Finality, Reorgs and Confirmation Depth

Wire a payment through a bank and, once it clears, the transaction is done — the bank's ledger will not later decide it never happened. Most blockchains don't offer that same instant certainty. A transaction can be included in a block, appear to have gone through, and then get erased a few minutes later because a different, longer chain of blocks wins out instead. Understanding how long you need to wait before treating a crypto transaction as truly settled is a basic operational question every exchange, custodian, and trading desk has to answer.

Why a block can get undone

In blockchains that build a chain by having many participants race to produce the next block, it's possible for two valid blocks to be created at nearly the same time, temporarily splitting the network into two competing versions of the chain. The network's rule is to eventually follow whichever chain is longer (or, more precisely, has accumulated more proof of work or stake behind it). When that resolution happens, blocks on the losing chain — along with any transactions inside them that don't also appear on the winning chain — are discarded. This event is called a reorg (reorganization). A one-block reorg is common and mostly harmless; a deep reorg, undoing many blocks at once, is rare but far more damaging, since it can reverse transactions people already treated as settled.

Finality is the property of a transaction becoming irreversible — no longer subject to being undone by a reorg. Some newer blockchain designs offer fast, explicit finality after a short delay; older proof-of-work chains like Bitcoin only offer probabilistic finality, where the chance of a reorg reversing a transaction shrinks the more blocks get built on top of it, but technically never reaches exactly zero.

Confirmation depth as the practical fix

Because finality is probabilistic on many chains, exchanges and custodians wait for a number of confirmations — additional blocks built on top of the one containing your transaction — before treating a deposit as final and crediting your account. A low-value deposit might only require one or two confirmations; a large withdrawal or an exchange listing might require dozens, because the cost of an attacker successfully reversing that transaction needs to exceed what they'd gain by cheating. The required confirmation depth is essentially a risk decision, traded off against how long a legitimate customer is willing to wait.