The mempool (memory pool) is each Bitcoin node's waiting room for unconfirmed transactions. When you broadcast a transaction, it sits in mempools across the network until a miner includes it in a block. Miners typically select transactions with the highest fee rates first.
What Is the Bitcoin Mempool?
2 min read
The short version
The mempool is like the queue at a post office where letters (transactions) wait to be processed. Letters with more postage (higher fees) get picked up first. If the office gets too busy, low-postage letters might wait hours or get returned if they sit too long.
How It Works
Each node maintains its own mempool, there is no single global mempool. When a transaction is broadcast, it propagates across the peer-to-peer network in seconds. Each node validates it independently (correct signatures, sufficient inputs, not a double spend) before accepting it into their mempool. Miners build block templates by selecting transactions from their mempool, optimizing for fee revenue (highest sat/vB first). The mempool has a configurable size limit (default 300 MB in Bitcoin Core); when full, the lowest-fee transactions are evicted. Transactions can expire from mempools after 14 days by default.
Fee estimation using mempool data
You want to send Bitcoin and need it confirmed within 2 blocks (~20 min). You check mempool.space and see: the next block template has 1,850 transactions at 25+ sat/vB, the second block has 1,600 at 15+ sat/vB. You set your fee at 18 sat/vB to land in the second block. For a 141 vbyte SegWit transaction, that's 2,538 sats ($1.52). If you're not in a hurry, you could set 5 sat/vB and wait 3-6 blocks (30-60 min) when demand subsides.
What People Get Wrong
There is one mempool
Each node has its own mempool. They're mostly similar due to transaction propagation, but can differ, especially during high traffic or when nodes have different policies.
Transactions in the mempool are confirmed
Mempool transactions are unconfirmed and technically reversible (via RBF or double-spend attempts). They become irreversible only after being mined into a block.
High fees are permanent
Fee rates fluctuate with demand. Weekends and off-peak hours often have 1-3 sat/vB rates. The mempool regularly clears completely during low-activity periods.
Keep Reading
Sources & Further Reading
- Mempool.space
The standard open-source mempool visualizer with real-time fee estimates
- Bitcoin Core Mempool Docs
Technical docs on mempool behavior, eviction, and transaction relay
Questions People Also Ask
- How do I speed up a stuck transaction?
- Use RBF (Replace-By-Fee) to broadcast a new version with a higher fee, or CPFP (Child-Pays-For-Parent) by spending the unconfirmed output with a high-fee child transaction.
- Can transactions be removed from the mempool?
- Yes, by expiration (14 days default), eviction (when mempool is full), by being mined, or by an RBF replacement. They're not permanent until confirmed in a block.
- What is a good fee to pay right now?
- Check mempool.space for real-time fee estimates. Rates vary from 1-2 sat/vB in quiet periods to 100+ sat/vB during high demand (NFT mints, market volatility).