## What is Check Sum Digit ?

Last digit of GST Number, i.e. checksum digit, is still a mystery to many people and GST experts. This article is for those enthusiast who want to know the mathematics behind that 15th digit. The logic is very simple and not a rocket science.

The checksum digit is a result of calculation applied on 14 digits. Checksum digit can range between [0–9] [A-Z]. So basically it is an “alphanumeric” character. So, let us prepare a alphanumeric coding system or conversion table for GST number digits as shown below:

Let’s say 14 digits of our GST Number are: 27AAPFU0939F1Z, and we want to calculate corresponding 15th digit.

So, we need to do calculate a hash of each digit of GST number. Steps for calculation of hash are mentioned below:

1. Obtain corresponding code [C] for each alphanumeric digit of GST number
2. Multiply “code” with number called Multiplier [M], which will be 1 and 2 for alternate digits of GST Number
3. On multiplication of “code” with “multiplier” [ C x M ] we get a number called Product [P]
4. Divide “product” with 36 [ P ÷ 36]. This division yields us a Quotient [Q] & a Remainder [R]
5. Add “quotient” and “remainder” [ Q + R ] to get Hash of that digit of GST number

Repeat the above steps for each digit of GST number, and obtain corresponding hash. After that, derive the checksum digit with below mentioned steps:

1. Add hash of all 14 digits calculated above to get Sum [S]
2. Now divide “Sum” with 36 and obtain it’s Remainder [Z]
3. Subtract from 36 the remainder obtained above [ 36 – Z ] to yield checksum code [Y]
4. Lookup corresponding alphanumeric digit for the checksum code [Y] obtained in above step, which is GST Number Checksum Digit

Illustration for the GST Number 27AAPFU0939F1Z is shown below:

Remainder of 221 ÷ 36 = 5

36 – 5 = 31

Corresponding alphanumeric digit for 31 in “Conversion Table” is V

So, V is the “checksum digit” or 15th digit of GST Number of our illustration

OneDrive link of Excel file for the above illustration with formulas for deriving checksum digit is given below. It calculates 15th checksum digit on entering 14 digits of GST Number. You can enter your own GST Number 14 digits and check the result.