To tell whether an RFID card is 125 kHz or 13.56 MHz, hold it against the back of an NFC phone running a tag-reading app. A phone’s NFC radio works only at 13.56 MHz, so if the app reads the card, the card is HF. If nothing happens, the card is probably 125 kHz LF, but confirm it on a reader before you reorder. Cards bought for the wrong band do not read at all, and no reader setting or firmware changes the band a card was built for.
The quick answer: what each result means
| Phone result | Printed on the card | Likely band and chip family | How to confirm |
|---|---|---|---|
| The tag app reads it and names an IC | Anything, or nothing | 13.56 MHz HF: ISO/IEC 14443 A (MIFARE Classic, Ultralight, DESFire, NTAG) or ISO/IEC 15693 (ICODE SLIX) | Already confirmed |
| No response | A 10-digit number | Probably 125 kHz LF: EM4100, EM4200, TK4100 or the rewritable T5577 | An LF or dual-frequency HF + LF reader |
| No response; the card works at a vehicle barrier from a distance | Varies | Possibly passive UHF (EPC Gen2), which a phone’s NFC radio cannot read | A UHF reader |
| Reads on the phone, yet an older 125 kHz door still accepts it | Varies | Dual-frequency HF + LF card | Read the LF side on an LF or dual-frequency reader |
The phone test, done properly
- Switch NFC on and unlock the screen. Android devices usually look for tags when the screen is unlocked, and not at all when NFC is switched off in Settings (Android Developers: NFC basics).
- Install a tag-reading app before you start. Android hands a tag with no NDEF data to whichever app has registered for its technology, so without a tag app a plain access card may produce no visible reaction. NXP’s free NFC TagInfo app identifies IC types and the IC manufacturer (Google Play: NFC TagInfo by NXP). NXP offers it for Android and iPhone.
- Take the card out of metal. A bare HF antenna detunes against metal, so remove the card from a metal cardholder or a steel badge reel, and take off a metal-backed phone case.
- Hold it flat and go slowly. Android’s documentation puts the usual NFC range at 4 cm or less (Android Developers: NFC overview). Lay the card flat on the back of the phone and move it slowly across, pausing for a few seconds rather than tapping once.
- Record the result. Screenshot the IC type, manufacturer and UID the app reports.
On an iPhone
Background tag reading on iPhone XS and later scans without an app, but it looks only for an NDEF URI record (a universal link or a supported URL scheme) (Apple: adding support for background tag reading). An access card carries no such record, so a plain tap shows nothing.
Why “nothing happened” does not prove 125 kHz
A blank result has at least four causes other than a 125 kHz chip.
- It is a UHF card. Passive UHF (EPC Gen2) cards, read from a distance at gates and barriers, are invisible to a phone’s NFC radio; our UHF vs HF vs NFC comparison covers that band.
- The phone does not support MIFARE Classic. Android lists MIFARE Classic as an optional technology that phones are not required to support (Android Developers: advanced NFC), so a phone can fail on a card that is plainly HF. Try a second phone.
- It was an iPhone. Core NFC’s MIFARE family list covers Ultralight, Plus and DESFire and otherwise reports “unknown” (Apple: NFCMiFareFamily), so treat a blank or “unknown” iPhone result as inconclusive and repeat on Android.
- Metal, a case or a rushed tap. Any of the phone-test conditions above can make a readable HF card fail.
Only a reader in the right band that is specified for the card’s chip family settles the question.
Reading the clues printed on the card
A 10-digit number is often printed on 125 kHz cards, and EM Microelectronic’s EM4100 datasheet shows why that length fits. The EM4100 is a read-only chip whose 64-bit memory is laser-programmed at the factory with a unique code on each chip. Of those 64 bits, 40 carry data; the rest are a 9-bit header, 14 parity bits and a stop bit. The 40 data bits are an 8-bit version or customer ID followed by a 32-bit number. A 32-bit field holds values up to 4,294,967,295, which is ten digits in decimal.
Treat a 10-digit number as a hint, not proof: printing conventions vary by card maker, and HF cards carry printed serials too.
Some cards carry a second figure, written as a facility code plus a card number, for example 188,24910. That figure can carry part of the same chip ID in the 26-bit Wiegand form access panels use: an 8-bit facility code and a 16-bit card number, taken from the lower 24 bits of the 32-bit number. 0012345678 in decimal is BC614E in hex; BC is 188 and 614E is 24910, so both printed numbers name one chip. The conversion runs only from the 10-digit number to the facility-code form, which drops the top 8 bits. Matching that format to an eSSL or ZKTeco terminal before printing is covered in India RFID Store’s card-printing compatibility guide.
Dual-frequency cards: when the answer is “both”
An HF + LF card normally laminates two separate chips, so it carries two identities, the HF chip’s UID and the LF chip’s ID, and each system has to hold the number its own reader sees.
One way to move off a 125 kHz prox estate without replacing every reader at once is a combination card that carries the prox chip alongside an HF 13.56 MHz or UHF chip, so old and new read points both work during the changeover. Our RFID cards page lists UHF + HF, LF + UHF and HF + LF dual-frequency cards. A single-IC part that puts NFC and UHF on one chip with shared memory is a different design, explained in our post on dual-frequency NFC + UHF tags.
Confirming on a reader
The band rule cuts both ways: neither a 13.56 MHz nor a 125 kHz reader sees a card from the other band.
- Use a dual-frequency desktop reader. The IDTS-DLH20 on our HF RFID readers page is a read-only USB reader specified for Mifare 1K, 4K, 2K and Ultralight at 13.56 MHz and for TK4100 and EM4200 at 125 kHz, so it confirms cards in those families at a desk. A card outside those lists, such as an ISO/IEC 15693 ICODE card or a 125 kHz card in a different format, may not read on it, so a blank result there does not prove the band. It does not write or re-encode cards.
- Check the reader you already have. Hold the card against the existing door or attendance reader. A reader that beeps or flashes red for an unenrolled card has read it, so the card is in a band that reader handles. No reaction points to a different band, but some readers stay silent for unknown cards, so confirm the band from the reader’s label or datasheet before you reorder.
- Present LF cards singly. LF has no anti-collision in practice, so a reader handles one card at a time.
- Keep 125 kHz and 134.2 kHz apart. Animal ID uses 134.2 kHz FDX-B, which the DLH20’s proximity-card LF side is not specified for; animal ID tags need a reader specified for that standard.
If the card turns out to be HF or UHF and still will not read, our tag-not-reading troubleshooting guide works through HF and UHF faults separately.
Why it matters before you reorder
The band narrows the choice; the chip family decides what a replacement card has to carry.
| EM4100 family (read-only) | T5577 (rewritable) | MIFARE Classic | |
|---|---|---|---|
| Band | 125 kHz | 125 kHz | 13.56 MHz, ISO/IEC 14443-3 Type A |
| What the chip holds | A 64-bit code laser-programmed at the factory, unique to each chip | Rewritable memory that can carry a chosen ID | 1 kB to 4 kB of EEPROM in key-protected sectors, plus a 4-byte NUID or 7-byte UID |
| Can the number be chosen? | No | Yes | Sector data, by whoever holds the keys |
| What the system checks | The fixed ID | Whatever ID was written | The UID, or data in protected sectors |
| Security note | No on-card authentication | No cryptographic authentication; an optional password guards writing, and on some chips reading, but it is sent in clear. A writable ID is why LF credentials can be copied | CRYPTO1 cipher; NXP’s MIFARE Classic page points security-relevant uses to its DESFire and Plus families |
What that means for each estate:
- EM4100 family. Every chip carries its own factory code, so a new card carries a new number and has to be enrolled. Nothing on the card authenticates it, which is workable for plain in and out but a reason not to extend LF into sensitive areas.
- T5577. Rewritable LF chips differ in detail. Microchip’s documentation for its ATA5577, a T5577-type chip, describes a 363-bit EEPROM in 11 blocks of 33 bits, lock bits that protect blocks and an optional password block against unauthorised writing. Ask for the datasheet of the exact chip your system expects.
- MIFARE Classic. Find out whether the system checks the UID or the sector data. If it checks the UID, note whether it is 4-byte or 7-byte: some readers return only 4 bytes of a 7-byte UID, so test a handful of cards on the actual reader first. If it checks sector data, reading the UID is not the same as reading protected sectors, and the sector keys belong in the application. If a previous integrator encoded the cards, the keys may not be in your possession. Ask that integrator or the system owner for them, since a new card supplier does not hold keys it did not set, or plan to re-issue the cards with keys you hold.
Moving doors to a stronger credential at the same time? Our RFID access control and biometrics page covers converting an estate one door at a time, and our guide to RFID cards for access control and ID compares the card technologies.
What to send us
Send us a sample card and we will check it against the reader it has to work with. Include:
- The card or fob itself, or a spare from the same batch.
- A screenshot from the tag app, if the phone read it.
- The make and model of the reader, terminal or panel the card has to work with.
- The number the system shows when a card is enrolled.
Tell us the chip your system expects and whether it checks the UID or sector data. Identium designs and manufactures UHF, HF and LF cards in New Delhi, and matching RFID key fobs for LF, HF and UHF readers. Cards are quoted against specification. Passive cards have no transmitter, so they need no WPC approval and no BIS registration. Contact us with the details.