Difference between 15 digit Id and 18 digit Id in Salesforce

In salesforce each record Id represents a unique record within an organisation. There are two versions of every record Id in salesforce :

• 15 digit case-sensitive version which is referenced in the UI
• 18 digit case-insensitive version which is referenced through the API

The last 3 digits of the 18 digit ID are a checksum of the capitalizations of the first 15 characters, this ID length was created as a workaround to legacy systems which were not compatible with case-sensitive IDs.

The API will accept the 15 digit ID as input but will always return the 18 digit ID.

You can create a custom formula field that returns type text, place the code below  in the formula field , you will got an 18 digit ID that you can easily add to any report.

Id
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,1,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,2,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,3,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,4,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,5,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,6,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,7,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,8,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,9,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,10,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)
& MID("ABCDEFGHIJKLMNOPQRSTUVWXYZ012345",(
    IF(FIND(MID(Id,11,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,1,0)
    +IF(FIND(MID(Id,12,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,2,0)
    +IF(FIND(MID(Id,13,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,4,0)
    +IF(FIND(MID(Id,14,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,8,0)
    +IF(FIND(MID(Id,15,1), "ABCDEFGHIJKLMNOPQRSTUVWXYZ")>0,16,0)
    )+1,1)

How to calculate the 18 Digit Id from 15 Digit Id programatically:

//Our 15 Digit Id
String id = '00570000001ZwTi' ;

string suffix = '';
integer flags;

for (integer i = 0; i < 3; i++) {
          flags = 0;
          for (integer j = 0; j < 5; j++) {
               string c = id.substring(i * 5 + j,i * 5 + j + 1);
               //Only add to flags if c is an uppercase letter:
               if (c.toUpperCase().equals(c) && c >= 'A' && c <= 'Z') {
                    flags = flags + (1 << j);
               }
          }
          if (flags <= 25) {
               suffix = suffix + 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'.substring(flags,flags+1);
          }else{
              suffix += '012345'.substring(flags - 26, flags-25);
          }
     }

//18 Digit Id with checksum
System.debug(' ::::::: ' + id + suffix) ;