How many magnets do I need? How to calculate it
Short answer: required holding force = weight × safety factor. Divide that by the holding force per magnet. The safety factor depends on how the magnet is loaded:
- Pulling perpendicular (something hanging from a steel ceiling or beam): factor 2 – 3.
- Sideways on a vertical wall (plate, panel, hook on the side of a cabinet): factor 5 – 10, because the magnet starts to slide at 15 – 25% of its holding force.
- Through paint, film, or a gap, or with vibrations: even higher.
Example: a 2 kg plate on a steel wall requires 2 × 8 = approx. 16 kg holding force. With Ø20 x 5 mm disc magnets (approx. 7.4 kg), you therefore need 3 of them.
Calculation examples
| Situation | Weight | Factor | Required | Example |
|---|---|---|---|---|
| Lamp under a steel beam | 3 kg | 3 | 9 kg | 1 pot magnet Ø25 mm (approx. 18 kg) or hook magnet Ø20 mm (16 kg) |
| Plate on a steel wall | 2 kg | 8 | 16 kg | 3 × disc Ø20 x 5 mm (7.4 kg) |
| Closing a cabinet door | – | – | 1 – 3 kg | 1 × disc Ø10 x 3 mm (2.1 kg) per door |
| Poster or A4 on a whiteboard | 0.05 kg | 10 | 0.5 kg | 1 × 5 mm cube or disc Ø8 x 2 mm |
| Tools on a steel cabinet | 1 kg | 8 | 8 kg | 1 rubber hook magnet Ø43 mm (approx. 14 kg) |
| Panel through 3 mm wood | 1 kg | 8, plus distance | > 15 kg | 2 × Ø20 x 10 mm, magnet against magnet |
You can find the holding force per size in how strong is a neodymium magnet? and with every product.
Step by step
- Weigh what you want to attach.
- Determine the direction: perpendicular from the steel, or sideways along the steel.
- Check the surface: thick steel, thin sheet metal, paint, or stainless steel? Thin or painted steel: extra factor 2. Stainless steel 304/316 does not work: see is stainless steel magnetic?
- Check the distance: the force drops off quickly through material. See do magnets work through wood and glass?
- Multiply by the factor and divide by the holding force per magnet. Round up.
- Distribute the magnets over the surface, for example in the corners, so that nothing can tilt.
- Test with the actual weight before mounting permanently.
Prefer not to calculate?
The Magnet Advisor calculates it for you based on your application, weight, and surface.
Frequently asked questions
Are two small magnets as strong as one large one?
Side by side, they add up approximately. Stacked on top of each other, the magnet becomes thicker and stronger, but not twice as strong. A thicker magnet often works better than two thin ones.
Can I use a magnet for something that is dangerous if it falls?
Not as the only attachment. Holding force is not working load. See holding force, pulling force, and working load.
