Magnet strength explained, pull force and Gauss

By Fridge Magnets Editorial

Pull force is the weight a magnet holds on flat bare steel in direct contact, and it is the only strength figure worth comparing. Gauss reads the field at a single point. A 12 mm N42 disc shows about 4,000 Gauss and holds 2,500 g; a flexible sheet shows about 300 Gauss and holds 60 g.

One listing says 4,000 Gauss. The next says 2.5 kg pull. A third says N42 and leaves it there. Only one of those three tells you whether the magnet will hold your kid's drawing on the fridge, and the other two are quoted far more often, usually by sellers who know that a four-digit number looks impressive next to a one-digit one.

What pull force measures, and under what conditions

Pull force is the weight a magnet supports before it separates from steel. It is measured by pressing the magnet against a plate and pulling it straight away, at right angles, until it lets go. The reading at the moment of release is the figure that goes on the listing.

The plate is specific, and the specification is generous. It is thick enough that the magnet cannot saturate it, so all of the field has somewhere to go. It is flat, ground rather than rolled. It is bare steel, with no paint, no powder coat, no lacquer. And the magnet is in direct contact with it, dry and clean. Every one of those conditions is better than your fridge, which is the reason the numbers on this site come with a correction factor a few paragraphs down.

Two more details change a pull figure without anyone lying about it. A magnet pulled off another magnet reads higher than the same magnet pulled off a plate, so a figure quoted magnet-to-magnet is not comparable with one quoted magnet-to-steel, and the listing rarely says which was used. And pull force is quasi-static, measured with a slow steady pull. Yank a magnet off and it comes away easier than the number suggests, which is worth knowing when you are trying to get one off a door without gouging the paint.

The numbers, side by side

One row is worth staring at. A 6 x 2 mm neodymium disc, small enough to lose in a rug, holds 700 g, more than a ferrite disc more than three times its diameter. Size and strength are separate axes, and only the pull column resolves them.

The pressed photo magnet is missing from that table on purpose. It is the magnet most people picture when they say photo magnet, and not one seller we cover publishes a pull force for it, so there is no figure to list and we have not put one on a gauge. All we will say is the comparison the build supports, which is that a solid ferrite disc in a steel shell holds several times what a flexible sheet magnet of the same footprint holds.

The other thing the table shows is where the steel goes. The 16 mm pot magnet at 6,000 g contains less neodymium than the 10 mm cube at 4,800 g. The pot has a steel cup around it that returns flux through the front face instead of letting it spill out the back, and that alone is worth more than the extra material. How fridge magnets work covers why.

Gauss reads one point and calls it strength

Gauss is a unit of magnetic flux density, and a gaussmeter probe reads it wherever you put the probe. Held against the center of a pole face, a 12 mm N42 disc reads about 4,000 Gauss. A flexible sheet magnet reads about 300 Gauss.

That ratio is roughly thirteen to one. The ratio of what those two hold is not thirteen to one, because surface field says nothing about how much steel the magnet is working on or how far the field reaches before it dies away. A wide, weak magnet spreads a modest field over a large area of door and can outhold a narrow, fierce one. The flexible sheet loses anyway, badly, but it loses for reasons the Gauss figure does not contain.

Surface Gauss also drops with distance far faster than most buyers expect, and it is measured at zero distance by definition. Two magnets with the same surface reading and different geometries will be nowhere near each other one millimeter off the face. Pull force at least integrates the whole field over the whole contact patch, which is the quantity you care about. Gauss is a useful number for engineers positioning a sensor. It is a marketing number on a fridge magnet listing.

What N42 is a number for

The N in N42 is neodymium. The 42 is the maximum energy product of the material, 42 MGOe, mega-gauss-oersteds, which is a measure of how much magnetic energy the material stores per unit of volume. It is a property of the alloy and the sintering, not of the magnet in front of you.

That distinction is the one people trip over. Grade tells you what a cubic millimeter of the stuff is worth. Pull force tells you what this particular magnet does. A 6 x 2 mm N42 disc and a 20 x 3 mm N42 disc are the same grade and differ by a factor of five in holding weight, because one has far more volume than the other and a much wider face to spread it across. Buying a higher grade at the same size buys you a modest percentage. Buying a bigger magnet at the same grade buys you multiples.

Grades also carry temperature suffixes, letters after the number that mark higher-temperature variants. Plain N42 is listed at about 80 C maximum working temperature, which covers every kitchen and rules out anything that sits on a warm motor. Ceramic ferrite has no N number at all. Its grades run C1 through C8 and up, and C8 is the one you meet in fridge magnets and craft supplies. The ferrite, neodymium, and flexible comparison puts the material properties side by side.

Thickness and diameter are separate levers

Neither dimension behaves the way a first guess suggests. Doubling the thickness of a small disc adds 40 to 70 percent, not 100 percent, because past a certain point the extra material sits too far from the steel to contribute much. Doubling the diameter does more, since it recruits new contact area as well as new volume: a 6 x 2 mm N42 disc holds 700 g and a 10 x 2 mm holds 1,400 g, an exact doubling for less than a doubling of width.

Flexible sheet is the exception that proves the thickness rule. Going from 0.5 mm to 0.75 mm raises a 50 mm square from 60 g to 110 g, an increase of more than 80 percent, because at those thicknesses the magnet is so thin that every extra fraction of a millimeter is still doing useful work close to the steel. This is why 0.75 mm sheet is worth asking for and why 0.5 mm feels like a fridge magnet that has given up.

The clearest lever of all is material. Same 20 mm diameter, same 3 mm thickness, ferrite against N42, and the figure goes from 500 g to 3,500 g. No amount of geometry closes a gap of seven times.

Paint, texture, and the 0.7 rule

Take any figure from the table and multiply it by 0.7. That is the number for a painted door, and it is not a hedge. Paint contributes 0.1 to 0.2 mm of gap on its own. A textured or brushed finish holds the magnet up on its high points, so the effective gap is set by the peaks rather than by the film. A curved door adds more for anything wide and rigid, which is why a 64 mm square pressed magnet can do worse on a real door than a 57 mm round one despite the extra face. Each tenth of a millimeter of gap costs a small disc roughly 10 percent, and painted doors in practice return 60 to 80 percent of the plate figure.

Anything you put underneath the magnet joins the gap. A thick photo print, a stack of receipts, a folded takeaway menu, all of them push the magnet away from the steel and all of them cost you at the same steep rate. A magnet holding twelve sheets of paper is a magnet working across a gap of a couple of millimeters, which is a different magnet entirely from the one in the test lab.

Then there is the door that gives back nothing. Austenitic stainless, grade 304, has a face-centered cubic structure stabilized by nickel and holds no magnetic domain, so a magnet slides straight down it. Ferritic stainless, grade 430, is magnetic and looks the same in the shop. Most fridges have painted mild steel side panels regardless, and 40 to 60 cm of usable width on the door itself when the door cooperates.

Working out what you need

Weigh the thing you want to hold, on a kitchen scale, before buying anything. Then take the plate figure for the magnet, multiply by 0.7 for a painted door, and leave a wide margin on top of that, because the load on a vertical door is shear and no manufacturer publishes a friction figure for their backing against appliance paint. A magnet that holds 350 g on a real door in a straight pull will start to creep with much less than 350 g hanging off it.

Run the usual cases through that. A photo or a postcard needs almost nothing, and a 0.5 mm sheet magnet at 60 g per 50 mm square is already overspecified. School notices, permission slips, and the accumulated paperwork of a family need a 20 x 3 mm ferrite disc at 500 g, which lands at about 350 g once the door has taken its share, or a 25 x 5 mm at 1,100 g if the pile is a thick one. A pressed photo magnet has a ferrite disc of its own behind the print and will beat any printed sheet by a wide margin, but no seller publishes what it holds, so buy that one for the picture rather than for a number. A tea towel, an oven glove, or anything with a loop wants a 16 mm neodymium pot with a hook at 6,000 g, where the margin is so large that shear stops being the limit. And a whiteboard or a magnetic knife strip is a different problem again, since the load hangs off the magnet rather than sitting under it.

Work in the other direction and the arithmetic still holds. If a 10 x 2 mm N42 disc at 1,400 g is not holding, the answer is rarely a stronger magnet of the same width. It is a wider one, a second one, or something between the magnet and the paint with more grip than nickel plating. The strength chart has every figure on this page in one sortable place.