How is the holding force of a magnet determined?

When a magnet is sold, a holding force is often stated. For example, you might see values like 5 kg, 20 kg, or even 100 kg of pull force. For many users, this seems like a simple indication of what a magnet can hold, but in practice, the actual holding force depends on many more factors.

The stated holding force is a laboratory value measured under ideal conditions. As soon as a magnet is used in a practical application, the results can deviate significantly.

To select a magnet correctly, it is important to understand how holding force is determined and which factors influence it.

What is holding force?

The holding force of a magnet is the maximum force required to pull the magnet perpendicularly from a steel surface.

This force is usually expressed in kilograms (kg) or Newtons (N).

When a manufacturer states that a magnet has a holding force of 20 kilograms, it does not mean that the magnet can hold a weight of 20 kilograms under all circumstances. It means that, under ideal test conditions, a pulling force of approximately 20 kilograms is required to detach the magnet from a steel plate.

How is holding force measured?

Manufacturers measure holding force under controlled conditions.

The magnet is placed on a thick, flat, high-quality steel plate. Then, a measuring device is used to determine the force required to pull the magnet directly off the surface.

During this measurement, the following assumptions are made:

• Perfect flat contact
• Unpainted steel
• No air gap
• Room temperature
• Optimal material thickness

Under these conditions, the maximum holding force is achieved.

However, these conditions are rarely present in practice.

Why does the actual holding force often differ?

Many users find that a magnet holds less weight than expected based on the specifications.

This is because every small deviation affects the magnetic circuit between the magnet and the steel.

Even a thin layer of paint, a protective film, or a small imperfection can noticeably reduce the holding force.

The greater the distance between the magnet and the steel, the faster the magnetic force decreases.

The influence of the contact surface

A magnet performs best when its entire surface makes contact with steel.

When only part of the magnet makes contact, the holding force decreases.

This is similar to a suction cup. The better the contact, the better the performance.

Therefore, magnets on smooth steel surfaces often perform better than on rough or damaged surfaces.

Material makes a big difference

Not every metal reacts the same to a magnet.

Mild steel or structural steel generally offers the highest holding force.

Materials such as aluminum, copper, and plastic are not magnetic and do not provide any holding force.

Stainless steel (SS), depending on its composition, can have a much lower magnetic effect than regular steel.

Therefore, the same magnet can yield very different results on two different metal surfaces.

Pull force and shear force

An important distinction is made between pull force and shear force.

Pull force occurs when the magnet is pulled perpendicularly from the surface.

Shear force occurs when the magnet slides parallel to the surface.

In vertical applications, shear force often plays a greater role than pull force.

As a result, a magnet that theoretically has 20 kilograms of pull force can start to slide in a vertical application at a significantly lower weight.

Friction between the magnet and the surface plays an important role here.

The influence of magnet size

Many people look exclusively at magnet quality, such as N35 or N52.

While magnet quality is important, the size of the magnet often has an even greater influence on the ultimate holding force.

A larger magnet has more contact surface and can therefore often generate more force than a smaller magnet of higher quality.

For this reason, dimensions are always taken into account when determining the performance of a magnet.

Temperature and holding force

Temperature also affects magnetic performance.

When a neodymium magnet is exposed to temperatures above its maximum operating temperature, its magnetic strength can decrease.

With prolonged overheating, this loss can be permanent.

For applications in warm environments, special temperature-resistant grades are therefore used.

Why safety margins are important

In practical applications, the maximum theoretical holding force is almost never assumed.

Professional users typically apply a generous safety factor.

For example, if an object weighing 10 kilograms needs to be attached, a magnet solution with a significantly higher theoretical holding force is often chosen.

This ensures sufficient reserve for vibrations, air gaps, wear, and other real-world influences.

Conclusion

The holding force of a magnet is measured under ideal laboratory conditions on a flat steel plate. In practice, the actual holding force is influenced by factors such as material type, contact surface, distance, temperature, and the way the load acts on the magnet.

Therefore, a stated holding force does not tell the whole story about performance in a specific application. A good understanding of these factors helps in choosing the right magnet and prevents disappointments during use.