What is the Mempool?
You send a Bitcoin transaction and your wallet says pending. Maybe it confirms in a few minutes, or maybe it takes much longer. So where is your transaction during that time? Most likely, it's sitting in a mempool, waiting to be included in a block.
The mempool is an important part of how Bitcoin transactions move through the network, but it isn't actually a single place. Here's how it works.
TL;DR
The mempool is where valid Bitcoin transactions wait before they're included in a block.
When a node receives a transaction, it checks that transaction against the rules it enforces. If the transaction is valid but hasn't been confirmed yet, the node can keep it in its mempool and relay it to other nodes. Each node maintains its own mempool.
What does “mempool” mean?
Mempool is short for memory pool. It's essentially a node's collection of valid, unconfirmed transactions that it knows about.
When you broadcast a Bitcoin transaction, it can spread from node to node across the network. Each node independently decides whether to accept that transaction into its mempool based on the rules and policies it follows. If accepted, the transaction waits there until something happens to it, most commonly being included in a block.
So where is the mempool?
This is where the term can be a little misleading. People often say things like, “Your transaction is in the mempool.” That makes it sound like the Bitcoin network has one giant waiting room where every unconfirmed transaction goes, but it doesn’t. Individual Bitcoin nodes maintain their own mempools.
Because transactions spread across the peer to peer network, many nodes will often have many of the same transactions in their mempools. But their mempools don't have to be identical. One node may know about a transaction another hasn't received yet. Nodes can also have different policies about which transactions they'll keep or relay.
So when people refer to “the mempool,” they're usually talking about the broader collection of unconfirmed transactions circulating around the Bitcoin network.
How does a transaction get into a mempool?
Before a node accepts an unconfirmed transaction into its mempool, it checks it. Among other things, the node makes sure the transaction is properly constructed, has the necessary authorization, isn't trying to spend bitcoin that has already been spent, and follows the rules the node enforces.
If the transaction doesn't pass the necessary checks, the node rejects it. If it does, the node may add the transaction to its mempool and relay it to other nodes. At this point, the transaction is valid, but it isn't confirmed yet.
Valid doesn't mean confirmed
A transaction sitting in a mempool has already passed important checks, but it hasn't been included in Bitcoin's blockchain yet. For that to happen, the transaction needs to be included in a valid block that becomes part of the chain. Until then, your wallet may describe the transaction as:
Pending.
Unconfirmed.
0 confirmations.
These are different ways of telling you essentially the same thing: the transaction has been broadcast, but it hasn't appeared in a confirmed block yet.
Why might a transaction wait?
Bitcoin blocks have a limited amount of space, and new blocks aren't produced on a perfectly predictable schedule. That means transactions aren't always confirmed immediately after they're broadcast. Sometimes a transaction may wait only a short time. Other times, particularly when many transactions are waiting for block space, it can take longer.
So if your transaction remains unconfirmed for a while, that doesn't necessarily mean something went wrong. It may simply still be waiting.
What happens when the transaction is confirmed?
Eventually, your transaction may be included in a new block. As nodes receive and accept that block, they can see that the transaction has now been confirmed.They no longer need to keep that transaction in their mempools, so it is removed. Meanwhile, transactions that weren't included remain unconfirmed and continue waiting.
The contents of each node's mempool are constantly changing.
Can a transaction disappear from the mempool?
Yes. Nodes don't necessarily keep unconfirmed transactions forever. Because each node manages its own mempool, it can remove transactions according to its own policies. A node might eventually drop a transaction after enough time has passed or when its mempool becomes crowded.
That doesn't automatically mean the transaction has disappeared from the entire Bitcoin network. Other nodes may still have it, and in some cases the transaction can be broadcast again.
Why does the mempool matter?
The mempool helps explain what’s happening in the space between sending a Bitcoin transaction and confirming it. It’s where valid transactions can wait for limited block space, and it’s why a transaction can be perfectly valid while still showing pending in your wallet.
It also helps explain why confirmation times can vary. Your transaction doesn’t go directly from your wallet into the blockchain. There’s a period in between when it has been broadcast to the network but hasn’t yet been included in a block.
So the next time you send bitcoin and see 0 confirmations, you’ll know what’s happening. Your transaction has been sent and verified by nodes that accepted it, but its journey isn’t finished yet. It’s still waiting to make it into a block.
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