In the application of magnets, magnetic flux or flux density is an important indicator to measure its performance (especially in motors), but in some application fields, such as magnetic sorting and salvage, magnetic flux is not an effective measure of sorting or adsorption effect. Magnetic attraction is a more effective indicator.
Magnetic attraction refers to the weight of ferromagnetic material that a magnet can adsorb. It is affected by factors such as the performance, shape, size and adsorption distance of the magnet. There is no mathematical formula to calculate the size of the attraction of a magnet, but we can measure the magnetic attraction value through a magnetic attraction measurement device (generally measuring the pulling force of the magnet and then converting it into weight), as shown in the figure below. The attraction of a magnet will gradually decrease as the distance of the adsorbed object increases.
If you search for magnet attraction calculations online, many websites will say “According to experience, the magnetic force of NdFeB magnets is 600 times their own weight (some say 640 times)”. Is this experience correct? We can verify it through experiments.
The experiment selected sintered NdFeB N42 magnets of different shapes and sizes, with nickel-copper-nickel coatings on the surface, and magnetized in the height direction. The maximum pulling force (N pole) of each magnet was measured and converted into adsorption weight. The measurement results are as follows:
| Shape | Diameter（mm） | Height（mm） | Volume（cm3） | Weight（g） | Attraction weight(kg) | Attraction weight/magnet weight |
| Disc Magnet | 6 | 1 | 0.03 | 0.21 | 0.4 | 1887 |
| 6 | 2 | 0.06 | 0.42 | 0.8 | 1887 | |
| 6 | 3 | 0.08 | 0.64 | 1.2 | 1887 | |
| 6 | 4 | 0.11 | 0.85 | 1.5 | 1769 | |
| 6 | 5 | 0.14 | 1.06 | 1.9 | 1793 | |
| 6 | 6 | 0.17 | 1.27 | 2.1 | 1651 | |
| 8 | 1 | 0.05 | 0.38 | 0.5 | 1327 | |
| 8 | 2 | 0.1 | 0.75 | 1 | 1327 | |
| 8 | 4 | 0.2 | 1.51 | 2 | 1327 | |
| 8 | 5 | 0.25 | 1.88 | 2.5 | 1327 | |
| 20 | 0.5 | 0.16 | 1.18 | 0.6 | 510 | |
| 20 | 1 | 0.31 | 2.36 | 1.3 | 552 | |
| 20 | 3 | 0.94 | 7.07 | 3.9 | 552 | |
| 20 | 5 | 1.57 | 11.78 | 6.4 | 544 | |
| 20 | 10 | 3.14 | 23.55 | 12.5 | 531 | |
| Rod Magnet | 8 | 20 | 1 | 7.54 | 10.1 | 1340 |
| 8 | 30 | 1.51 | 11.3 | 14.9 | 1318 | |
| 10 | 10 | 0.79 | 5.89 | 6.2 | 1053 | |
| 10 | 20 | 1.57 | 11.78 | 12.6 | 1070 | |
| 10 | 30 | 2.36 | 17.66 | 18.8 | 1064 |
| Shape | Length（mm） | Width（mm） | Height（mm） | Volume（cm3） | Weight（g） | Attraction weight(kg) | Attraction weight/magnet weight |
| Block Magnet | 3 | 3 | 3 | 0.03 | 0.2 | 0.7 | 3457 |
| 3 | 3 | 8 | 0.07 | 0.54 | 1.5 | 2778 | |
| 5 | 5 | 2 | 0.05 | 0.38 | 0.7 | 1867 | |
| 5 | 10 | 2 | 0.1 | 0.75 | 1 | 1333 | |
| 20 | 10 | 5 | 1 | 7.5 | 5.1 | 680 | |
| 22 | 7 | 2 | 0.31 | 2.31 | 1.8 | 779 | |
| 22 | 11 | 2 | 0.48 | 3.63 | 2.2 | 606 | |
| 50 | 25 | 10 | 12.5 | 93.75 | 24.8 | 265 | |
| 50 | 50 | 5 | 12.5 | 93.75 | 18.1 | 193 | |
| 50 | 50 | 25 | 62.5 | 468.75 | 88.5 | 189 |
It is not difficult to find from the measurement results:
The ratio of the weight that magnets of different shapes and sizes can absorb to their own weight varies greatly, some are less than 200 times, some are more than 500 times, and some can reach more than 3000 times, so the 600 times written on the Internet is not completely correct
For cylinders or round cakes with the same diameter, the greater the height, the greater the weight that can be absorbed, and the suction force is basically proportional to the height
For cylinders or round cakes with the same height (blue cells), the greater the diameter, the greater the weight that can be absorbed, and the suction force is basically proportional to the diameter
For cylinders or round cakes of the same volume and weight (yellow cells), the diameter and height are different, and the weight that can be absorbed varies greatly. Generally, the longer the orientation direction of the magnet, the greater the suction force
For magnets of the same volume, the suction force is not necessarily equal. Depending on the shape, the suction force may vary greatly
The same is true in reverse. Magnets that absorb the same weight of ferromagnetic materials may have different shapes, volumes and weights
No matter what the shape, the length of the orientation direction has the greatest influence on the attraction force.