Does a larger magnet lead to stronger holding force? Probably, yes. But as you will learn in this article, the steel thickness of the plate you’re using for the magnet could be just as important as the magnet itself for a strong holding force. You can also use our magnet holding force calculator to see how steel thickness affects different magnets.
In short: If the steel is too thin, it reaches magnetic saturation, reducing the magnet's holding force even if the magnet itself is very powerful. To understand why this happens, we first need to look at how a magnet works.
A Magnet Needs a Complete Magnetic Circuit
A permanent magnet creates magnetic flux (yes, that’s what it’s called) that travels in a closed magnetic circuit from one pole to the other. In general, more magnetic flux crossing the contact surface results in greater holding force.
The field lines traveling from pole to pole are somewhat analogous to an electrical circuit, where the current is affected by the resistance along the way.
An electrical current is limited by electrical resistance. High resistance is caused by materials low conductivity, long distance within that material, and small cross-sectional area.
Magnetic flux is limited by magnetic reluctance. High reluctance is caused low magnet permeability, long distance and small cross section area.
Why Steel Is So Important
As we have explained our article on how air gap affects magnetic holding force, air has very low magnetic permeability, almost as low as pure vacuum. That’s why you want to keep the airgap as thin as possible.
Steel, on the other hand, has very high permeability – hundreds or even thousands of times greater than air. But it is not unlimited.
Inside the steel are countless tiny regions called magnetic domains. When no magnet is nearby, these domains point in many different directions.
As the magnet is placed against the steel, more and more of these domains align with the magnetic field. This allows the steel to carry increasing amounts of magnetic flux.
Eventually, nearly all of the domains are aligned. At this point, the steel has reached magnetic saturation.
Once the steel approaches magnetic saturation, its permeability decreases significantly. It can no longer carry additional magnetic flux efficiently, forcing more of the magnetic field to take less effective paths through the surrounding air. What happens is that the steel's effective permeability decreases.
In other words, the steel is no longer such an efficient path for the magnetic field. The result is a weaker magnetic field at the contact surface—and therefore a lower magnetic holding force.
Why Thin Steel Reduces Magnetic Holding Force
Thin steel has a smaller cross-sectional area available for carrying magnetic flux. For the same magnet, this means the magnetic flux is concentrated into a smaller volume of steel, producing a higher magnetic flux density.
For typical low-carbon steels, magnetic saturation begins at a flux density of around 1.6–2.1 T, depending on the material. This leads to lower permeability, reduced flux and reduced magnetic holding force. We also see field lines “leaking through” the steel and into the air on the other side.
This is why the same magnet often performs noticeably worse on thin sheet steel than on a thick steel plate.
How Thicker Steel Increases Magnetic Holding Force
A thicker steel plate provides a larger path for the magnetic flux.
Instead of forcing the same amount of magnetic flux through a thin section of steel, the flux is distributed through a greater volume of material.
This keeps the magnetic flux density lower, allowing the steel to maintain its high permeability.
Looking at the animation below, you'll see how the flux density increases when the plate thickness is reduced, and an increasing amount of flux travels through the air instead of the steel plate:
Is There an Ideal Steel Thickness and How do I Find it?
More powerful magnets generally require thicker steel to achieve their full rated holding force. Every magnet has a minimum steel thickness required to achieve its full holding force. We call this the saturation thickness. Below this thickness, the steel begins to saturate and limits the magnetic circuit.
You can estimate the saturation thickness using our magnet calculator. Keep the air gap constant while varying the steel thickness. Once increasing the thickness no longer increases the predicted holding force, you have reached the saturation thickness for that magnet.
