What is an IMEI Number and How is the Check Digit Constructed?
The International Mobile Equipment Identity (IMEI) is a globally standardized 15-digit numerical identifier assigned to every 3GPP (GSM, LTE, 5G) cellular terminal in the world. Defined under the international telecommunications standard 3GPP TS 23.003, the IMEI functions as a unique electronic fingerprint for smartphones, cellular smartwatches, cellular tablets, and IoT modems.
A standardized 15-digit IMEI consists of three distinct structural segments:
- Type Allocation Code (TAC — Digits 1 to 8): Issued by designated GSMA Reporting Bodies (such as TÜV SÜD or CTIA). The first two digits indicate the Reporting Body Identifier (RBI), while the remaining six digits specify the precise brand, model, and hardware variant (e.g. iPhone 15 Pro Max A3106).
- Serial Number (SNR — Digits 9 to 14): A sequential 6-digit individual serial number assigned by the manufacturer to identify each specific handset.
- Check Digit (CD — Digit 15): A single computed digit derived from the preceding 14 digits using the Luhn Modulo 10 Algorithm. When the device is transmitted over cellular radio interfaces, the check digit prevents corrupted data from creating invalid network registrations.
Mathematical Mechanics of the Luhn Algorithm (Mod 10)
Invented in 1954 by IBM scientist Hans Peter Luhn (US Patent 2,950,048), the Luhn algorithm is an error-detection checksum formula designed to detect single-digit transcription errors and almost all transpositions of adjacent digits:
- Step 1: Count the 14 IMEI digits from left to right (positions 1 through 14).
- Step 2: Leave odd-positioned digits ($1^{\text{st}}, 3^{\text{rd}}, 5^{\text{th}}, 7^{\text{th}}, 9^{\text{th}}, 11^{\text{th}}, 13^{\text{th}}$) as they are (multiply by 1).
- Step 3: Double every even-positioned digit ($2^{\text{nd}}, 4^{\text{th}}, 6^{\text{th}}, 8^{\text{th}}, 10^{\text{th}}, 12^{\text{th}}, 14^{\text{th}}$).
- Step 4: If doubling results in a two-digit number ($\ge 10$), sum the two individual digits together (for instance, $7 \times 2 = 14 \rightarrow 1 + 4 = 5$).
- Step 5: Add all 14 resulting values together to obtain total sum $S$.
- Step 6: Calculate the Check Digit $\text{CD} = (10 - (S \pmod{10})) \pmod{10}$.
100% Privacy & Local Browser RAM Execution
Your smartphone's IMEI is sensitive hardware metadata that can be linked to your cellular subscription and identity. Many third-party IMEI checking websites log user submissions to build commercial marketing databases.
Collabsource guarantees 100% in-browser mathematical execution. This utility runs purely inside your browser's JavaScript engine. Zero network requests, analytics payloads, or backend server connections are initiated. You can verify this in Chrome DevTools Network Tab or run this page completely disconnected from the internet.
How to Find Your Device's IMEI Number
- Universal Dialer Code: Open your phone dialer app and dial
*#06#. The device's 15-digit IMEI, EID, and serial number will immediately display on screen. - Apple iOS (iPhone / iPad Cellular): Navigate to Settings > General > About and scroll down to the Primary / eSIM IMEI field.
- Google Android: Navigate to Settings > About Phone > IMEI (Slot 1 / Slot 2).
- Hardware SIM Tray / Box: Eject the physical SIM tray to view the micro-engraved 15-digit IMEI, or inspect the barcode label on your phone's retail packaging.