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Block Size | Bitcoin Glossary | Mapping Bitcoin

Block Size

Protocol

Also known as: block weight, block size limit

The maximum amount of data that can be included in a single Bitcoin block. After SegWit, the limit is measured in weight units (4 million weight units per block), which translates to roughly 1-4 MB of data depending on the transaction types included.

Overview

Block size is one of the most consequential parameters in Bitcoin's protocol, directly affecting transaction throughput, fee pressure, and the feasibility of running a full node. The original 1 MB limit was replaced by a weight-based system with the activation of SegWit in 2017, allowing blocks to contain up to 4 million weight units.

Weight Units

SegWit introduced a new way to measure block capacity called "weight":

Transaction data type        Weight multiplier
────────────────────────────────────────────────
Non-witness data (legacy)    4x (each byte = 4 WU)
Witness data (signatures)    1x (each byte = 1 WU)

Maximum block weight: 4,000,000 weight units (4 MWU)

Example block compositions:
  All legacy transactions:     ~1.0 MB    (4 MWU / 4 = 1 MB)
  All SegWit transactions:     ~2.0-2.3 MB
  Theoretical maximum:         ~4.0 MB    (if almost all witness data)

The Block Size Debate

The appropriate block size was one of the most contentious debates in Bitcoin's history (2015-2017). The core tension was between two approaches:

  • Bigger blocks: Increase the limit to accommodate more transactions per block, potentially sacrificing decentralization by making nodes more expensive to run
  • Optimize + layer: Keep blocks small to preserve decentralization, and scale through protocol optimizations (SegWit) and Layer 2 solutions like the Lightning Network

The Bitcoin community ultimately chose the second approach.

Impact on Fees

When blocks are full, transactions compete for limited space by offering higher fees. This creates a fee market where users balance urgency against cost. During periods of high demand, fees can spike significantly, incentivizing the use of fee-efficient practices like batching and SegWit address formats.

Why the Limit Exists

The block size limit serves a critical role in maintaining decentralization. Larger blocks require more bandwidth, storage, and processing power to validate. Keeping the limit manageable ensures that ordinary users can run full nodes on consumer-grade hardware, maintaining the network's resistance to censorship and manipulation.

Related Terms

BlockProtocol

A data structure containing a set of validated transactions, a reference to the previous block, and a proof-of-work solution. Blocks are appended to the blockchain approximately every 10 minutes and form the permanent, ordered record of all Bitcoin transactions.

SegWitProtocol

Segregated Witness, a soft fork upgrade activated in 2017 that separates (segregates) signature data (witness) from transaction data. SegWit fixes transaction malleability, enables the Lightning Network, increases effective block capacity, and reduces fees for compliant transactions.

Witness DiscountProtocol

A fee calculation rule introduced by SegWit that counts witness data at one-quarter the weight of non-witness data. This discount incentivizes moving signature data to the witness section, effectively increasing block capacity and reducing fees for SegWit transactions.

TransactionProtocol

A signed data structure that transfers bitcoin from one or more inputs to one or more outputs. Transactions reference previous UTXOs, include scripts or witness data proving authorization, and are broadcast to the network for inclusion in a block.