How many magnets do I need? How to calculate it
Short answer: required holding power = weight × safety factor. Divide that by the holding power per magnet. The safety factor depends on how the magnet is loaded:
- Pulling straight (something hangs under a steel ceiling or beam): factor 2 – 3.
- Sideways on a vertical wall (sign, panel, hook on the side of a cabinet): factor 5 – 10, because the magnet already slides at 15 – 25% of its holding power.
- Through paint, film or a gap, or with vibrations: even more.
Example: a 2 kg sign on a steel wall needs 2 × 8 = approx. 16 kg holding power. With disc magnets Ø20 x 5 mm (approx. 4.9 kg) you need 4 of them.
Calculation examples
| Situation | Weight | Factor | Needed | 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) |
| Sign on a steel wall | 2 kg | 8 | 16 kg | 4 × disc Ø20 x 5 mm (4.9 kg) |
| Closing a cabinet door | – | – | 1 – 3 kg | 1 × disc Ø10 x 3 mm (1.6 kg) per door |
| Poster or A4 on a whiteboard | 0.05 kg | 10 | 0.5 kg | 1 × cube 5 mm or disc Ø8 x 2 mm |
| Tool 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 to magnet |
The holding power per size can be found in how strong is a neodymium magnet? and on every product.
Step by step
- Weigh what you want to attach.
- Determine the direction: straight away from the steel, or sideways along the steel.
- Check the surface: thick steel, thin sheet metal, paint or stainless steel? Thin or painted steel: an extra factor of 2. Stainless steel 304/316 does not work: see is stainless steel magnetic?
- Check the distance: through material the force drops quickly. See do magnets work through wood and glass?
- Multiply by the factor and divide by the holding power per magnet. Round up.
- Distribute the magnets over the surface, for example in the corners, so nothing can tilt.
- Test with the actual weight before final mounting.
Rather not 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 roughly add up. Stacked, the magnet becomes thicker and stronger, but not double. One thicker magnet often works better than two thin ones.
May I use a magnet for something that is dangerous if it falls?
Not as the only fastening. Holding power is not a working load. See holding power, pull force and working load.
