A Mempool is where a Bitcoin node keeps accepted transactions that are not yet confirmed in a block. A node is a computer connected to the Bitcoin network. Each has its own pool, so there is no single shared queue with a guaranteed order. An explorer is a website where you can look up transactions and blocks.
After you press Send, your wallet passes a signed transaction to the network. Nodes receiving it check whether it meets the network rules and their local admission conditions. If it passes, they may keep it in their mempool and relay it onward. Seeing a payment in an explorer therefore means its operator knows about it, not that it is confirmed. Its first confirmation comes only when it enters a valid block in the current chain. Even that can change in a reorganization, when the network moves to another valid branch with more accumulated work. [2]
A mempool is not a checkout queue. Block space is limited, and miners have an incentive to select transactions that pay more for it. The relevant rate is sat/vB: satoshis, the smallest BTC units, per virtual byte, the unit Bitcoin uses to account for transaction size. For example, 100 vB at 10 sat/vB means a fee of 1,000 satoshis. It is not a percentage of the amount sent. If a transaction spends an output of another unconfirmed transaction, the fee of the whole dependent package also matters. Even a high rate promises no particular block or confirmation time. [4]
Each node receives transactions slightly differently and may have different memory settings or rules for accepting and forwarding them. It may also lack an earlier transaction that a new payment depends on. Pending counts and fee estimates can therefore differ. These local operating rules are distinct from consensus, the shared conditions for valid blocks. A node can accept a valid block containing a transaction it never held in its own mempool. [2]
Once confirmed, a transaction no longer waits in the mempool. But it can also leave because memory is scarce, it has waited too long, or another transaction spending the same funds replaces it. Removal from one node is not a network-wide cancellation: another node may retain it and a miner may confirm it later. Do not infer from one missing record that it is safe to pay again using different funds. First check the wallet’s status and any replacement. After a reorganization, some disconnected transactions can return to the pending pool if they pass the checks again. [1] [2]
For an ordinary check, distinguish an unconfirmed payment from one in a block and inspect its fee rate and any preceding transactions. If you run your own Bitcoin Core node, getmempoolentry shows information about one locally known transaction, while getmempoolinfo summarizes the pool. These commands query your node, not a central network-wide list. A public explorer likewise offers its own view; its waiting-time estimate can change as new transactions and blocks arrive. [3]
For the clearest picture, read this entry together with Transaction, Transaction Fees, Mining, Full Node, UTXO. The reverse links also lead from Mining, Transaction, UTXO, Transaction Fees.
01Is there one global Bitcoin mempool?+
No. Each node has its own pool, shaped by the transactions it receives, its settings and memory, and which transactions it accepts and forwards.
02What happens if a transaction disappears from a mempool?+
First check whether it has already been confirmed in a block. If not, one node may have removed or replaced it. Another may still keep it and a miner may confirm it later; disappearing from one mempool does not, by itself, cancel the payment.