To convert a Salesforce ID from 15 characters to 18 characters, you can use the following algorithm:

1. Take the 15-character Salesforce ID.
2. Append three characters of zeros (“000”) to the end of the ID.
3. Calculate the last three characters of the 18-character ID using a modified Base 62 conversion algorithm.

Here’s an example to illustrate the conversion:

15-character Salesforce ID: 003ABCDEF123456

Conversion steps:

1. Append three zeros: 003ABCDEF123456000
2. Extract the case-sensitive base-62 encoded value from the last three characters: 000
   • ‘0’ in base 62 represents 0 in decimal
   • ‘0’ in base 62 represents 0 in decimal
   • ‘0’ in base 62 represents 0 in decimal
   • Combine these three decimal values: 000
3. Insert the decimal value from step 2 into the 16th, 17th, and 18th characters of the ID: 003ABCDEF123456000

The resulting 18-character Salesforce ID after conversion would be: 003ABCDEF123456000

Please note that this conversion process is not reversible. If you have an 18-character Salesforce ID and want to convert it back to a 15-character ID, the original 15-character ID would be lost as the conversion involves additional characters to represent the ID.