How the IMEI checksum works

The algorithm, in four steps

The IMEI check digit uses Luhn, the same scheme that validates bank card numbers. Over all 15 digits:

1. Double every second digit, starting from the second.
2. If a doubled value goes above 9, subtract 9 from it.
3. Add all fifteen resulting values together.
4. The IMEI is well-formed if that total is divisible by 10.

Worked through

Taking 490154203237518. Doubled positions are marked:

• 4 → 4  ·  9 ×2 = 18 → 9  ·  0 → 0  ·  1 ×2 → 2
• 5 → 5  ·  4 ×2 → 8  ·  2 → 2  ·  0 ×2 → 0
• 3 → 3  ·  2 ×2 → 4  ·  3 → 3  ·  7 ×2 = 14 → 5
• 5 → 5  ·  1 ×2 → 2  ·  8 → 8

Total: 60. Divisible by 10, so the number is well-formed. Change that final 8 to a 9 and the total becomes 61 — no longer divisible, and the check fails.

What it catches

Luhn is designed for the two mistakes humans actually make:

• Any single wrong digit. Always caught.
• Two adjacent digits swapped — typing 21 for 12. Caught in almost every case.

So if you read an IMEI off a label and it fails the checksum, the overwhelmingly likely explanation is a misread character — an 8 for a B, a 5 for an S, a 1 for a 7 — not a counterfeit device.

What it absolutely does not prove

This is where the checksum gets over-interpreted. A passing checksum means the number is arithmetically consistent. Nothing more.

• It does not mean the device exists. Anyone can generate a number that passes Luhn — the maths is public and takes seconds.
• It does not mean the device is genuine, unstolen, or safe to buy.
• It does not mean the IMEI belongs to the phone in front of you. That you verify by dialling *#06# on the device itself.

Treat it as spell-check for a number, not proof of anything about hardware. The fuller picture is in what an IMEI check cannot tell you.

Checking one

Our free lookup runs this calculation as you type and shows the result alongside the TAC, serial and check digit split out.