When Satoshi launched Bitcoin, the currency had virtually no value, and only a few people were mining for new coins. It was just a novelty at that point, with people trading around UTXOs as a way to distribute ownership of the coins and bootstrap interest in the network.
For the first few years of Bitcoin, individual transactions were relatively small from a data standpoint, so the size limit was never an issue since it did not affect the network.
Fast forward a few years, and that bootstrap network started to develop a market price users were exchanging them for fiat currency, which turned into speculating on this asset. Exchanges made it easier for users to get access to trade and transfer Bitcoin, which began to put pressure on the underlying blockchain.
As more people rolled in, transactions started to fill up those 1MB blocks regularly, and something needed to be done to improve throughput.
The obvious option was to increase the block size, which kicked off the block size wars. Satoshi never indicated why the limit was put in place; it is speculated that it was to keep the blockchain small and discourage large amounts of spam transactions.
So, the size was always up for debate; some wanted 4MB blocks, some 8MB blocks, and others wanted a more nuanced approach with Bitcoin’s block weight.
The concept was introduced with the SegWit (Segregated Witness) upgrade in 2017, and it fundamentally changed how Bitcoin measures and limits the size of blocks in the blockchain.
What is Block Weight?
Block weight is a measurement system that determines how much data can fit into a Bitcoin block. It replaced the simple block size limit of 1MB that existed before SegWit. The block weight is measured in “weight units” (WU), with a maximum limit of 4 million weight units per block.
The primary purpose of block weight is to provide a more nuanced approach to calculating how much space a transaction occupies.
Under this system, the signature data within a transaction receives a “discount,” making it less costly compared to other types of transaction data. This means that, although signature data is vital for the validation of a transaction, it doesn’t take up as much space inside a block.
The calculation of a transaction’s weight involves multiplying the sizes of different parts of the transaction data by specific values.
How Block Weight is Calculated?
The block weight calculation uses a weighted formula that differentiates between witness (SegWit) data and non-witness data:
- Non-witness data (transaction data without signatures) is counted as 4 weight units per byte
- Witness data (primarily signatures) is counted as 1 weight unit per byte
The total block weight is calculated using this formula:
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Block Weight = (Base size × 4) + Witness size
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Why Block Weight Was Introduced?
Before Segwit, the block limit was 1MB, as measured in bytes, and was called block size.
The introduction of block weight solved several critical issues:
- Transaction Malleability: By segregating witness data, SegWit fixed the transaction malleability problem, which was crucial for implementing second-layer solutions like the Lightning Network.
- Backward Compatibility: The block weight system allowed older Bitcoin nodes to continue operating without upgrading, as they would still see blocks as being within the traditional 1MB limit.
- Effective Block Size Increase: While maintaining backward compatibility, the new system effectively allowed blocks to be larger than 1MB, increasing transaction throughput.
Real-World Impact on Bitcoin Transactions
The block weight system has several practical implications:
Transaction Capacity
- A block can now theoretically contain up to 4MB of data if it consists purely of witness data
- In practice, blocks typically range from 1.5MB to 2MB in size
- This increased capacity allows for approximately 2,000 to 2,500 transactions per block
Fee Economics
- Witness data is discounted in the weight calculation
- This incentivizes the use of SegWit addresses and transactions
- Users can save on transaction fees by using SegWit-compatible wallets
Technical Details
Bitcoin transactions can fall into different weight categories:
- Legacy Transactions: These use the original transaction format and consume the most weight units
- SegWit v0: The first version of SegWit transactions (P2SH-P2WPKH and native P2WPKH)
- SegWit v1 (Taproot): The newest transaction type, offering further optimizations
Block Weight Examples
To illustrate how block weight works in practice:
Example 1: Legacy Transaction
- Transaction size: 250 bytes
- Block weight: 250 × 4 = 1,000 weight units
Example 2: SegWit Transaction
- Base size: 200 bytes
- Witness size: 100 bytes
- Block weight: (200 × 4) + 100 = 900 weight units
Future Implications
The block weight system continues to shape Bitcoin’s development:
- Scalability Solutions: The system provides a foundation for future scalability improvements and layer-2 solutions.
- Network Efficiency: By incentivising more efficient transaction types, the network gradually becomes more optimised.
- Future Upgrades: The weight system’s flexibility allows for potential future upgrades without requiring fundamental changes to the block size limits.
Worth your weight in Digital Gold
Block weight is more than just a technical parameter—it was a crucial innovation that enabled Bitcoin to scale while maintaining decentralisation and backward compatibility. It is part of the upgrade that gives us the Lightning Network and keeps blocks small enough to encourage the use of second-layer solutions.
While it has given the network some additional runway in scaling without eliminating large portions of the nodes on the network that cannot store a full archive of all transactions, it isn’t going to be enough to stop users from having to make the decision to either fork over extra cash and purchase larger hard drives or pruning their nodes.
This runway has only been further eroded by the launch of ordinals, which encouraged speculators to fill blocks with metadata such as images and other files or use it to create tokens and NFTs via meta-protocols like Runes, BRC-20 and more.

Do your own research.
If you want to learn more about Block Weight, use this article as a starting point. Don’t trust what we say as the final word. Take the time to research other sources, and you can start by checking out the resources below.