Free tool / no upload
RFID UID converter for hex, decimal and reversed byte order
Paste a card UID and see every representation a reader or backend can produce: normalized hexadecimal, reversed byte order, decimal value and the full byte map. Everything runs in this browser.
UID / CONVERSION
UID format converter
Normalize a hexadecimal identifier, reverse its byte order and calculate the decimal representation without sending data to a server.
Accepts 4, 7 or 10 bytes with spaces, colons, hyphens or a 0x prefix.
- Normalized hexadecimal
04A1B2C3D4E5F6- HEX / SPACED
04 A1 B2 C3 D4 E5 F6- Reversed byte order
F6E5D4C3B2A104- Decimal value
1303689068602870
| B0 | B1 | B2 | B3 | B4 | B5 | B6 |
|---|---|---|---|---|---|---|
| 04 | A1 | B2 | C3 | D4 | E5 | F6 |
| 4 | 161 | 178 | 195 | 212 | 229 | 246 |
Why the same card shows two different numbers
An RFID UID is a sequence of bytes, and different systems print those bytes differently. Many readers output the UID least-significant byte first, so 04 A1 B2 C3 becomes C3 B2 A1 04. Others convert the value to decimal, drop leading zeros or group the digits in blocks.
None of these systems is wrong: they all describe the same chip. Integration fails when two sides of a project exchange identifier lists without agreeing on representation and byte order. This converter makes each representation visible so the mismatch can be identified in seconds.
4-byte, 7-byte and 10-byte UIDs
Contactless chips following ISO/IEC 14443 use single (4-byte), double (7-byte) or triple (10-byte) size identifiers. Most modern charging and access credentials carry a 7-byte UID, while many installed readers still report only 4 bytes.
When a 7-byte credential passes through a 4-byte reader path, the value is truncated and no longer matches the personalization file. Compare the byte length reported by your backend with the byte length in the production file before assuming the cards are faulty.
How to use the converter
Choose hexadecimal or decimal input, select the byte length, then paste the identifier. Spaces, colons, hyphens and a 0x prefix are accepted. The panel shows the normalized value, the reversed byte order, the decimal representation and each byte with its decimal equivalent.
Copy any representation with one click, or export the analysis as CSV to attach to a support ticket or an integration handover. If a value cannot be interpreted at the selected length, the tool flags it instead of guessing.
UID conversion questions
UID conversion questions
Many reader chipsets transmit the UID least-significant byte first. If your backend shows the same bytes in the opposite order to the production file, the reader path reverses byte order. Reverse it once, consistently, on one side.
Treat the full UID as one unsigned integer and convert base 16 to base 10. A 7-byte UID can exceed 32-bit limits, so use 64-bit or arbitrary-precision handling. This tool does the conversion locally for 4-, 7- and 10-byte identifiers.
Usually one of three causes: the backend stores the decimal value while the card shows hexadecimal, the byte order is reversed somewhere in the chain, or leading zeros were dropped by a spreadsheet. Compare all representations before re-encoding cards.
No. All parsing and conversion runs in your browser. Identifiers are not transmitted to or stored by ChargeRFID, which also means the tool works offline once the page is loaded.
Related tools
Ordering credentials? Fix the format before production.
Every ChargeRFID order ships with a machine-readable data contract and acceptance vectors, so your backend team can verify the identifier format before the cards arrive.