Mempool
A mempool is the waiting area where unconfirmed cryptocurrency transactions sit before miners include them in a block.
Definition
The mempool is a temporary waiting area for cryptocurrency transactions that have been broadcast to the network but have not yet been confirmed in a block. Each node keeps its own version of the mempool, so there is not one single global queue. In mining, the mempool is where miners look for transactions to package into the next candidate block.
How It Works
When someone sends a cryptocurrency transaction, their wallet broadcasts it to nearby nodes. Those nodes check basic rules, such as whether the transaction is properly signed, whether the coins have not already been spent, and whether the fee meets local policy. If the transaction passes those checks, the node adds it to its mempool and relays it to other nodes.
Miners build new blocks by selecting transactions from their mempool. They usually prioritize transactions that pay higher fees per unit of block space, often called the fee rate. This is why a transaction with a higher fee may confirm faster during busy periods.
The mempool changes constantly. New transactions enter, confirmed transactions leave, and some low-fee transactions may be dropped if a node’s mempool becomes full. Because every node can set its own memory limits and relay policies, two nodes may have slightly different mempool contents at the same time.
Why It Matters
The mempool is a real-time signal of network demand. When the mempool is small, transactions often confirm quickly and fees tend to be lower. When the mempool is crowded, users compete for limited block space, which can push fees higher.
For miners, the mempool affects revenue. The block reward includes both the subsidy and the transaction fees from the transactions included in the block. Choosing higher-fee transactions can increase a miner’s total payout.
For users, the mempool helps explain confirmation delays. A transaction is not final just because it has been broadcast; it becomes much harder to reverse only after miners include it in a block and later blocks build on top of it.