Salesforce has 2 versions of Id, 15 and 18 characters. Originally, all Salesforce IDs were 15-characters and case-sensitive whereas 18 character Id is case insensitive.
Suppose, we are exporting report data from salesforce onto excel and using the Excel VLOOKUP function to match up records. Since VLOOKUP doesn’t handle case-sensitive data well, we would have to convert 15 character Salesforce Id to 18 character Id.
As part of Spring ’12 release, Salesforce introduced a new formula function CASESAFEID, which accepts a 15-character ID as a parameter and returns the 18-character version. For instance, CASESAFEID(Id).
Also, I use the link https://www.adminbooster.com/tool/15to18 to convert 15 character Id to 18 character Id instantly.
There is a knowledge article on converting 15 to 18 character Id and the link for the same is https://help.salesforce.com/apex/HTViewSolution?id=000005288&language=en_US