The rule of thumb, and why it matters
A magnet holding to a mating magnet is roughly twice as strong as the same magnet holding to a steel plate. If a datasheet quotes magnet-to-magnet and your assembly is magnet-to-steel, you have about half the force you planned for — and you will only find out on the bench.
The steel is part of the specification
Thickness matters: too thin and the plate saturates, so extra magnet buys you nothing. So does the alloy — mild steel pulls well, most stainless grades pull far less or not at all, and nobody discovers that until the prototype. If the mating part is stainless, say so before anyone quotes.
Surface, coating and gap
Paint, powder coat, plating and even a sticker are all an air gap, and force falls away fast with gap. A quoted contact force assumes bare metal touching. Half a millimetre of paint is a real reduction, not a rounding error.
What to give a supplier
The mating material and its thickness, the finish on it, the working gap, the force you need at that gap, the temperature, and whether it is outdoors. Those six answers turn a guess into a quote.
Off-the-shelf, if that is all you need
General magnets and mounting hardware are on Amazon and those are affiliate links. Engineered coded parts are not — describe the job instead and we will tell you what fits.
Magnetic bases and holdersSteel mounting platesMagnetic viewing filmNot off the shelf?
Describe what the part has to do — the force, the gap, the material it mates to and where it lives. We will tell you what fits, including when the answer is an ordinary magnet.
Describe the jobMore on the underlying idea: how coded magnets are built pixel by pixel · choosing the material · the catalogue, sorted by behaviour