What Is the Difference between Block Size and Block Weight in Bitcoin?

Before SegWit, blocks had a strict size limit of 1MB. After SegWit, the concept of "block weight" was introduced.

Block weight is calculated as (base size 3) + total size, where base size excludes witness data and total size includes it. The maximum block weight is 4 million weight units (4MWU), effectively allowing for a larger capacity block than the original 1MB.