Block size weight

Since SegWit activated (August 2017), Bitcoin's block capacity limit is 4,000,000 weight units, not a simple byte count. Each non-witness byte counts 4 weight units; each witness byte (signatures and related data) counts 1. Consequences: a block of purely legacy transactions still maxes out near 1 MB, typical mixed blocks run ~1.5–2 MB of raw data, and the theoretical ceiling approaches 4 MB only for witness-dominated blocks. The old fixed "1 MB limit" no longer exists as such.

Nuance: The 4:1 weighting (the "witness discount") was designed to rebalance the cost of spending versus creating outputs and to raise capacity without a hard fork. Fee estimation uses "virtual bytes" (1 vbyte = 4 weight units, so 1M vbytes per block).

Common misstatements:

• "Bitcoin blocks are limited to 1 MB." — True before August 2017; the limit is now 4M weight units (~2 MB typical, ~4 MB ceiling).
• "SegWit raised the block size to 4 MB." — Only witness-heavy blocks approach 4 MB; the effective capacity gain for ordinary payment traffic is roughly 2x.
• "Block size and block weight are the same thing." — Weight is the consensus metric; size in bytes varies for the same weight depending on the witness share.