Block Size and Block Weight

💡 The Plain-English Definition

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

🤔 But Why Though?

Satoshi originally added a 1MB block-size limit as a temporary anti-spam measure — a way to stop bad actors from flooding the network with enormous blocks and making it expensive to run a node, a computer that independently validates the entire blockchain. As Bitcoin grew, that limit became a real constraint: at 1MB per block and roughly ten-minute blocks, Bitcoin can process only about 3–7 transactions per second.

The limit became the centre of what’s called the block size wars — a years-long fight over whether to raise it and, if so, by how much. The debate was never purely technical; it was philosophical. Bigger blocks would allow more transactions at lower fees, but they’d also make running a full node more expensive, which could concentrate node operation among well-resourced players and reduce decentralisation — the spreading of control across many independent participants. The side that wanted bigger blocks eventually split off, in a hard fork, to create Bitcoin Cash. Bitcoin itself took a different path: SegWit.

SegWit, activated in August 2017, didn’t simply raise the limit. It changed what counts toward it, introducing “block weight” — a measurement where a transaction’s signature data (its witness data) counts as only one quarter of the weight of the rest. The hard cap became 4 million weight units. A block full of legacy transactions reaches that at around 1MB; a block full of SegWit transactions can hold roughly 1.5–2MB; and a block full of Taproot transactions — from the 2021 upgrade that improved privacy and efficiency — can approach 4MB in extreme cases.

The reason not to simply make blocks bigger is that larger blocks mean more data for every node to store, process, and pass on, which raises the hardware and bandwidth needed and can thin out the number of independent node operators. The Lightning Network — Bitcoin’s second layer for fast, cheap off-chain payments — is the scaling path Bitcoin chose instead: keep the base layer conservative and build speed and capacity in layers on top.

🌍 The Real-World Analogy

Think of Bitcoin’s base layer like a settlement system for banks — not built for your daily coffee, but for final, irreversible settlement of large amounts. Banks don’t push every debit-card swipe through the Federal Reserve’s settlement system. They tally transactions up internally and settle the net amounts in bulk. Lightning does the equivalent for Bitcoin: handle the coffee instantly off-chain, then settle the net position on-chain from time to time.