ESI stands for Employee State Insurance, managed by the Employee State Insurance Corporation (ESIC), which is an autonomous body created by law under the Ministry of Labour and Employment, Government of India.

Organizations that are registered under ESI are given an ESI Number, which they have to use while doing transactions.

ESI number follows a 17 digit code format. For explaining the format, let's take a code, e.g. 31–00–123456–000–0001.

1. 31 - stands for the region where the company/establishment is situated. e.g. 31 is for Mumbai, 52 is for Hyderabad.

2. 00 - stands for main region code. It is used in the case where the company's head office is situated in some other region. Suppose a company is situated in Mumbai, having its HO at Hyderabad, then the code would be 31–53-******-***-****.

3. 123456 - this represents the company's unique code; it can have a maximum of 6 digits.

4. 000 - These three digits denote the number of branches.

5. 0000 - These four digits depend upon the type of business activity in which the company is involved.
e.g. For Service providers- 0001, for Restaurants-1102.