How IMEI Check Works
Every IMEI verification follows a three-step process: format validation, Luhn algorithm check, and TAC database lookup. Here is exactly what happens behind the scenes when you check an IMEI.
Published April 2026 Β· ~6 min read
The Three Layers of IMEI Verification
A complete IMEI check is not a single operation. It stacks three independent verification layers, each catching a different class of errors or fraud.
Checks that the input is exactly 15 digits, numeric only. Rejects letters, spaces, and wrong-length strings immediately.
Applies the Luhn algorithm to verify mathematical integrity. Detects single-digit typos and most transposition errors.
Matches the first 8 digits against the global TAC registry to identify the device brand, model, and capabilities.
Step-by-Step: The Luhn Algorithm
The Luhn algorithm (also called the "modulus 10" algorithm) is used to compute and verify the 15th digit of every IMEI. Here is how it works:
The 15th digit is the check digit itself, so the algorithm operates on the first 14 digits of the IMEI.
Starting from the rightmost digit of the 14, double every second digit. For example in 35209900176300, the 2nd, 4th, 6th... digits from the right get doubled.
If doubling produces a number greater than 9, subtract 9 (equivalent to summing the two digits). Then add all 14 values together.
Check digit = (10 - (sum mod 10)) mod 10. If the sum is a multiple of 10, the check digit is 0. The result must match the 15th digit for the IMEI to be valid.
TAC Database: Identifying the Device
The TAC (Type Allocation Code) is the first 8 digits of any IMEI. It acts as a device fingerprint that maps to a specific manufacturer and model.
The GSMA maintains the official TAC allocation registry and assigns new TAC codes to manufacturers before devices can be sold. When you run an IMEI check, the TAC portion is looked up against this database to reveal:
- Brand & model (e.g., Samsung Galaxy S24 Ultra)
- Device type (smartphone, tablet, modem, IoT module)
- Network capabilities (supported bands, CA combinations, VoLTE)
- Chipset information (Qualcomm, MediaTek, Exynos, etc.)
The HiCellTek TAC database contains over 250,000 device records, enriched with RF capabilities, chipset details, and band support data not available in standard TAC databases.
Blacklist Screening
The final layer of IMEI verification is blacklist screening. The GSMA CEIR (Central Equipment Identity Register) is a global shared database where carriers report stolen, lost, or fraudulent devices.
When a device is reported stolen, the carrier adds its IMEI to the national blacklist. Through the CEIR, this information is shared internationally, meaning a phone stolen in France can be blocked across networks in over 150 countries.
A blacklisted IMEI results in the device being unable to register on any participating carrier network, effectively rendering it useless for calls, texts, and mobile data.
When is IMEI Verification Used?
Always verify the IMEI before purchasing a second-hand device. A quick check confirms the phone is not blacklisted, matches the advertised model, and has a valid serial number.
Insurers verify the IMEI to confirm the device model, purchase date, and whether the same IMEI has been claimed before. It prevents duplicate or fraudulent claims.
Police use IMEI tracking to locate stolen devices and identify suspects. The IMEI links a physical device to network records, call logs, and location history.
Telecom engineers use IMEI/TAC data to identify device capabilities (band support, CA, MIMO) when troubleshooting network performance issues in the field.
Frequently Asked Questions
Can an IMEI pass the Luhn check but still be fake?
How fast is an IMEI check?
Is IMEI checking free?
Can I check IMEI without the phone?
What happens if an IMEI is blacklisted?
How many IMEI numbers exist?
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