Mempool
The mempool is the queue of unconfirmed transactions. Your transfer waits there until a miner or validator includes it in a block.
How it works
When you send a transfer, it first lands in the mempool (short for memory pool). A miner or validator picks transactions from the pool and puts them into a new block. Until your transfer is in a block, it is not confirmed and the recipient has not received the money yet.
Every node keeps its own mempool, so they can differ a little. Block space is limited, so when the network is busy a queue builds up. Transactions with higher fees tend to go first: in Bitcoin the fee is measured in satoshis per virtual byte (sat/vB), in Ethereum in gwei.
A stuck transaction can often be sped up. Bitcoin has RBF (Replace-by-Fee, resending the same transaction with a higher fee) and CPFP (the recipient spends the unconfirmed output with a high fee). On Ethereum you send a new transaction with the same nonce (sequence number) and a higher fee. If a very low-fee transaction sits too long, nodes may drop it and the coins stay with you.
The network is congested and you sent a transfer with a minimal fee. It sits in the mempool longer than the rest. Open a block explorer, paste your hash and check the status. Many wallets let you resend with a higher fee; follow your own wallet instructions.
Your BTC transfer is 140 vB. At 5 sat/vB you pay 700 sats, but the mempool is currently clearing transactions from 20 sat/vB. To get into the next few blocks you need about 140 × 20 = 2,800 sats.
Do not send the same amount again until you are sure the first transaction failed. Otherwise you can pay twice, because both transactions may confirm. Check the status in a block explorer first and use your wallet's speed-up feature if it has one.
Questions and answers
What is the mempool in simple terms?
It is a waiting room for transactions. A transfer sits there until it is included in a block, and only then is it confirmed.
Why is my transaction stuck in the mempool?
Most likely its fee is lower than others in the queue and the network is busy. You can speed it up by replacing it with a higher fee or wait for the queue to clear.
How long can a transaction stay in the mempool?
From minutes to several days, depending on network load and your fee. If nobody picks it up, nodes may drop it, and the coins are not lost: they stay at your address.
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Updated: 2026-10-19