Every node has its own mempool
A node first checks that an unconfirmed transaction does not violate consensus rules and that it meets the node's local acceptance and relay policy. If accepted, the node may keep it in its memory pool—the mempool—and announce it to peers. Another node may have received it later, rejected it under different settings or evicted it in the meantime.
There is therefore no global mempool with an official position in a queue. A block explorer shows the view of its own node. A transaction may be known to much of the network but still not belong to consensus history. Consensus governs valid blocks; mempool and relay rules are local node policy.
The fee follows data, not the purchase price
A transaction with one input and two outputs can occupy roughly the same space whether it transfers ten euros or ten thousand euros worth of BTC. Wallets therefore compare fee rates, commonly stated in satoshis per virtual byte. More inputs and more complicated spending conditions add data weight.
The fee itself is the difference between the value of the inputs and outputs. When few transactions are waiting, a low fee rate may be enough. Under heavy demand, users offer more for the same space and estimates rise. It is neither a compulsory percentage of the payment nor a charge dictated by one Bitcoin company.
- the absolute fee is counted in satoshis
- the fee rate compares that fee with the transaction's virtual size
- the required rate changes with demand for block space
A miner does not include transactions strictly by arrival time
A block producer selects transactions from its own mempool. It has an economic reason to build a valid package that earns as much in fees as possible. Effective fee rates and relationships between parent and child transactions matter, rather than a simple first-in, first-out queue.
Fee estimates use recent blocks and current conditions, but they cannot know future demand. A new wave of transactions may extend the wait, while a quieter mempool may shorten it. Even the average interval of roughly ten minutes does not mean that the next block must arrive in exactly ten minutes.
A waiting transaction can sometimes be accelerated
Some wallets can create a replacement version with a higher fee or spend an unconfirmed output in a new transaction that gives miners an incentive to include the whole package. These approaches are called replace-by-fee and child-pays-for-parent. Whether they work depends on the transaction, wallet and node policies.
A low fee does not automatically mean that bitcoins are lost. The transaction may confirm later, be evicted from mempools or be replaced by a conflicting version under applicable rules and policies. Check its state in your own wallet before acting. Never give recovery words to an online service that claims it can ‘accelerate’ a transaction.