How close and strong a magnet must be to disrupt phone compass, cards

Part of our guide on magnetic mounts and overheating

Measured and conditional guidance — how magnet grade, size, and distance determine compass errors, card demagnetization, and sensor interruptions, with sources and test methods.

How phone compass (magnetometer) sensitivity and magnets interact

Phone magnetometers detect the Earth’s magnetic field (typically ≈25–65 µT). Interference happens when a nearby magnet produces a local field at the sensor location that is comparable to, or larger than, the Earth’s field or the sensor’s expected operating range. Magnetometer modules used in phones (examples: Bosch BMM150, AKM AK8963) have documented ranges and resolutions — consult the sensor datasheet for specific sensitivity and tolerance (see references).

Because a permanent magnet’s field drops quickly with distance (a dipole field falls off roughly with the inverse cube of distance at distances larger than the magnet), the distance at which a given magnet perturbs a phone’s magnetometer varies with magnet grade (N35–N52), size, orientation, and any magnetic shielding or phone chassis layout. For small neodymium discs, surface fields can be tens to several hundred millitesla (mT); how that translates into microtesla at the sensor depends on distance and geometry. That is why different phones (different sensor placement and shielding) show different susceptibility.

Authoritative manufacturer guidance and sensor datasheets emphasize variability. For example, Apple discusses magnetic accessories and potential interference with compass and sensors in its support notes; sensor manufacturers publish measurement ranges and resolution. Rather than quoting a single ‘2–3 cm’ rule as universal, treat any specific distance as conditional: some small, strong magnets can affect magnetometers at a few centimeters on some phones, while weaker magnets or phones with different shielding may show no measurable error until much closer.

• Earth’s field: ≈25–65 µT (microtesla)
• Phone magnetometer specs vary— check module datasheets (e.g., Bosch BMM150, AKM AK8963) for sensitivity and range
• Magnet field falls off approximately with distance^3; practical interference distance depends on magnet size/grade and phone design

Magnetic stripe cards: coercivity, standards, and demagnetization risk

Magnetic-stripe cards encode data using magnetic domains with a defined coercivity — the field strength required to change or erase those domains. ISO/IEC standards (ISO/IEC 7811 series) document magnetic stripe formats and coercivity categories (commonly referred to as low-coercivity (LoCo) and high-coercivity (HiCo) stripes). LoCo stripes typically require lower fields to alter than HiCo stripes, so LoCo cards are more easily erased by a nearby magnet. There is more on potential camera damage from magnets in a separate guide.

The practical consequence is that there is not a single distance threshold that applies to all cards. A strong rare-earth magnet in direct contact or within millimeters of a LoCo stripe can erase or corrupt data; HiCo stripes resist much higher local fields. Card readers and stripe formulations also vary in robustness. Manufacturer guidance for payment cards and access control often advises keeping strong magnets away from cards and using contactless/RFID where security is needed.

For reliable assessment, compare the local field at the card surface (measured with a gaussmeter) to the card’s specified coercivity or test the card in a reader while in the wallet. This is the only way to quantify risk for a given magnet and card combination.

• Card coercivity varies; see ISO/IEC 7811 specifications for stripe types and coercivity guidance
• LoCo cards are more easily demagnetized than HiCo cards
• Risk is greatest when the magnet is in contact or within millimeters; distance sensitivity depends on magnet strength and stripe coercivity

Camera and other sensor interference — what matters

Some camera modules use small internal magnets (e.g., for optical image stabilization) and may include shielding. Magnetic fields can perturb mechanical elements or magnetically sensitive components (Hall sensors, some proximity sensors). The distance at which interference appears again depends on magnet strength, module shielding, and component design. Before you commit to anything, it is worth looking at do magnets damage phones.

Temporary operational glitches (focus issues, stabilization problems) are more probable than permanent damage unless the magnet is unusually strong and in direct contact. Manufacturer notes for phones and camera modules generally advise avoiding very strong magnets near lenses and sensors and provide examples of tested accessory designs (e.g., MagSafe guidelines describe engineered field distributions to avoid unintended interference).

Because device sensitivity varies, test functionally (take photos, observe stabilization/focus) while moving the magnet away to establish a practical safe distance for your particular phone and accessory.

• Shielding and module design determine susceptibility
• Temporary image or focus issues are possible under sufficiently strong local fields; permanent damage is uncommon without contact
• Test your device to determine the distance at which normal operation resumes

Putting it together: ranges, examples, and why single distances are unreliable

Many online summaries quote simple distances (e.g., “2–3 cm” for compass interference) as a rule of thumb. These can be useful as a starting heuristic but are not universal. Good sources (sensor datasheets, Apple’s accessory guidance, ISO card standards, magnet manufacturers) consistently show that the actual threshold depends on: magnet grade (N35–N52 and physical size), magnet surface field, how the field attenuates with distance, the phone’s magnetometer placement and shielding, and the card stripe coercivity. The other half of this decision is safe distance for neodymium magnet and keycard.

To give concrete but conditional examples based on published magnet manufacturer data and sensor specs: small neodymium discs (depending on diameter and grade) can have surface fields ranging from a few tens to several hundred millitesla; at a few millimetres from the surface the local field can still be thousands to tens of thousands of µT (orders of magnitude above Earth’s field), but at several centimeters it often falls to levels comparable to the Earth’s field. Those numbers explain why the same magnet can cause interference on one phone but not another at the same nominal distance.

Therefore, present any specific distance as a conditional range and, when practical, verify by measurement or functional testing for your devices.

How to test your own magnetic mount or wallet safely

The only reliable assessment is measurement and functional testing for your particular combination of magnet and devices. Recommended steps below use a handheld gaussmeter/magnetometer and functional tests (compass app, card readers, camera tests). Gaussmeters measure magnetic flux density in tesla units (usually millitesla or microtesla); reputable instrument manufacturers and suppliers provide handheld units and application notes (examples: Hirst, Lake Shore, AMETEK — see references).

If you do not have a gaussmeter, you can still perform simple functional tests: observe compass stability with and without the accessory near the phone, test cards on a reader before relying on them, and photograph test scenes while moving the magnet to find when camera artifacts cease. These practical tests are what manufacturers and standards imply when they recommend keeping strong magnets away from sensitive items.

1. Obtain or borrow a handheld gaussmeter capable of measuring µT and mT ranges (check manufacturer specs).
2. Measure field strength at the location of the phone magnetometer and at the card position inside the wallet with the accessory in place, sampling at incremental distances.
3. Compare measured values to the Earth’s field (≈25–65 µT) and the device sensor datasheet ranges; for cards, consult ISO/IEC 7811 or card issuer guidance on coercivity when available.
4. Perform functional tests: compass app behavior, card reader tests, and camera focus/stabilization tests while moving the magnet away until normal function returns.

Balancing magnet strength, grades, and safety for mounts and wallets

Magnet grade (commonly N35–N52 for commercial neodymium magnets) and physical size determine how strong the field is at the magnet surface; magnet manufacturers (e.g., K&J Magnetics) publish typical surface fields and recommended safe uses. Accessory designers (MagSafe style systems, Qi2) engineer magnet position, spacing, and field distribution to provide secure hold while minimizing interference with sensors and cards.

If an accessory uses visibly large or high-grade neodymium magnets (e.g., N48, N52), expect stronger fields that remain significant farther from the magnet than a small N35 part. Where possible, choose designs that isolate cards from magnets (physical spacing, non-magnetic pockets) or use contactless payment options to mitigate risk.

When in doubt, test your specific cards and phone — the combination of magnet grade/size and device construction determines whether you’ll see recalibrations, card errors, or camera glitches.

Questions people still ask

I reviewed sensor datasheets and accessory manufacturer notes and recommend measuring field strength and testing devices rather than relying on single-distance rules.