Fridge Magnets

The physics of fridge magnets

Put two magnets on the counter. A 12 mm neodymium disc reads about 4,000 Gauss at its face. A printed sheet magnet cut from a takeaway menu reads about 300 Gauss. That is thirteen to one on the field reading. On weight held, the disc beats the sheet by roughly forty to one. The Gauss figure never told you that, and it never will.

Two ideas carry the rest of this section, and once they are in place the individual pages are mostly arithmetic.

Pull force is the number that matters

Pull force is the weight a magnet holds before it comes away from steel. Manufacturers measure it with the magnet in direct contact with a thick, flat, bare steel plate, pulled straight off at right angles. Grams or kilograms, one number, comparable between a ferrite disc and a neodymium one because the test does not care what the magnet is made of.

Gauss measures flux density at a single point, usually the center of the pole face. It tells you how hard the field is right there and nothing about how much steel the magnet is gripping. Widen a disc from 6 mm to 10 mm at the same 2 mm thickness and the surface reading barely moves, while the hold goes from 700 g to 1,400 g. Two magnets can share a Gauss figure and differ by a factor of ten in what they will carry. Magnet strength explained works through the whole business, grades included.

The door takes some of it back

Every pull force figure you read was measured under conditions your fridge does not meet. The plate was bare. Your door is painted, and paint adds 0.1 to 0.2 mm of gap. It may be textured, which means the magnet rides on the high points. It is probably slightly curved, so a rigid 64 mm square touches at two edges rather than across its face.

Each tenth of a millimeter of gap costs a small disc roughly 10 percent of its pull. Add texture and curvature and a painted door gives back 60 to 80 percent of the plate figure. Multiply by 0.7 and you will not be far off. That correction applies to every number on this site, so we quote the plate figure and let you do the multiplication once you know your own door.

Some doors take all of it. Austenitic stainless, grade 304, is not magnetic at all: nickel stabilises a face-centered cubic structure that holds no magnetic domain, and a magnet slides off it like it slid off the tiles. Ferritic stainless, grade 430, is magnetic and looks identical in the showroom. The side panels of most fridges are painted mild steel whatever the door is doing, which is the usual rescue.

What each family holds

Family What it is Typical hold
Flexible sheet, 0.5 mm ferrite powder in a vinyl binder 60 g at 50 x 50 mm
Flexible sheet, 0.75 mm the same, thicker 110 g at 50 x 50 mm
Pressed photo magnet flat ferrite in a crimped steel shell not published, several times a sheet magnet
Ceramic ferrite disc, C8 pressed and sintered iron oxide ceramic 500 g at 20 x 3 mm
Neodymium disc, N42 sintered NdFeB, nickel plated 3,500 g at 20 x 3 mm
Neodymium pot with hook an N42 core in a steel cup 6,000 g at 16 mm

The spread inside that table is larger than most buyers expect. A 20 x 3 mm neodymium disc holds about seven times what a ceramic ferrite disc of exactly the same dimensions holds, and about sixty times a 0.5 mm sheet magnet covering a 50 mm square of door. A pressed photo magnet, which is only a flat ferrite disc in a steel shell, holds several times a 0.75 mm sheet magnet of the same footprint, and that row stays wordy because no seller publishes a pull force for the pressed build. Nobody is cheating. The materials store different amounts of energy per unit of volume, and the steel shell in a pressed magnet channels the field forward instead of letting half of it leak out the back.

Heat and time sort them differently. Ceramic ferrite works up to about 250 C and does not care. Neodymium starts losing strength permanently above about 80 C, which rules it out of anything that sits on a hot appliance, and it drops roughly 1 percent per decade at room temperature. That is a slow enough loss that the plating failing and the magnet rusting will matter first. How long magnets last covers the failure modes in order.

What we have not measured

Every figure here is a published or manufacturer number. We have not put any of these magnets on a gauge ourselves, and until we do, no page on this site carries a star rating or a test result. Where a brand does not publish a thickness, a price, or a minimum order, we write that it is not published rather than filling the gap with something plausible. The strength chart lists everything we have in one place, and how we test describes what would have to change for those numbers to become our own.

Ferrite and neodymium disc magnets with a digital force gauge and iron filings showing field lines