What is an IMEI number? Digits, checks and uses

IMEI stands for International Mobile Equipment Identity. It is a 15-digit number given to every piece of hardware that connects to a mobile network: phones and tablets, but also the cellular modems inside trackers, meters, alarm panels and other IoT devices. The number identifies the equipment itself. It stays the same whichever SIM is inside, and a phone with two SIM slots has two IMEIs, one for each. This article explains what the digits mean, how the last digit is checked, how the IMEI differs from the other identifiers on a cellular device, and what it is used for. It sits alongside the ICCID and IMSI guides, and the connectivity hub collects the rest of the cellular articles.

What the IMEI identifies

A cellular connection involves three separate things, and each has its own number:

• The device, the radio hardware: identified by the IMEI.
• The subscription, the account on a mobile network: identified by the IMSI, stored on the SIM.
• The SIM card or chip itself, as a manufactured part: identified by the ICCID, its serial number.

Move a SIM from one phone to another and the IMSI and ICCID move with it, while each phone keeps its own IMEI. That separation is the whole reason the number exists: it lets a network tell which handset or modem is in use, independently of who is paying for the connection.

The format is defined by 3GPP, the body that writes the mobile standards, in its numbering specification TS 23.003. The first part of every IMEI is allocated through the GSMA, the mobile operators’ industry association, which runs the Type Allocation Code scheme for manufacturers.

The IMEI digit by digit

Type Allocation Code (TAC), 8 digits. Allocated to a manufacturer for one model, or for a variant of a model. Every unit of that model shares the same TAC, which is why a database of TACs can tell a network or a shop what kind of device a given IMEI belongs to. The first two digits are the Reporting Body Identifier, which shows which organization issued the code.

Serial number (SNR), 6 digits. Assigned by the manufacturer to each unit. Combined with the TAC, it makes the IMEI unique.

Check digit (CD), 1 digit. Calculated from the first fourteen digits with the Luhn formula, the same check used on payment card numbers. It catches a single mistyped digit and most swaps of two neighboring digits. It is not sent over the air; the network sees only the first fourteen.

Older IMEIs had a different split, with a six-digit type approval code followed by a two-digit final assembly code. The eight-digit TAC replaced that in 2004, so any IMEI still in use reads correctly with the current split.

Checking an IMEI with the Luhn formula

The check digit is simple to verify by hand, which makes it a quick test of whether a number typed into a form or read off a label is plausible.

To check a full 15-digit IMEI, run the same steps over all fifteen digits, doubling every second digit from the second as before. If the total is a multiple of ten, the number passes. A pass does not mean the device exists; it only means the number was not mistyped.

IMEISV: the version with software

Networks often ask a device for its IMEISV rather than its IMEI. It has 16 digits: the same TAC and serial number, followed by a two-digit software version number in place of the check digit. That lets an operator see which firmware version a model is running, which helps when a network problem turns out to affect one software release.

In 5G, the standards use the general term Permanent Equipment Identifier (PEI). For an ordinary 3GPP device, the PEI is its IMEI or IMEISV, so nothing changes in practice.

IMEI vs IMSI vs ICCID vs EID

An eSIM does not replace the IMEI. A phone or module with an eSIM still has an IMEI for its radio, and the eSIM chip has its own 32-digit EID as well. The IoT SIM card guide explains eSIM and iSIM, and the IMSI article covers how the subscription identity is protected over the air.

Where to find the IMEI

On a phone. Dial *#06# and the IMEI appears on screen; this works on almost every handset. It is also in the settings, usually under the “about” section, on the original box, and on the SIM tray or the back of some models.

On a cellular module or IoT device. The module’s label normally carries the IMEI as digits and as a barcode or QR code, and the finished product often repeats it on its own label. From software, send the standard AT command AT+CGSN to the modem, which returns the IMEI on modules that follow the 3GPP command set; check the module’s AT command manual for its exact response format. Many IoT platforms also report the IMEI of each device in their dashboard, because it is the one identifier that stays fixed when SIMs are swapped.

What the IMEI is used for

Network checks. When a device attaches, the network can ask for its IMEI or IMEISV and compare it against an equipment register. Operators use this to refuse devices that have been reported stolen or that are not allowed on the network.

Stolen-phone blocking. In many countries, operators share lists of IMEIs reported lost or stolen, so a blocked phone cannot be used on any participating network even with a new SIM. Changing an IMEI to get around this is illegal in some countries, including the United Kingdom.

Compatibility. Operators and online checkers read the TAC to see the model, and from that which bands and technologies it supports, before activating a service.

Device identity in IoT fleets. For a fleet of trackers or meters, the IMEI is a convenient permanent key in an asset register: printed on the device, readable remotely, and unaffected by SIM changes.

IMEI locking of IoT SIMs. Many IoT SIM providers let you lock a SIM to the IMEI of the device it was installed in. If someone removes the SIM and puts it in a phone, the network refuses the connection, which stops a stolen data SIM from being used elsewhere. It is worth enabling on devices installed in public or unattended places, and worth remembering before a field technician swaps a modem. The MNO and MVNO guide covers who provides IoT SIMs and what they control.

Is it safe to share an IMEI?

The IMEI is not a password. Networks authenticate the SIM, using a secret key stored on it, and never treat the IMEI as proof of anything. Someone who knows your IMEI cannot take over your phone number or read your messages with it.

It is still reasonable to keep it private. A known IMEI can be used to file a false stolen-device report against it, or cloned into a counterfeit device in places where that is not policed. Share it with your operator, an insurer or a repair shop when they need it, and avoid posting it publicly, for example in a photo of a phone’s box on a sales listing.

For the other numbers on a cellular device and how a connection is set up, read the ICCID and IMSI guides, and browse the rest of the connectivity guides.