What is the Bitcoin mempool?
The waiting room every Bitcoin transaction passes through — and why yours might be stuck in it.
Definition
The mempool (memory pool) is the set of valid, unconfirmed transactions a Bitcoin node keeps in memory while they wait to be mined. There isn't one global mempool: every node has its own, and they're usually very similar because nodes relay transactions to each other.
How miners use it
A miner builds a block template from its mempool, picking the transactions (and packages of parent + child) with the highest fee rates until the block is full. Everything else stays in the mempool and competes again for the next block.
Why it fills up
Demand for block space is uneven. Market volatility, exchange withdrawals, inscriptions and token mints can add hundreds of thousands of transactions in hours. Live numbers — transaction count, size in vMB and the fee rates for the next blocks — are in our fee estimator.
Limits and eviction
- Size limit: Bitcoin Core keeps up to 300 MB of mempool data by default. When full, the lowest-fee transactions are evicted and the node's minimum accepted fee rises.
- Age limit: transactions older than 14 days are dropped by default.
- Minimum relay fee: most nodes don't accept transactions paying under 1 sat/vB, though some now relay lower.
An evicted transaction isn't cancelled — anyone with the signed copy can rebroadcast it with our push tool. Until it confirms or is replaced, the coins are in limbo.
Reading your position
Projected blocks show which fee range will make it into each of the next blocks. If your fee rate is above the lower edge of the first block, you're likely next. If it's below the last projected block, you'll wait until the backlog clears — or until you speed it up.