The difference between WLL, SWL and MBL

Do you work with lifting equipment every day? Then you've probably come across abbreviations such as WLL, SWL, and MBL. For mechanical engineers, safety coordinators, and riggers, these are far more than technical terms—they can make the difference between a safe lifting operation and a serious incident.

At Mennens, we regularly see these terms being confused in practice. That's why we've put together a clear overview of the definitions, applicable standards, and common misconceptions, so you can work safely and with confidence.

What is WLL (Working Load Limit)?

WLL stands for Working Load Limit.

It is the maximum load specified by the manufacturer that a lifting device is designed to lift safely. It is important to remember that the WLL applies to straight vertical lifting under the conditions specified by the manufacturer.

Example: If a shackle is marked WLL 2 t, it may safely lift a maximum of 2 tonnes, provided the load is applied in line with the shackle.

Please note: When lifting at an angle—for example with a two-leg chain sling—the forces in each sling leg increase. The WLL shown on the equipment does not change, but the effective lifting capacity of the sling arrangement is reduced.

What is SWL (Safe Working Load), and why is it rarely used today?

SWL stands for Safe Working Load.

Historically, this was the standard term used throughout the lifting industry. Today, however, it has largely been replaced by WLL in European and ISO standards.

The reason is simple: the word "safe" can be misleading from both a technical and legal perspective. Whether a lift is safe depends not only on the lifting equipment but also on factors such as the lifting method, environment, load distribution, and operator competence.

Where is SWL still used?

You may still encounter the term SWL in:

• Offshore and maritime applications, where cranes and winches are often rated using SWL.

• Specific lifting plans, where SWL may refer to the maximum allowable load for a particular lifting configuration after applying reduction factors for sling angles or operating conditions.

Mennens expert tip: If modern below-the-hook lifting equipment—such as shackles, hooks, or webbing slings—is marked SWL, pay extra attention. The current international standard is WLL. For cranes and hoists, the preferred term is Rated Capacity.

What is MBL (Minimum Breaking Load)?

MBL stands for Minimum Breaking Load. It is also sometimes referred to as Minimum Breaking Force (MBF).

This is the minimum load at which the lifting equipment will break or deform to such an extent that it can no longer safely support the load. The value is established during destructive testing performed by the manufacturer.

Load capacities are often expressed in tonnes or kilograms, while breaking forces may also be expressed in kilonewtons (kN).

During normal lifting operations, the applied load should never approach the MBL. The MBL is used solely as the basis for determining the equipment's safety factor.

The safety factor

The Safety Factor (SF)—also known as the Design Factor—provides the safety margin built into lifting equipment. It is the ratio between the Minimum Breaking Load (MBL) and the Working Load Limit (WLL).

Formula:

WLL = MBL ÷ SF

The required safety factor depends on the type of lifting equipment, as different materials respond differently to wear, fatigue, and damage.

Typical industry values are:

• Chain slings and lifting accessories: 4:1

• Wire rope: 5:1

• Synthetic webbing slings and round slings: 7:1

Example: A webbing sling with a WLL of 1 tonne has a theoretical Minimum Breaking Load of 7 tonnes. This generous safety margin is essential to accommodate dynamic loading, wear, and unexpected operating conditions.

Proof load testing and EKH certification

The Proof Load is the load applied during testing by the manufacturer or by an inspection body such as Mennens.

According to EKH guidelines and the Machinery Directive, lifting equipment must be inspected and, where applicable, proof load tested at prescribed intervals.

During proof load testing, the equipment is typically loaded to a value above its WLL—for example 125% or 150%, depending on the equipment type—to verify that no permanent deformation occurs.

The weakest component determines the WLL

One of the most common mistakes during lift planning is assuming that lifting capacities can be added together.

Consider the following lifting assembly:

• Crane hook — WLL 10 tonnes

• Master link — WLL 6 tonnes

• Chain leg — WLL 4 tonnes

What is the Working Load Limit of the complete assembly?

The answer is 4 tonnes.

The component with the lowest Working Load Limit always determines the maximum capacity of the entire lifting assembly.

Summary: the key terms at a glance

Want to be sure you're lifting safely?

Understanding the terminology is the first step. Applying it correctly in day-to-day lifting operations is the next.

How do you deal with complex sling angles? When should a webbing sling be removed from service? And are you complying with the latest EKH guidelines?

Download our white paper: Safe Lifting in Practice

In this white paper, our experts cover:

• The most common mistakes when slinging loads.
• Practical tips for carrying out pre-use inspections.
• Legislation, maintenance, inspections, and certification.

This whitepaper is only available in Dutch!