How Do Magnetic Door Catches Work?

Push a kitchen cupboard door almost closed and, in the last few millimetres, it seems to pull itself shut with a soft click. Open it again and you feel a brief resistance before it lets go. There is no latch, spring or hook involved, only a small plastic or metal block on the cabinet and a thin metal plate on the door.

Magnetic catches are so ordinary that they are easy to overlook, yet they are a neat piece of applied magnetism. Understanding how they work explains why they suddenly ‘grab’ the door, why some stop holding properly over time, and why the same catch can feel strong on one door and weak on another.

The basic principle: completing a magnetic circuit

A permanent magnet produces a magnetic field that loops out of one pole and back into the other. That field travels far more easily through iron and steel than through air. Engineers often describe this using the idea of a magnetic circuit: steel parts act like low-resistance ‘wires’ that guide the field, and air gaps act like resistors that weaken it.

Ordinary steel is attracted to a magnet because it is ferromagnetic. Inside it are tiny regions called magnetic domains, each behaving like a miniature bar magnet. In an unmagnetised piece of steel these domains point in random directions and cancel out. When a magnet comes near, the domains in the steel align so that an opposite pole forms on the side nearest the magnet, and unlike poles attract. A magnetic catch is designed to make that attraction as strong as possible at the exact moment the door is closed, and to give it a short, all-steel path to follow.

How it works, step by step

1. The magnet and its pole pieces. Inside the catch body, a block-shaped permanent magnet is sandwiched between two flat steel plates. Each plate collects the field from one face of the magnet and carries it to an edge at the front of the catch.
2. The open door. With the door open, the field has to leap across air from one pole piece to the other. Air is a poor path, so the field spreads out and the pull at any distance is modest.
3. The door approaches. As the steel strike plate on the door comes within a few millimetres of the pole pieces, the field starts to pass through it. The attraction rises steeply as the gap shrinks, which is why the door seems to be pulled the last part of the way.
4. Contact. When the strike plate touches both pole pieces, it completes the loop: magnet, first pole piece, strike plate, second pole piece, back to the magnet. Almost the entire path is now steel, and the holding force is at its maximum.
5. Opening. Pulling the door separates the plate from the pole pieces. The force needed falls off rapidly once a small gap opens, so the door releases cleanly rather than dragging.

The science behind it

Why pole pieces make a small magnet stronger

An early US patent for a magnetic catch, filed by Elmore A. Heppner in 1954 and granted in 1960, describes a pressed ferrite magnet held between two rectangular pole pieces whose straight edges stick out to meet a steel plate on the door. The same principle appears in ‘pot’ magnets used in industry. Eclipse Magnetics explains that the steel casing of a pot magnet directs the field straight to the surface being held, with the steel acting as the opposite pole, so the field completes its loop over a very short path. Removing air gaps from that path gives the most holding force for the size of magnet.

Why gaps matter so much

Manufacturers rate a magnet’s pull by pressing it flat against a thick, smooth, low-carbon steel plate and pulling straight off. Real catches rarely meet those ideal conditions. Radial Magnets, a US supplier, gives the following indicative figures for how much of the rated pull a small disc magnet keeps as a gap is introduced. Paint, plating, dirt and plastic covers all count as gap.

The exact curve depends on the magnet’s shape, but the pattern is the important part: a gap thinner than a sheet of card can cost a large share of the force.

Approximate worked example: imagine a catch whose pull is listed as 3 kg in ideal conditions. If a painted strike plate leaves an effective gap of about 0.5 mm, the table suggests it might keep roughly 40–60% of that, or about 1.2–1.8 kg of pull. These are illustrative numbers only; real catches vary.

The steel matters too

Thin steel can become magnetically ‘full’ (saturated) and cannot carry the whole field, so a thin strike plate gives less pull than a thick one. The type of steel matters as well. Radial Magnets notes that low-carbon steel performs best, 400-series stainless steel holds noticeably less, and 300-series stainless steel is essentially non-magnetic. That is one reason a decorative stainless plate may not work as a strike plate. The site’s article on why stainless steel does not rust easily explains how these steel families differ.

Sliding versus pulling

A catch is strongest when the door pulls straight away from it. If the load tries to slide the plate sideways instead, only friction resists it. Eclipse Magnetics states that a standard magnet begins to slide at about 20% of its straight pull, and a pot-style magnet at about 33%. This is why magnetic catches are suited to holding doors closed rather than supporting weight.

Which magnet is inside?

Heppner’s patent specifies a pressed ferrite (ceramic) magnet, with alnico as an alternative, and the Baermann fridge-seal patent below also uses ferrite. Ferrite and neodymium are two widely used permanent-magnet materials, and they trade strength against other properties:

Figures from Magnet Expert’s comparison of magnetic materials. Because ferrite is weaker, ferrite magnets need to be relatively large to do the same job, which suits the chunky body of a traditional catch.

Real-world applications

• Cupboards and furniture. The classic use: a catch on the carcass and a strike plate on the door.
• Push-to-open (touch) latches. A 1970 US patent by Harold D. Hutchinson describes a catch in which the magnet and its pole pieces can slide backwards. Pressing the door pushes the magnet onto a fixed steel piece, which gives the field an easy short-cut path at the back and weakens the pull at the front. A spring then pushes the door open, so no handle is needed.
• Refrigerator door seals. A 1968 US patent by Max Baermann describes a door gasket containing a flexible magnetic strip made from barium ferrite particles in a flexible binder, with alternating north and south poles along its face. The strip pulls the gasket against the steel cabinet all the way round, sealing the door without a mechanical latch.

Common misconceptions

‘A stronger magnet always makes a better catch’

Holding force is only useful up to a point: a door also has to be opened by hand without wrenching the hinges or the cabinet. In practice, alignment and contact usually matter more than raw magnet strength, because a small gap can remove most of the available force.

‘Magnetic catches wear out because the magnet loses its strength’

Ferrite magnets are described by suppliers as having good magnetic stability. A permanent magnet can be weakened by strong heating or hard blows, but a catch that stops holding has more often shifted out of alignment. Heppner’s 1954 patent noted exactly this problem with early catches: fittings moved over time, reducing face-to-face contact, and magnets could crack when doors were slammed.

‘The rated force tells you what the door will hold’

Rated figures assume perfect contact with thick, clean steel and a straight pull. Paint, thin plates, misalignment and sideways loads all reduce the real-world holding force, sometimes to a small fraction of the rating.

Limitations and exceptions

• Alignment. If the strike plate meets only one pole piece, or sits at an angle, the circuit is incomplete and the catch feels weak. Adjusting the catch or plate usually restores the hold.
• Non-magnetic doors. Aluminium, brass, wood and plastics are not attracted, so a separate steel strike plate is always needed.
• Heat. Permanent magnets lose strength if heated too much, and neodymium magnets have relatively low operating limits, so catches should not be placed next to ovens or heat sources beyond their rating.
• Child safety and security. A magnetic catch is not a lock. Where children or hazardous contents are involved, use purpose-made safety latches, and seek advice from a qualified installer for anything structural or safety-critical.
• Loose magnets. As a general precaution, keep spare magnets and broken catches away from young children.

Header photo: Janay Peters on Unsplash.