Overview
A reorg (reorganization) occurs when a Bitcoin node receives a valid chain of blocks that has more cumulative proof of work than its current chain tip. The node switches to the heavier chain, "undoing" the blocks it previously considered canonical and replacing them with the new chain. Transactions that were confirmed in the abandoned blocks may become unconfirmed, which is why Bitcoin users wait for multiple confirmations before considering a payment settled.
How a Reorg Happens
Normal chain progression:
Block 100 ── Block 101 ── Block 102 ── Block 103
▲
chain tip
A competing chain is discovered:
Block 100 ── Block 101 ── Block 102 ── Block 103
│
└── Block 101' ── Block 102' ── Block 103' ── Block 104'
▲
heavier chain
After reorg:
Block 100 ── Block 101' ── Block 102' ── Block 103' ── Block 104'
▲
new chain tip
Blocks 101, 102, 103 are abandoned (orphaned)
Transactions in those blocks return to the mempool
(unless they conflict with transactions in the new chain)
Types of Reorgs
- Natural reorgs (1-block) — These happen occasionally when two miners find a block at nearly the same time. The network briefly has two competing chain tips, and one is abandoned when the next block is found. These are normal and harmless.
- Deep reorgs — Reorganizations of 2+ blocks are extremely rare under normal conditions and may indicate an attack or major network issue.
- Attack reorgs — A malicious miner with sufficient hashrate could attempt to secretly mine a longer chain to reverse their own transactions (double-spend attack).
Impact on Transactions
When a reorg occurs, transactions in the orphaned blocks fall into one of two categories:
- Still valid — The transaction is returned to the mempool and will likely be included in a future block on the new chain
- Conflicting — If the new chain includes a transaction that spends the same inputs (a double-spend), the original transaction becomes invalid
Confirmation Depth and Security
The probability of a reorg decreases exponentially with each additional block built on top of a transaction. This is why merchants and exchanges typically require:
- 1 confirmation — Sufficient for small, trusted transactions
- 3 confirmations — Common for moderate-value transactions
- 6 confirmations — Traditional standard for high-value transactions (approximately 1 hour)
Common Misconception
Small, one-block reorgs are not attacks. They are a natural consequence of the decentralized mining process where two miners can independently find valid blocks at nearly the same time. The protocol handles this gracefully by always following the chain with the most cumulative proof of work.