why these numbers matter for aerial rigging

A single piece of equipment never tells the whole story. When you build a rigging system, the weakest link sets the limit for the entire setup. If your carabiner has a higher SWL than your swivel, the swivel's rating is what counts.

That's why every component in your rig—carabiners, swivels, span sets, rescue-rated hardware—needs to be assessed together. Always rig to the lowest SWL in the chain, and always factor in the dynamic nature of aerial work. A static hang and a hard drop place very different demands on the same equipment.

Performing under the spotlight should feel effortless and free. Knowing your numbers is what makes that freedom possible.

Rig with confidence

Understanding SWL, MBS, and Safety Factor turns rigging from guesswork into a clear, calculable process. Remember the three essentials: MBS is where gear breaks, SF is your built-in margin, and SWL is the limit you work to—found by dividing MBS by SF.

Before your next session, take a moment to check the ratings on every piece of equipment in your system, identify the weakest link, and confirm your safety factor suits the demands of your discipline. When your rigging is sound, you're free to focus on the only thing that should be on your mind: your performance.