Key Takeaways
Segregated Witness (SegWit) is a soft fork upgrade to the Bitcoin protocol that separates signature data from transaction data, increasing the number of transactions that can fit in each block.
By reorganizing how witness data is stored and weighted, SegWit increased practical block capacity by roughly 1.7 to 2 times, with a theoretical ceiling equivalent to 4 MB of total data.
SegWit fixed transaction malleability, a bug that prevented reliable second-layer development. This fix enabled the Lightning Network and opened the door for more advanced scripting on Bitcoin.
Three SegWit-based address formats exist: Nested SegWit (P2SH, starting with "3"), Native SegWit (bech32, starting with "bc1q"), and Taproot (bech32m, starting with "bc1p").
Since its activation in 2021, the Taproot upgrade has built on SegWit v0 foundations, introducing Schnorr signatures and improved privacy for complex transactions.
Introduction
Segregated Witness, commonly known as SegWit, is a protocol upgrade for the Bitcoin blockchain. It was first proposed by developer Pieter Wuille in 2015 and formally specified as BIP 141, then activated as a soft fork in August 2017 after a community-driven process.
The core idea is straightforward: remove the witness data (digital signatures) from each transaction and store it in a separate structure, rather than inside the transaction input itself.
This change had a significant impact on Bitcoin. It increased the number of transactions that can fit in a single block, lowered transaction fees during periods of high demand, and fixed a long-standing bug that had limited what developers could build on top of the Bitcoin network.
SegWit is now widely adopted across wallets, exchanges, and nodes. It also laid the groundwork for subsequent Bitcoin upgrades, including the Taproot soft fork activated in November 2021.
What Is SegWit?
Bitcoin blocks have a capacity limit. The original 1 MB limit was intended to prevent spam, but it also created a bottleneck as Bitcoin grew in popularity. Blocks filled up quickly during periods of high demand, leading to long confirmation times and rising fees. The network could process only around seven transactions per second, far below conventional payment networks.
SegWit addresses this by restructuring how transaction data is organized. A Bitcoin transaction has two main components: the transaction data (who sends how much to whom) and the witness data, which includes the digital signature proving the sender owns the funds.
Before SegWit, the signature was bundled inside the transaction input and counted fully against the block capacity. Signatures could account for up to roughly 65% of a block's data.
SegWit moves the signature outside the main transaction data into a separate witness field. This witness data is counted differently under a new block weight system. Each byte of non-witness transaction data is weighted at four weight units (WU), but each byte of witness data counts as only one WU.
The total block limit is 4 million weight units rather than the old 1 MB byte limit. Because witness bytes carry less weight, blocks can effectively hold more transactions without changing Bitcoin's fundamental security model.
In practice, a block filled entirely with SegWit transactions typically reaches around 1.6 to 2.3 MB, roughly 1.7 to 2 times the capacity of pre-SegWit blocks. The theoretical maximum of 4 MB would require a block composed entirely of witness data, which cannot occur because every block requires non-witness transaction data. SegWit was deployed as a soft fork, meaning nodes running older software could still follow the chain, even if they did not enforce the new rules.
What Are the Main Benefits of SegWit?
Increased block capacity
By discounting the weight of witness data, SegWit allows more transactions to be included in each block. While Bitcoin's 10-minute average block interval does not change, the number of transactions confirmed per block increases. This means more users can have their transactions processed in each 10-minute window, reducing the competition for block space that drove up fees before SegWit was widely adopted.
Lower transaction fees
With more transaction capacity available per block, the average fee users need to pay for timely confirmation tends to be lower than it would have been under the original 1 MB limit. During peak demand periods before SegWit was widely used, transaction fees sometimes exceeded $30, and confirmation delays stretched to hours or even days. While Bitcoin fees still rise and fall with network demand, SegWit added headroom that helps absorb spikes in activity.
Transaction malleability fix
A notable flaw in the original Bitcoin protocol was transaction malleability. A third party could theoretically alter a transaction's digital signature in a way that changed the transaction ID (TxID) without invalidating the transaction itself. This made it difficult to build reliable smart contracts and second-layer protocols on Bitcoin, since a transaction could be modified after being broadcast but before being confirmed.
By separating signatures from the data used to compute the TxID, SegWit eliminated this vulnerability for SegWit transactions. The TxID for a SegWit transaction is calculated from only the core transaction data, meaning it cannot be altered by modifying the witness. This fix made it possible to develop protocols that depend on unmodifiable, predictable transaction IDs.
SegWit and the Lightning Network
One of the most important outcomes of the malleability fix was enabling the Lightning Network. The Lightning Network is a second-layer payment protocol that operates on top of Bitcoin. It allows users to open payment channels, conduct many transactions off-chain among themselves, and then settle only the final balance on the Bitcoin blockchain.
The Lightning Network depends on the certainty that transaction IDs cannot be altered while channels are open. Without the SegWit malleability fix, a channel partner could potentially modify a transaction in transit, breaking the trust assumptions the protocol relies on. By making TxIDs trustworthy, SegWit provided the foundation the Lightning Network required.
Lightning Network transactions are typically near-instant and carry very low fees, making smaller payments practical on Bitcoin. This scaling approach can dramatically increase throughput without every transaction needing to be recorded on the main chain.
SegWit vs. SegWit2x
While SegWit was being debated, a competing proposal called SegWit2x (or S2X) emerged. SegWit2x proposed implementing SegWit while also doubling the base block size limit to 2 MB. This would have required a hard fork, meaning nodes that did not upgrade would no longer be compatible with the new chain.
SegWit, by contrast, was deployed as a soft fork. This made it backward-compatible: nodes running older software could still process transactions and follow the chain. The adoption was driven through a process called a User-Activated Soft Fork (UASF), where economic participants coordinated to enforce the new rules.
SegWit2x was ultimately abandoned in November 2017 after its proponents could not reach sufficient consensus among miners, developers, and node operators. The debate highlighted how contentious proposed changes to Bitcoin's core rules can become, given its decentralized governance.
SegWit Address Formats
There are three address formats associated with SegWit outputs on Bitcoin. Nested SegWit, also known as Pay-to-Script-Hash (P2SH), was designed for compatibility with older wallets. It wraps the SegWit structure inside a traditional address format, so wallets that had not yet adopted SegWit could still send funds to it. Nested SegWit addresses start with the number "3."
Native SegWit v0, also called bech32, is the more efficient format. Bech32 addresses start with "bc1q" and are entirely lowercase. They support better error-detection mechanisms and produce slightly smaller transactions, which means lower fees compared to Nested SegWit. Since Native SegWit omits the compatibility wrapper, transactions are cheaper and the address format is more compact.
Native SegWit v1, introduced with the Taproot upgrade in November 2021, uses an updated encoding called bech32m. Taproot addresses start with "bc1p." Bech32m is similar to bech32 but corrects an edge-case weakness in the original encoding. Taproot builds on SegWit v0 by adding Schnorr signatures and improved scripting capabilities.
Transactions between Legacy (non-SegWit), Nested SegWit, Native SegWit v0, and Native SegWit v1 addresses are fully compatible on the Bitcoin network. However, not all wallets and exchanges have updated to support bech32m, so you should check compatibility before withdrawing to a bc1p address.
SegWit Adoption and What Came Next
SegWit adoption grew gradually after activation in August 2017. By 2025, the large majority of Bitcoin transactions are estimated to use SegWit or Taproot outputs, with Native SegWit (bech32) addresses accounting for the largest share. This widespread adoption has helped reduce average transaction fees and made the Lightning Network more practical.
In November 2021, Bitcoin activated Taproot, a soft fork upgrade that builds directly on SegWit's foundations. Taproot introduced Schnorr signatures, which are more efficient and compact than the ECDSA signatures used in earlier Bitcoin transactions.
It also expanded Bitcoin's scripting capabilities and improved privacy for complex transactions, such as multi-signature setups where a single Taproot transaction can look indistinguishable from an ordinary payment.
Together, SegWit and Taproot represent two of the most significant protocol upgrades in Bitcoin's history. They reflect the cautious, consensus-driven approach the Bitcoin development community takes to evolving the protocol.
FAQ
What does "segregated witness" mean?
"Segregated" means separated, and "witness" refers to the digital signature data that proves ownership of the funds being spent. SegWit separates this signature data from the main transaction data so that it counts less toward the block weight limit.
Does SegWit actually increase the block size?
Instead of increasing the byte limit directly, SegWit replaced Bitcoin's 1 MB block size limit with a 4 million weight unit (WU) system. Non-witness data counts as 4 WU per byte and witness data counts as 1 WU per byte. In practice, this allows blocks of roughly 1.6 to 2.3 MB under typical conditions. The old 1 MB limit on raw non-witness data no longer exists as a standalone rule; it was superseded by the weight unit calculation.
Is Native SegWit better than Nested SegWit?
For most users, Native SegWit (bech32) addresses are preferred because they offer lower transaction fees, better error detection, and a cleaner address format. Nested SegWit exists primarily for backward compatibility with older wallets and services that do not yet support native bech32 addresses. Taproot addresses (bech32m, starting with bc1p) offer additional privacy and efficiency benefits for more advanced use cases.
How does SegWit relate to Taproot?
Taproot is a 2021 Bitcoin upgrade that builds directly on SegWit. It requires SegWit to function, since Taproot outputs use the SegWit v1 format with bech32m addresses. Taproot added Schnorr signatures and improved scripting flexibility, making complex transactions more private and efficient than SegWit v0 alone.
Why was SegWit controversial?
SegWit was debated extensively in the Bitcoin community, primarily because different stakeholders disagreed on how to scale Bitcoin. Some participants wanted to increase the base block size directly through a hard fork (as proposed by SegWit2x), while others favored the more conservative soft-fork approach SegWit took.
The debate ultimately demonstrated Bitcoin's decentralized governance, as the community coordinated through a User-Activated Soft Fork (UASF) to deploy the upgrade without requiring miner approval.
Closing Thoughts
SegWit was a landmark upgrade for Bitcoin, and the way it was adopted through community coordination rather than top-down decision-making serves as a notable example of decentralized protocol governance in practice.
By restructuring how transaction data is stored and weighted, SegWit increased Bitcoin's practical throughput, reduced fee pressure, and eliminated the transaction malleability bug that had blocked second-layer development for years.
The Lightning Network, which depends on SegWit's malleability fix, has continued to grow and represents one of the more practical approaches to scaling Bitcoin for smaller, everyday payments.
Meanwhile, Taproot extended SegWit's foundations further in 2021, improving privacy and scripting capabilities. While scalability remains an ongoing challenge for blockchain technology broadly, SegWit and the upgrades it enabled represent meaningful steps in that direction.
Further Reading
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