There are two types of magnets commonly used for fridge magnets. Ferrite magnets are the dark coloured hard magnets that are often used for magnetic souvenirs as they are cheap however they are not very strong. If you wish to use the magnet to hold up paper on the fridge then Ferrites may not be strong enough.
Neodymium magnets have a chrome finish and are considerably stronger than Ferrite magnets however they are more expensive. As Neodymium is a stronger magnetic material you would achieve the same or greater holding power from a magnet with smaller dimensions.
A couple of other factors to consider are:
- The Pull Force listed for each magnet is based on lifting 10mm thick steel vertically. Fridges normally have only 1mm or 2mm thick steel and the magnetic field is not able to achieve full saturation of the steel or its full holding power.
- Magnets on a vertical surface such as a fridge are generally able to hold around 30% of this strength due to the effects of gravity and the lack of traction on the smooth fridge surface.
- Also, any gap created (such as paper) between the magnet and the metal it is magnetically adhered to will also slightly diminish the holding strength of the magnet.
There are a surprising number of variables at play when using magnets and most applications require some trial and error to select the correct magnet for your project.