How is the holding force of a magnet determined?
Short answer: the holding force of a magnet is the force required to pull it perpendicularly off a thick, flat, bare steel plate, expressed in kg (1 kg ≈ 9.81 N). It is a laboratory value under ideal conditions. In practice, you will almost never achieve 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 the polar side on a steel plate made of mild steel (for example, S235), ground and flat, and thick enough that the steel does not saturate: at least 10 mm, and more for large magnets.
- A pull tester slowly and exactly perpendicularly pulls the magnet off the plate.
- The highest force before separation is the holding force, at room temperature (approx. 20 °C).
This is why the stated value is an upper limit: any deviation from this situation results in less force. Manufacturers state the value as "approx." because there is a variation of a few percent between magnets.
What reduces the holding force in practice?
| Factor | Effect (indicative) |
|---|---|
| Air gap / paint / foil of 0.5 mm | small magnets: 25 – 40% less |
| Air gap of 1 mm | small magnets: often half as much; large magnets: 20 – 30% less |
| Thin steel (1 mm plate, for example a refrigerator door) | strong magnets: 50 – 80% less, the steel saturates |
| Sideways load (vertical wall) | magnet slides at 15 – 25% of the holding force |
| Rough, rusty, or bent surface | less contact, therefore less force |
| Stainless steel (most types), cast iron | Stainless steel 304/316: almost none; cast iron: clearly less |
| Temperature above 20 °C | approx. 0.12% less per °C (reversible up to 80 °C) |
A uniformly loaded magnet on a thick steel beam comes close to the stated value. A magnet on a lacquered, thin door that has to hold a coat may only achieve 10% of it.
Pulling force and sliding force
Pulling force is perpendicular to the surface; the stated holding force applies there. Sliding force is along the surface, such as with a magnet on a vertical wall where something is hanging. Then friction determines when it will slide: with steel on steel, that is at approximately a quarter of the holding force. Rubber increases friction. Therefore, rubber-coated magnets and magnets with a rubber protective disc slide away less quickly.
Why do holding forces differ between stores?
If you search for the same magnet at different stores, you will often see different holding forces. This has a few causes:
- Measured or calculated. Some values are measured on a pull tester, others are calculated using a physics formula based on size and quality. A calculation is a good approximation, but not an exact measurement. Especially with very thin magnets, ring magnets with a large hole, and pot magnets, the actual force can differ significantly.
- A different measuring setup. The thickness and type of the steel plate, how flat and smooth it is, and how quickly it is pulled affect the result. There is no fixed standard that all suppliers use.
- Rounding generously. Some suppliers round up considerably, or state the value of a stronger quality than they supply.
- Production variation. Even magnets from the same batch differ by a few percent.
We deliberately use conservative values. If we are between two numbers, we state the lower one: we would rather have a magnet that holds more than we promise than the other way around. Does a magnet clearly hold less than stated under the ideal conditions above? Let us know, and we will investigate it.
How much safety margin?
- Pulling perpendicularly on thick steel: calculate with a maximum of half the stated holding force.
- On a vertical wall: calculate with 10 – 20%.
- In case of vibrations, impacts, or if a fall is dangerous: never use a magnet as the only fastening.
The holding force is therefore not the permitted working load. We explain the difference in holding force, pulling force, and working load.
Frequently asked questions
Why does my magnet hold less than stated on the packaging?
Usually due to the substrate (thin or lacquered steel), a small gap, 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 between two magnets instead of on steel?
Two identical magnets attracting each other hold approximately as strongly or slightly stronger than one magnet on thick steel. A magnet on a 1 mm steel plate (such as a metal backing plate) holds much less.
How thick must the steel 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 achieve the full force.
Why does another store state a higher holding force for the same magnet?
Because the value was determined differently: measured or calculated, on different steel, or rounded more generously. A higher value does not mean the magnet is stronger. Therefore, mainly compare size and quality (such as N42 or N52), and always calculate with a safety margin.
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?
Need help choosing? Use the Magnet Advisor or contact us.
