How is the holding force of a magnet determined?
Short answer: the holding force of a magnet is the force needed to pull it perpendicularly off a thick, flat, bare steel plate, expressed in kg (1 kg ≈ 9.81 N). It is a lab value under ideal conditions. In practice you almost never reach it: a layer of paint, thin steel, a small air gap or a sideways load can halve the force or reduce it even further.
How is holding force measured?
- The magnet is placed with its pole face on a mild steel plate (for example S235), ground flat and thick enough that the steel does not saturate: at least 10 mm, more for large magnets.
- A tensile tester pulls the magnet slowly and exactly perpendicularly off the plate.
- The highest force before release is the holding force, at room temperature (approx. 20 °C).
That is why the stated value is an upper limit: every deviation from this situation gives less force. Manufacturers state the value as "approx." because there is a spread of a few percent between individual magnets.
What reduces holding force in practice?
| Factor | Effect (indicative) |
|---|---|
| Air gap / paint / film of 0.5 mm | small magnets: 25 – 40% less |
| Air gap of 1 mm | small magnets: often half; large magnets: 20 – 30% less |
| Thin steel (1 mm sheet, e.g. a fridge door) | strong magnets: 50 – 80% less, the steel saturates |
| Sideways load (vertical wall) | magnet slides at just 15 – 25% of the holding force |
| Rough, rusty or curved surface | less contact, so less force |
| Stainless steel (most types), cast iron | stainless 304/316: virtually none; cast iron: clearly less |
| Temperature above 20 °C | approx. 0.12% less per °C (reversible up to 80 °C) |
An evenly loaded magnet on a thick steel beam comes close to the stated value. A magnet on a painted, thin door that has to hold a coat might reach 10% of it.
Pull force and shear force
Pull force is perpendicular away from the surface; that is what the stated holding force applies to. Shear force is along the surface, as with a magnet on a vertical wall with something hanging from it. Then friction determines when it starts to slide: for steel on steel that is at about a quarter of the holding force. Rubber increases friction. That is why rubber-coated magnets and magnets with a rubber protective pad slide away less easily.
How much safety margin?
- Pulling perpendicularly on thick steel: count on at most half of the stated holding force.
- On a vertical wall: count on 10 – 20%.
- With vibrations, impacts or where a fall is dangerous: never use a magnet as the only fixing.
So holding force is not a permitted working load. We explain the difference in holding force, pull force and working load.
Frequently asked questions
Why does my magnet hold less than the packaging says?
Usually because of the surface (thin or painted steel), a small distance or because the load is sideways. The magnet itself is rarely weaker. See also why do magnets lose their strength?
How much does a magnet hold against another magnet instead of steel?
Two identical magnets attracting each other hold about as strongly or slightly more than one magnet on thick steel. A magnet on a 1 mm steel plate (such as a metal counter plate) holds much less.
How thick does the steel need to be?
For small magnets up to approx. Ø10 mm, 1 – 2 mm of steel is enough. For magnets of Ø20 mm and larger you need 5 – 10 mm to reach the full force.
Read also
- How strong is a neodymium magnet? Holding force per size
- Do magnets work through wood, glass and plastic?
- Why are pot magnets so strong?
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