How the GSTIN check character works, and why it catches typos

A GSTIN is 15 characters: a 2-digit state code, the 10-character PAN, an entity number, the letter Z (usually), and a final check character.

The calculation

1. Give each character a value: 0 to 9 for the digits, 10 to 35 for A to Z.
2. Multiply the values by 1, 2, 1, 2… working from the first character.
3. For each product, add its quotient and remainder when divided by 36.
4. Add them all up. The check character is the one that makes the total a multiple of 36.

27AAPFU0939F1Z + V  → valid
33AAACC1206D1Z + N  → valid
33AAACC1206D1Z + M  → not valid (should be N)

What this catches

Changing any single character always changes the check character, so every one-character typo is caught. Some swaps of two neighbouring characters are not caught.

What it doesn't prove

A well-formed GSTIN isn't necessarily registered, active, or the one on your customer's invoice. Check status on the GST portal before relying on it for input tax credit.

Misread characters

Scans and photos confuse O with 0, I with 1, S with 5, B with 8, Z with 2 and G with 6. If swapping those look-alikes produces a GSTIN that passes the check, that's a good candidate for the original, and the portal error RET191113 (invalid customer GSTIN) is often exactly this.