# What is the Bitcoin mempool?

The mempool is each node's waiting room for valid transactions not yet in a block. Learn how it works, why it fills up and how to read it live.

Canonical page: https://mybitcoinexplorer.com/learn/what-is-the-mempool
Topic: Nodes and the network
Published: 2026-10-29
Updated: 2026-10-30


## A waiting room
When a [node](https://mybitcoinexplorer.com/learn/what-is-a-bitcoin-node) receives a valid transaction that isn't in a block yet, it keeps it in memory: its **mem**ory **pool**. Miners choose from their mempool when building blocks.

> Diagram: Transactions wait in the mempool; miners take the best-paying ones first — 

## Every node has its own
There's no master mempool. Each node's view depends on what it has heard and its own settings, so two nodes can disagree slightly. "The mempool" usually means the typical view of well-connected nodes.

## Why transactions leave
- **Confirmed** in a block.
- **Replaced** by a conflicting, higher-fee version. See [Replace-by-fee (RBF)](https://mybitcoinexplorer.com/learn/replace-by-fee-rbf).
- **Evicted** when the mempool is full (Bitcoin Core's default limit is 300 MB of memory): the lowest fee rates go first.
- **Expired** after two weeks unconfirmed, by default.

## Reading mempool data
| Metric | Meaning |
|---|---|
| Transaction count | How many are waiting |
| Virtual size | Total vbytes waiting: about 1,000,000 vB fills one block |
| Minimum fee | The lowest rate this node currently accepts |

> A node that is still syncing can't show a meaningful current mempool. Ours is fully synced: see what is waiting right now on our [mempool page](/mempool).


## Reading the mempool live
The MyBitcoinExplorer [mempool page](/mempool) shows what our fully synced node is holding right now: how many transactions are waiting and their total virtual size, and lets you open individual waiting transactions. Our [fee page](/fees) turns that into live estimates. When the waiting total is well under 1,000,000 vB, almost anything paying the minimum gets in soon.

## Mempool and fees in one picture
> Diagram: When the mempool holds more than a block's worth, only the highest fee rates make the next block — 


## Key takeaways

- The mempool holds valid, unconfirmed transactions.
- Each node keeps its own mempool, so views can differ.
- Transactions leave when confirmed, replaced, evicted or expired.

## Common questions

### Why did my transaction disappear from the mempool?

It may have been evicted for a low fee, expired after two weeks, or been replaced. Your coins are still yours; you can rebroadcast or send a new transaction.

### How many blocks' worth is waiting?

Divide the mempool's total vsize by roughly 1,000,000 vB per block.

### How long can a transaction stay in the mempool?

By default Bitcoin Core drops unconfirmed transactions after two weeks, or sooner if the mempool fills and its fee rate is too low.

### Why is the mempool sometimes empty?

When demand is low, every waiting transaction fits into the next block, so the backlog clears.

## Related lessons

- [Fee rate: sats per vbyte](https://mybitcoinexplorer.com/learn/fee-rate-sat-per-vbyte)
- [Replace-by-fee (RBF)](https://mybitcoinexplorer.com/learn/replace-by-fee-rbf)
- [Relay policy and standardness](https://mybitcoinexplorer.com/learn/relay-policy-and-standardness)
- [What is a Bitcoin node?](https://mybitcoinexplorer.com/learn/what-is-a-bitcoin-node)