Block Size and Block Weight

💡 The Plain-English Definition

Block size refers to how much data a Bitcoin block can contain. For most of Bitcoin’s history this was capped at 1 megabyte. After the SegWit (Segregated Witness — a 2017 upgrade that changed how transaction data is stored and measured) upgrade, the measurement shifted to “block weight,” with an effective limit equivalent to roughly 2–4 megabytes depending on transaction types.

🤔 But Why Though?

Satoshi originally included a 1MB block size limit as a temporary spam prevention measure — a way to stop bad actors from flooding the network with enormous blocks and making it expensive to run a node (a computer independently validating the entire Bitcoin blockchain). As Bitcoin grew, this limit became a genuine constraint: at 1MB per block and roughly 10-minute block times, Bitcoin can process approximately 3–7 transactions per second. The limit became the central battle of what’s known as the block size wars — a years-long conflict about whether to simply increase the limit and, if so, by how much. The debate was never purely technical. It was philosophical: a larger block size would allow more transactions at lower fees, but it would also make running a full node more expensive, potentially concentrating node operation among well-resourced participants and reducing decentralisation. The side that wanted larger blocks eventually hard-forked to create Bitcoin Cash. Bitcoin itself took a different path: SegWit.

SegWit, activated in August 2017, didn’t simply raise the block size limit. It changed what counted toward the limit by introducing “block weight” — a new measurement where transaction signature data (witness data) counts at one quarter of the weight of other transaction data. The hard limit became 4 million weight units. A block filled with legacy transactions hits this at around 1MB. A block filled with SegWit transactions can contain roughly 1.5–2MB of data. A block with Taproot (a 2021 Bitcoin upgrade improving privacy and efficiency) transactions can approach 4MB in extreme cases. Larger blocks would mean more data for nodes to store, process, and relay — raising hardware and bandwidth requirements and potentially reducing the number of independent node operators. The Lightning Network (Bitcoin’s second-layer payment system for fast, cheap off-chain transactions) represents the path Bitcoin chose for scaling: keep the base layer conservative and build speed and capacity in layers above it.

🌍 The Real-World Analogy

Think of Bitcoin’s base layer like a settlement system for banks — not designed for your daily coffee purchase, but for final, irrefutable settlement of large value. Banks don’t send individual transactions through the Federal Reserve’s settlement system for every debit card swipe. They net them out internally and settle net positions in bulk. Lightning does the equivalent for Bitcoin: handle the coffee at lightning speed off-chain, settle the net position on-chain periodically.