A chain reorganization (reorg) occurs when a node discovers a longer or heavier valid chain and switches to it, abandoning the blocks it previously considered canonical. Transactions in orphaned blocks return to the mempool. Reorgs are a normal (if rare) occurrence in probabilistic-finality systems.
What Is a Chain Reorg?
2 min read
The short version
A reorg is like realizing the road you were on isn't actually the main highway, there's a longer road that branched off a few miles back. Your GPS recalculates and you switch routes. The transactions you "saw" on the abandoned road go back to the waiting pool to be re-included on the correct route.
How It Works
Reorgs happen naturally when two miners find valid blocks at roughly the same time. Part of the network builds on one, part on the other. Once one fork gets an additional block first, the shorter fork is orphaned. Depth-1 reorgs are common (happen daily on Bitcoin). Deeper reorgs (2+ blocks) are rare under normal conditions, a depth-6 reorg on Bitcoin would indicate an attack or severe network partition. On Ethereum PoS, reorgs deeper than 1 slot are extremely rare due to the attestation mechanism, and anything finalized cannot be reorged at all.
A natural 1-block reorg on Bitcoin
Miners A and B both find block 850,001 within 2 seconds of each other. Half the network receives A's block first and starts mining 850,002 on top of it. The other half builds on B's block. After 8 minutes, a miner on team A finds 850,002. Team B's nodes see A's chain is now longer (2 blocks vs 1) and reorganize: they drop B's block 850,001, adopt A's chain, and return B's unique transactions to the mempool. Those transactions are included in the next block. Users with 1 confirmation on B's block temporarily saw their transaction "unconfirmed" again.
What People Get Wrong
Reorgs mean the blockchain is broken
Shallow reorgs (1-2 blocks) are a normal part of how distributed systems resolve temporary disagreements. They're expected and handled gracefully.
Reorgs always indicate an attack
Most reorgs are natural race conditions, two miners finding blocks simultaneously. Only unusually deep reorgs (3+ on Bitcoin) suggest malicious activity or severe network issues.
Finalized transactions can be reorged
On Ethereum PoS, finalized transactions (2+ epochs old) cannot be reorganized. Only the most recent unfinalized slots are susceptible to reorgs.
Keep Reading
Sources & Further Reading
- Bitcoin Reorg Monitor (Fork Monitor)
Live monitoring for Bitcoin chain reorganizations and stale blocks
- Understanding Reorgs (BitMEX Research)
Research analysis of historical Bitcoin reorgs and their frequency
Questions People Also Ask
- How often do reorgs happen on Bitcoin?
- 1-block reorgs happen roughly once per week on average. 2-block reorgs are very rare (few times per year). Anything deeper than 2 would be major news.
- Do I lose my coins in a reorg?
- No. Your transaction returns to the mempool and will likely be re-included in the next block. You just need to wait for re-confirmation. The coins never left your control.
- Can reorgs be prevented?
- They can be minimized by faster block propagation and finality mechanisms, but not completely eliminated in probabilistic systems. Absolute-finality chains (Tendermint) don't have reorgs at all, but trade other properties.