Luhn Checksum Calculator

Calculate the ISO/IEC 7812 check digit for any payload, or verify a complete number — the same maths that backs ICCIDs, IMEI numbers and ID numbers.

Calculate a check digit

Paste a number without its check digit. The digit added at the end makes the completed number pass Luhn.

Verify a number

Paste a complete number, check digit included. See whether it passes Luhn, and the full digit-by-digit working if it doesn't.

What is the Luhn algorithm?

The Luhn algorithm, also called modulus 10, produces a single check digit from a string of digits. Starting from the digit immediately to the left of where the check digit goes and working leftwards, every second digit is doubled — subtracting 9 from any result over 9 — then every digit is summed. The check digit is whatever number makes that total divisible by 10.

It was designed to catch the errors a human makes copying a long number by hand: a single mistyped digit, or two adjacent digits swapped. It is not designed to catch anything more sophisticated, and was never intended to.

Where you'll meet it

Any identifier long enough that people mistype it tends to end in a Luhn digit. On this site alone: the ICCID printed on every SIM, and the South African ID number. Beyond telecoms and identity, it's the same maths behind IMEI numbers and most payment card numbers.

Frequently asked questions

What is the Luhn algorithm?

The Luhn algorithm (ISO/IEC 7812, also called modulus 10) is a checksum formula that produces a single check digit from a string of numbers. It catches the errors people actually make when copying a number by hand — a mistyped digit or two adjacent digits swapped — not fraud or forgery.

Which identifiers use a Luhn check digit?

ICCIDs (SIM serial numbers), IMEI numbers, South African and many other national ID numbers, and payment card numbers all end in a Luhn check digit. Any place a long number gets typed, scanned or read aloud benefits from one.

Is Luhn a security feature?

No. It is a transcription-error check, not encryption or fraud prevention — anyone can compute a valid check digit for any payload. Its job is to reject numbers that were mistyped or misread, not to verify who issued the number or whether it is genuine.

Why does doubling a digit sometimes subtract 9?

Doubling a single digit (0–9) can produce a two-digit result (up to 18). The algorithm needs a single digit to sum, so it takes the digit sum of the doubled value — and subtracting 9 from any result over 9 is a fast way to get the same answer as adding its two digits together.