How to Read an RFID Card Number: It’s Usually Simpler Than You Think

2026-02-03Cykeo News RFID FAQ 10

You need to read an RFID card number. Maybe you’re setting up a new door lock, troubleshooting an access system, or just curious what’s on that old keycard floating in your drawer. The good news? Getting the basic serial number (the “card number”) is the easiest part of RFID. The bad news? If you grab the wrong tool, you’ll get nothing. Let’s cut through the noise and get you that number.

First, understand what you’re after. The “card number” you want is most likely the UID (Unique Identifier) or Facility Code/Card Number combo. This is the tag’s built-in serial number, not custom data stored on it. It’s like reading the VIN off a car, not the contents of the glovebox. For 90% of use cases—like programming an entry system—this is the only number you need.

Step 1: Know Your Enemy (The Card Type)

This is the critical step everyone skips, leading to wasted money. RFID cards come in two common flavors that don’t talk to each other:

125 kHz (Low Frequency – LF): These are the classic, thick plastic keycards. They have a simple, unchangeable number. This is what many older apartment buildings and gyms use.
13.56 MHz (High Frequency – HF/NFC): These are newer, often thinner, and might have logos like “MIFARE” or “NFC.” Your office badge or modern hotel key is probably HF.

How to tell? No surefire way without a reader, but LF cards often have a wider, visible coil under light, while HF cards may have a more complex, dense pattern. The safest bet: If you know the system was installed in the last 10-15 years, lean towards HF. If it’s ancient, lean LF. When in doubt, a dual-frequency reader solves this.

Step 2: Pick Your Tool (The Right Reader)

You don’t need a $500 lab device. Here are your practical options, from cheapest to most reliable:

The ‘Just Give Me the Digits’ Method (For LF Cards): Buy an RDM6300 module ($5). Connect its TX pin to an Arduino’s RX pin, power it, and open the Serial Monitor. Tap your LF card. It will spit out a decimal number like 2001501234. That’s your card number. Done.
The ‘I Might Have HF Cards’ Method: Get a PN532-based USB reader ($25-$40). It reads both HF and NFC. Plug it in, download a free tool like “NFC Tools” for PC/Mac or a similar app for your phone. Tap the card. The app will show the UID in hex format (e.g., 04:A1:B2:C3:D4:80:80). The app might also show the card type, confirming your guess.
The ‘No Fuss, Just Work’ Method (For Pros & Frustrated Beginners): Use a dedicated handheld like a CYKEO diagnostic reader. These are pre-configured to identify the card type and display its number clearly on screen. You pay for the convenience and reliability. This is what I use on service calls because time is money.

Step 3: The Actual Reading Process

With the PN532 and NFC Tools as our example:

Plug the reader into a USB port.
Open the NFC Tools application.
Click the “Read” tab.
Place your card flat on the reader. Hold it still. Don’t tap and lift.
The software will beep and display information. Look for “UID”, “Serial Number”, or “ATQA”/“SAK” values which identify the chip type. The UID is your primary card number.

What Does This Number Mean? Decoding the Output.

LF Card Output (from RDM6300): You get a plain decimal number, e.g., 345678901. This is often a combination of a facility code and card ID. Your access system software will ask for this full number.
HF Card Output (from PN532): You get a hexadecimal UID like 04:A1:B2:C3:D4:80:80. The first byte (04) often indicates the manufacturer. The middle bytes are the unique serial. The last byte is a check digit. Most systems will let you input this with or without the colons or with just the central unique part.

“It’s Not Working!” – Quick Fixes

No Response: 99% chance your reader and card frequency are mismatched. Try the other type of reader.
Intermittent Reads: You’re tapping. Don’t tap. Place the card on the reader and hold it for a full second.
Weird or Multiple Numbers: Some older HF cards (MIFARE Classic) have a 7-byte UID. Some new ones have a 10-byte UID. This is normal. Record the whole thing.
Software Doesn’t See Reader: Install the drivers that came with your USB rfid reader. On Windows, check Device Manager for unrecognized devices.

Pro Tip: Once you have the number, write it on the card with a permanent marker. You’ll thank yourself later.

Ultimately, learning how to read an RFID card number is about matching the right tool to the card and knowing where to look in the software output. Start by borrowing a known card from a system that works (like your office) and practice reading it. Once you can reliably pull the UID from that, you can tackle any unknown card. For one-off tasks, a cheap module is fine. If you do this regularly, a purpose-built tool like a CYKEO handheld is worth every penny for the time and headache it saves.

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