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Mining

What Is a Chain Reorganization (Reorg)? When the Best Chain Changes

A chain reorganization, or reorg, happens when a node replaces its current best chain tip with a competing branch that has more proof of work, removing some blocks from the best chain and adding others.

Key takeaways

How a reorg happens

Two miners can find blocks at nearly the same time, so for a short while the network has competing tips. When one branch gets another block, it has more work and nodes on the other branch switch to it. That switch is a chain reorganization. The documentation describes the process as removing stale blocks one by one from the tip, adding their transactions back to the memory pool, then adding the replacement blocks in order.

What happens to transactions

A transaction that was in a removed block is not lost. If the new branch does not already contain it, it returns to the pool and is mined again. If the new branch contains a conflicting transaction, one that spends the same coins differently, the original becomes invalid. For a recipient that means a payment can briefly show a confirmation and then lose it.

Depth and risk

One-block reorgs happen regularly on busy networks and are harmless. The risk grows with depth, because a deeper reorg means more work was replaced. That is why services wait for several confirmations before treating a payment as final, and why chains with little hashing power are more exposed than large ones.

What users should do

Wait for a number of confirmations that fits the value at stake. Be most careful with irreversible actions, such as handing over goods, depositing to an exchange or bridging, until the transaction is buried deeply. If you ever see a transaction disappear from the explorer after showing as confirmed, check again later: it is probably in the memory pool or in a different block.

Example

Your payment is in block 5,000 on branch A. A longer branch B appears that does not include it. Nodes reorg to B, your payment returns to the mempool, and a few minutes later it is mined into B's block 5,002.

Put it into practice

Wait for enough confirmations

Reorgs are why one confirmation is not final. With Scrypt Wallet you hold your coins directly, and you can check a block explorer until the confirmations match the amount.

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Sources and verification

Last reviewed 2026-10-02 by the Scrypt Wallet team. The factual claims on this page, and where each comes from:

  1. Peers follow the most difficult chain and discard stale blocks on shorter forks. [1]
  2. During a reorganization, stale blocks are removed from the tip and their transactions are added back to the memory pool. [1]
  3. The paper treats the chain with the most proof of work as valid. [2]

Sources

  1. Bitcoin developer guide: Block chain · Bitcoin.org · primary source
  2. Bitcoin: A Peer-to-Peer Electronic Cash System · Satoshi Nakamoto, 2008 · primary source

Primary sources are protocol papers, specifications, source code or a project's own documentation; secondary sources are wikis and explainers. We checked each claim against the source text on the review date. Found an error? Tell us and we will correct it.

Frequently asked questions

Is a reorg the same as a 51% attack?

No. Small reorgs happen naturally. A 51% attack is a deliberate attempt to rewrite deeper history with majority hashing power.

Can a reorg cancel my payment?

Only if the new branch contains a conflicting spend of the same coins. Otherwise your transaction is mined again.

How many confirmations are safe?

It depends on the amount and the chain's hashing power. Bigger amounts and smaller chains need more.

Related terms

Go deeper

Written by the Scrypt Wallet team · Updated 2026-10-02. General education, not financial, tax or legal advice.