Bitcoin fees, mempool and confirmations

The essentials

Fees are commonly quoted in satoshis per virtual byte. A satoshi is one hundred-millionth of a bitcoin; virtual size reflects transaction weight. Cost depends more on transaction structure and competing demand than on the amount transferred. A large-value payment can be cheaper than consolidating many small outputs.

How it works

Nodes maintain their own pools of pending transactions; there is no single authoritative global mempool. A transaction included in the current tip has one confirmation. Each additional block increases the count, but the appropriate acceptance threshold depends on the payment and network conditions.

What to watch

Replace-by-fee and child-pays-for-parent can improve the fee incentive for pending transactions, subject to relevant policies and available outputs. They do not reverse a confirmed payment. Distinguish throughput, time to inclusion and settlement confidence: faster inclusion does not by itself eliminate the possibility of a reorganisation.

Understand the details

Bitcoin fees depend largely on transaction weight and competition for block space. Wallets commonly quote satoshis per virtual byte. A payment of a large amount can be smaller in data size than a small payment consuming many inputs. Fee estimates are forecasts of demand, not reservations guaranteeing a block.

Boundaries and common mistakes

Replacement policies and child-pays-for-parent can help some pending transactions, but support and eligibility vary. A replacement changes the transaction identifier and requires careful recipient tracking. A confirmation policy should also account for value and reorganization risk; paying more affects inclusion incentives rather than making invalid transactions acceptable.

The mechanism at a glance

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