When someone sends a crypto transaction, it is not confirmed instantly. It first gets broadcast across the network and sits in the mempool of each node that receives it, waiting to be picked up by a miner or validator and included in the next block. Every node keeps its own version of the mempool, so it is not always perfectly identical across the whole network at any given moment.
Miners and validators generally have an incentive to prioritise transactions that offer higher fees, since block space in any given block is limited. When network activity is high and many transactions are competing for that limited space, the mempool fills up and typical fees tend to rise, since transactions offering only low fees may sit unconfirmed for longer. This means the fee needed to get confirmed reasonably quickly is not fixed, but shifts constantly with real-time demand for the limited space in each new block.
Once a transaction is included in a block, it leaves the mempool of nodes that have received that block. Transactions can also be dropped from the mempool over time if they go unconfirmed for too long, or replaced with a new version offering a higher fee, depending on what the network's software allows. Many block explorers offer a live view of the mempool, letting anyone see pending transactions before they are confirmed.
Key takeaways
- The mempool holds transactions that have been broadcast but not yet confirmed in a block.
- Transactions offering higher fees are typically prioritised when the mempool is congested.
- Each node keeps its own version of the mempool, so it can vary slightly across the network.
Mempool — frequently asked questions
Why is my transaction stuck in the mempool?
This usually happens when the fee attached is low relative to current network demand, so other transactions are being prioritised ahead of it. Depending on the network, options may include waiting, or resubmitting with a higher fee.
Can I check the mempool myself?
Yes. Many block explorers show pending, unconfirmed transactions in something close to real time, which gives a general sense of current network congestion and typical fee levels at that moment. The exact contents can vary slightly between different nodes' individual views of the mempool.
New to crypto, or filling in the gaps? Work through the essentials in Learn, browse every term A–Z, or see live prices for the coins these concepts power.