Put a magnet against a copper housing and nothing happens. Copper is not magnetic in the sense that a buyer means when they ask the question, but the physics is more interesting than a simple no, and the answer changes as soon as the copper is an alloy, is cold worked, or is sitting next to a steel part.
Is copper magnetic?
No. Copper is diamagnetic: it is very slightly repelled by a magnetic field, with a relative permeability just below 1 (about 0.999994). It cannot be picked up by a magnet and it cannot be magnetised and hold a field. That is true of pure copper (C11000) and of most copper alloys, including brass and bronze, which are also non-magnetic in practical terms. The practical consequence is that copper is used where a part must not disturb a magnetic field: sensor housings, shielding, busbars, motor and transformer windings. It is also why a magnet is a quick shop check - if a magnet sticks to a "copper" part, the part is not copper, or there is a steel insert, plating or fitting inside it.
Diamagnetic, paramagnetic, ferromagnetic - what the words mean
- Diamagnetic - slightly repelled. Copper, silver, gold, lead, water. Relative permeability a hair below 1. You will never feel this with a hand-held magnet.
- Paramagnetic - very slightly attracted. Aluminium and most austenitic stainless steels sit here; permeability a hair above 1.
- Ferromagnetic - strongly attracted and able to hold a field. Iron, nickel, cobalt, and the ferritic stainless grades such as 430 and 430F, where permeability runs from hundreds to thousands.
- Work-induced magnetism - austenitic stainless (304, 316) is paramagnetic in the annealed state, but cold forming or drawing converts some of the structure to martensite, so a deep drawn 304 housing can show a weak attraction while the same material in sheet form does not.
Relative permeability of materials we machine and draw
| Material | Behaviour | Relative permeability | Magnet test |
|---|---|---|---|
| Copper C11000 | Diamagnetic | ≈ 0.999994 | No attraction |
| Brass (CuZn) | Diamagnetic | ≈ 0.99999 | No attraction |
| Aluminium 1050 / 5052 | Paramagnetic | ≈ 1.00002 | No perceptible attraction |
| 304 stainless, annealed | Paramagnetic | ≈ 1.002–1.02 | Very weak or none |
| 304 stainless, deep drawn | Paramagnetic + strain martensite | Up to ≈ 1.5+ locally | Weak attraction possible |
| 430 / 430F stainless | Ferromagnetic | ≈ 500–1,700 | Strong attraction |
| Pure iron / DT4E | Ferromagnetic | Several thousand | Strong attraction |
Values are the normal published ranges and vary with grade, cold work and field strength. They are here to show the orders of magnitude - copper and aluminium are effectively transparent to a magnetic field, 430F and pure iron are not.
Why this matters when a part is specified
- Sensor housings - a copper or 304 housing does not bend a magnetic field around the sensing element, so a Hall-effect or inductive sensor reads what it is supposed to read. A 430F fitting on the same assembly does the opposite, which is why the material of each part on the assembly has to be stated.
- Shielding - non-magnetic metals shield by eddy currents rather than by flux diversion, so copper and aluminium work for RF and high-frequency fields while a ferromagnetic steel is the choice for low-frequency magnetic shielding.
- Assembly and test - a magnet is the fastest incoming-inspection check on a mixed box of fittings. It separates 430F from 304 in a second, and it catches steel inserts or plated parts that were not declared.
- Welding and joining - ferromagnetic behaviour changes arc behaviour and fixture design, which is one reason a copper part and a 430F part are rarely welded in the same fixture.
Where copper housings actually get used
Copper earns its place where thermal conductivity matters as much as the magnetic answer. Temperature sensor housings for coolant circuits and battery thermal loops are the clearest example: the housing has to move heat to the sensing element quickly, hold a thread and a seal, and keep out of the magnetic argument altogether. We draw copper housings in progressive dies tooled in-house - twelve housing variants on tooling we own, built to customer drawings - and we also turn copper and stainless fittings on the same plant, so a mixed assembly can be quoted as one route.
Key point
Copper is not magnetic, and neither is brass, bronze or annealed 304. The materials that are magnetic are the ones people forget: 430 and 430F stainless and pure iron. If a part number does not state the grade, the magnet test on the assembly is where the mistake shows up - usually after the fittings have already been bagged.
Frequently asked questions
Is copper magnetic yes or no?
No. Copper is diamagnetic - very slightly repelled - so a magnet will not pick it up and copper cannot be magnetised.
Is brass magnetic?
No. Brass is a copper-zinc alloy and is non-magnetic in all normal compositions, so brass fittings behave like copper in a magnetic field.
Is 304 stainless steel magnetic?
Annealed 304 is essentially non-magnetic. Cold working - drawing, bending, thread rolling - can form a little strain-induced martensite, so a heavily drawn 304 housing may show a weak attraction while flat sheet does not.
Which stainless grade is magnetic?
The ferritic and martensitic families, including 430 and 430F. They are strongly attracted to a magnet, which is why we quote sensor hex nuts in either 430F or 304 depending on whether the assembly can tolerate a magnetic fitting.