Guide
What Is the Mempool? Bitcoin's Waiting Room for Unconfirmed Transactions
The short answer: the mempool is a "waiting room" for unconfirmed transactions — when it fills up, fees rise and confirmations slow down
The mempool (short for memory pool) is where unconfirmed transactions wait temporarily before they're included in a block. Every Bitcoin node keeps its own mempool, and miners pick the highest-fee transactions from it to pack into the next block.
Key takeaways
- The mempool is the waiting room for unconfirmed transactions, held in memory by each node
- When more transactions arrive than there's room for, the mempool gets congested and fees (sat/vB) go up
- Because miners prioritize higher-paying transactions, a fee market emerges
- You can check current congestion and feerates at mempool.space
How a transaction gets confirmed
- You send from your wallet → the transaction is broadcast to the network
- It enters each node's mempool as an unconfirmed transaction
- A miner selects higher-fee transactions, includes them in a block, and they become confirmed (see What Is Bitcoin Mining?)
- Once in a block, the transaction leaves the mempool
For how confirmations are counted, see What Is a Bitcoin Confirmation?.
Why congestion happens
There's a limit to how much data fits in a single block, and blocks are produced on average about once every 10 minutes (see How Bitcoin Works). When incoming transactions outpace that rate, they pile up in the mempool — that's "congestion." When it's congested, people who want to get in quickly bid higher feerates, pushing up the price per unit in the fee market.
By default, Bitcoin Core allocates about 300MB of memory to the mempool. Once that limit is reached, it stops accepting new transactions below a certain feerate. Low-fee transactions that linger for a long time may eventually be dropped from the mempool. If your own transaction is stuck waiting with a low fee, see what to do when a Bitcoin transaction won't confirm for ways to speed it up.
How to read mempool.space
On mempool.space, you can read roughly the following information at a glance:
| Item | What it tells you |
|---|---|
| Mempool size | How many unconfirmed transactions are waiting (level of congestion) |
| Fee Estimates | The feerate (sat/vB) you should pay right now |
| Recent blocks | What feerates are actually getting included |
The heavier the congestion, the higher the recommended feerate. For how fees are determined, see How Bitcoin Fees Work.
Frequently asked questions
Where is the mempool? Is there just one?
It doesn't live on a single central server. Every Bitcoin node keeps its own mempool in memory. Because nodes have different settings and receive transactions differently, their contents don't match perfectly.
What happens when the mempool gets congested?
Unconfirmed transactions pile up, and the going rate of the fee (sat/vB) needed to get included by a miner rises. Low-fee transactions wait longer in line, so they take more time to confirm.
Do unconfirmed transactions stay in the mempool forever?
No. Once a transaction is included in a block, it leaves the mempool. On the other hand, a transaction with a low fee that goes unconfirmed for a long time may be dropped or expired from a node's mempool — in which case the funds simply remain with the sender.
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A note on investing
This article is for informational purposes only and is not investment advice. Bitcoin carries risks including price volatility, hacking, and loss. Make investment decisions at your own responsibility and only with funds you can afford to lose. Tax rules and regulations can change, so always confirm the latest details with official primary sources.
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FAQ
- Where is the mempool? Is there just one?
- It doesn't live on a single central server. Every Bitcoin node keeps its own mempool in memory. Because nodes have different settings and receive transactions differently, their contents don't match perfectly.
- What happens when the mempool gets congested?
- Unconfirmed transactions pile up, and the going rate of the fee (sat/vB) needed to get included by a miner rises. Low-fee transactions wait longer in line, so they take more time to confirm.
- Do unconfirmed transactions stay in the mempool forever?
- No. Once a transaction is included in a block, it leaves the mempool. On the other hand, a transaction with a low fee that goes unconfirmed for a long time may be dropped from a node's mempool, in which case the funds simply remain with the sender.
This article is informational only and is not financial, investment, or trading advice. Prices are reference snapshots and may be outdated. Always do your own research.
