Every neodymium magnet has a pull force, which tells you exactly how strong that magnet is. Measured in pounds or kilograms, the pull force is the force required to pull that magnet straight free from a 1/8 inch (3.175 mm) thick steel plate. This means that the pull force also tells you the limit on the holding power of the magnet.
Generally, any magnet with a pull force above seven pounds (3.175 kg) can pinch your fingers. Stronger magnets can be even more dangerous and should only be handled by experienced individuals.
Magnet placement can dramatically affect pull force.
While the pull force rating enables you to calibrate how much weight or tension a magnet will hold, placement is also an important part of the equation.
A magnet placed vertically, such as on the underside of a steel beam or table, will hold it's listed pull force weight.
A magnet mounted horizontally, such as on the side of a refrigerator or file cabinet, will only hold 30 percent or less of it's listed pull force due to gravity and the potential for the magnet to slide down the attachment surface.
Also, magnets stick best to ferromagnetic surfaces and do not stick to chrome, brass, aluminum, silver, gold, wood, plastic or tile. Attaching magnets to these surfaces is best accomplished with countersunk or self-adhesive magnets.