Cat Microchip Registration: 134.2 kHz and 15 Digits

A microchip is a passive radio tag, not a tracker: it holds 1 number and broadcasts nothing until a scanner powers it from 3-8 cm away. The chip itself rarely fails; the registration behind it does. That is why the useful work is 3 steps in order: confirm the frequency (134.2 kHz ISO or 125 kHz legacy), confirm the 15-digit number on paper, and confirm which database holds your current phone number.

Two frequencies, one scanner problem

ISO 11784 and 11785 define the 134.2 kHz FDX-B chip, which carries a 15-digit code and is the standard in the EU, UK, Australia and 60-plus countries. The 125 kHz chips sold in North America before roughly 2010 are a different standard with 9-10 digit codes, and a reader built for one frequency hears nothing from the other.

Read range is short: 3-8 cm for a handheld scanner, less through fur and fat on a 6 kg cat. A clinic that scans for 5 seconds at 20 cm and finds nothing has not tested the chip, it has tested the scanner technique.

What the 15 digits mean

An FDX-B chip stores 64 bits, and the printed code usually shows a 3-digit country code plus a 12-digit national ID. The country code 999 means the chip was sold without a national prefix, which is common on chips bought online and implanted abroad. Write the full 15 digits on paper the day of implantation and photograph the paperwork; the number is the only key to the registry.

Implanting is not registering

The implant takes about 30 seconds: a 12 mm needle under the skin on the dorsal midline between the shoulder blades, at 8-12 weeks of age or any time after. The clinic records the number in its own file. The registry is a separate company, often a separate fee of 10-30 USD once or 5-15 USD a year, and nothing links the two automatically.

Three fields decide whether a found cat gets home: 2 phone numbers, a street address, and an alternate contact who will answer on a Sunday. A chip with 1 disconnected number is functionally the same as no chip, which is the single most common failure in shelter intake.

Which database, and when to update

Register in the national database of the country where the cat lives, then add an international registry if you travel. The EU requires an ISO 11784/11785 chip for a pet passport, implanted before or on the same day as the rabies vaccine, and travel is allowed 21 days after that dose.

Update within 10 days of any move or phone change, and re-verify every 12 months with a 2-minute call or login. Rehoming means a transfer of ownership, not a new chip: the old record must be changed the same week, or the next scan calls your number for a cat you no longer own.

Three reasons a scan finds nothing

First, migration. A chip can drift 1-10 cm from the implant site, usually down the left side or toward the armpit, and a scan limited to the shoulder blades misses it. Second, a dead or incompatible chip: a 125 kHz chip under a 134.2 kHz-only reader, or a chip that stopped responding after a fall. Third, technique: a sweep that takes under 10 seconds, a reader held more than 10 cm away, or 1 orientation instead of 3.

Standard	Frequency kHz	Code digits	Reader needed	Common in
ISO 11784/11785 FDX-B	134.2	15	any ISO reader	EU, UK, AU
FDX-A legacy	125	10	125 kHz reader	US, Canada
AVID legacy	125	9	AVID-compatible	US clinics pre-2010
ISO with 999 prefix	134.2	15	any ISO reader	online, export chips
ISO 11784 3-digit country	134.2	15	any ISO reader	national registries

The recheck flow, 6 steps

When a cat arrives with no readable chip, work the list in order. Step 1: a full-body sweep, 60-90 seconds, 3 orientations, from nose to tail base and down all 4 legs. Step 2: a second scanner, ideally a multi-frequency unit that reads 125 and 134.2 kHz. Step 3: the clinic record, with implant date and number, from the previous owner or shelter. Step 4: 2-view radiographs, since a chip shows as a 12 mm rod even when it is silent. Step 5: call the registry with any partial number to search. Step 6: re-implant at a new site, usually 2-3 cm off the midline, and register the new number the same day.

Repeat the scan at every annual exam, 20-30 seconds, and write the result in the file. A chip that read last year and does not read this year is a reason to re-implant, not to argue with the reader.

Migration, lumps, and the annual check

Drift of 1-3 cm is common and harmless; a chip that travels to the armpit is still readable if the scan covers the whole trunk. A new lump over the implant site should be measured and checked at 1 month, 2 months and 3 months, because a fast-growing mass at the site needs a sample, and slow-growing ones are usually fat or scar tissue.

Kittens implanted at 8-12 weeks keep the same number for life, so the registration is the durable part. If the cat came from a breeder or a rehome with no papers, the age estimate in /tools/cat-age/ tells you when the chip was likely placed, and the routine exam schedule for a specific breed, such as the /care/arabian-mau-care-calendar/ or the /care/asian-tabby-care-calendar/, is the right place to fold the annual scan into.

Five steps to lock it in

1. Ask the clinic for the full 15-digit number and photograph the implant certificate.
2. Confirm the frequency with a multi-frequency reader before you assume 134.2 kHz.
3. Register 2 phone numbers and 1 alternate contact, and add the chip number to your own phone contacts.
4. Set a 12-month reminder to verify the record, and update it within 10 days of a move.
5. Ask for a 20-30 second scan at every annual exam and keep the result in the file.

Frequently asked