How long is a Salesforce ID?

15 digit , it may be number and letters.
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15 digit , it may be number Or letters.

There are 2 Salesforce IDs
you have a 15 digit ID - which you can get when you extract IDs through reporting
you have an 18 digit ID - which you extract through Dataloader Export
The difference is that the 18 digit ID is completely unique and is preferable option to use when you want to do any data manipulation in Excel. The 15 digit ID you can't use in Excel as Excel ignores case sensitive.