BIP141 specifies the following:
Blocks are currently limited to 1,000,000 bytes (1MB) total size. We change this restriction as follows:
Block weight is defined as Base size * 3 + Total size. (rationale)
Base size is the block size in bytes with the original transaction serialization without any witness-related data, as seen by a non-upgraded node.
Total size is the block size in bytes with transactions serialized as described in BIP144, including base data and witness data.
The new rule is block weight ≤ 4,000,000