Will it stick? Magnet strength, thickness and what it can hold

Pull force is the measured attraction force between a magnet and a steel surface. Manufacturers usually quote it in kilograms, meaning the straight-line weight needed to pull the magnet directly off the plate it was tested on. A figure such as 1.5 kg pull force tells you the magnet resisted a 1.5 kg pull before separating, under the specific test conditions the manufacturer used.

Why the rating assumes a thick steel plate

A magnet's field needs somewhere to go. When a magnet sits against steel, the steel carries the magnetic field between the two poles, a path engineers call the magnetic circuit. If the steel plate is thick enough, it completes that circuit fully and the magnet performs at its rated strength. Manufacturers test pull force on a plate of solid, thick mild steel for this reason, because it lets the magnet reach its full potential without the steel itself becoming the limiting factor.

Why a fridge door is not that plate

A fridge door is not a solid steel block. It is a thin steel sheet, often under a millimetre, wrapped around an insulating foam core, and on many models the front panel is stainless steel or even a glass composite. A thin sheet cannot carry as much magnetic field as a thick plate, so it does not complete the circuit the same way, and the magnet delivers less than its rated pull force against it. This is why a magnet rated at 1.5 kg on a test plate can feel noticeably weaker on an actual kitchen door, and it is a mismatch between the test and the target surface.

If your magnets are sliding down the door despite a reasonable pull force rating, the door itself is worth checking before you assume the magnet is underpowered. The stainless steel and glass-door guide covers which fridge surfaces are magnetic at all, and the guide to what fridge magnets are made of explains the material differences behind the strength figures printed on packaging.

Three things decide whether a magnet holds: the thickness of the magnetic material, the area of the magnet in contact with the door, and the gap between the magnet's face and the steel underneath it. Get any one of these wrong and the magnet slides, however good the artwork looks.

Thickness and what it can lift

Flexible magnetic sheet is sold in standard thicknesses, and each step up genuinely changes what the magnet will hold. A 0.4 mm magnet will hold its own printed face flat against a clean ferritic steel door but has little spare capacity: it will not also hold a sheet of paper. A 0.6 mm magnet has enough spare pull to hold a single sheet or two behind it, which covers a takeaway menu or a business card. A 0.75 mm or 0.85 mm magnet is what you specify for a calendar, where the printed card itself has weight and the magnet needs to hold that plus a few sheets pinned behind it through the year.

Area matters as much as thickness

A large thin magnet can outperform a small thick one, because pull force is generated across the whole area in contact with the door. A credit-card-sized 0.4 mm magnet has very little surface to work with, so it holds itself and nothing more. An A5 magnet at the same 0.4 mm thickness has roughly four times the contact area and will comfortably hold a couple of sheets of A4. If a magnet needs to carry weight rather than just sit flat, increasing the area is often the cheaper fix compared with jumping a thickness band, though a supplier will quote both options against the finished size you need.

The gap you cannot see

Lamination, the clear protective film bonded over the printed face, and any air gap from an uneven fridge surface both sit between the magnet and the steel, and pull force drops sharply as that gap widens. A magnet rated for direct steel contact can lose a noticeable share of its holding power once it is working through 0.2 mm of laminate and a door that is not perfectly flat. This is why a magnet that held well in a supplier's sample pack sometimes disappoints on a textured or curved fridge door: the gap has grown.

What to specify

• Single sheet or business card behind it: 0.6 mm flexible magnetic sheet, standard business-card or A6 size
• Calendar with card stock and monthly tear-offs: 0.75 mm to 0.85 mm, sized to the calendar backing
• Large format, decorative use only, nothing held behind it: 0.4 mm is normally sufficient, provided the door itself is a magnetic grade

Confirm the exact thickness and lamination spec with your printer before ordering, since finishes vary between suppliers and the figures above are a starting point for the conversation. For the reasoning behind pull force ratings and why a fridge door behaves differently from the flat steel plate suppliers test against, see Will It Stick? Magnet Strength, Thickness and What It Can Hold. If the door itself might be the problem rather than the magnet, the stainless steel and glass-door guide covers which fridge surfaces are magnetic at all.