A mempool, short for memory pool, is the set of transactions a node has received and checked but that no block has included yet. A wallet broadcasts a payment, nodes pass it along, and it sits in their mempools until a miner or validator picks it up. Because block space is limited, the queue is where users bid against each other with fees. The crypto hub covers the wider market, and how blockchain works follows one payment from broadcast to confirmation.
How does a mempool decide what confirms next?
Nothing decides centrally: block producers usually take the transactions that pay the most per unit of block space, so fee rate, not total fee, sets the order.
On Bitcoin, space is counted in virtual bytes (vbytes) and fee rates in satoshis per vbyte (sat/vB), and a block holds about 1 million vbytes. Since Bitcoin Core 31.0, released in April 2026, nodes rank transactions in connected groups called clusters, so a low-fee parent and the high-fee child that spends it are judged together, at their combined rate. Ethereum ranks by the tip each transaction adds on top of the base fee, the system gas fees explained walks through.
Size is a poor guide to price. At 06:20 UTC on November 2, 2026, mempool.space counted 45,181 waiting transactions, about 24.5 million vbytes or roughly 25 blocks' worth, yet its projected next block had a median of 0.13 sat/vB, because most of the backlog bid between the 0.1 sat/vB relay floor and that level. The same site suggested 1 sat/vB for every priority, a reminder that estimates can sit well above what actually clears.
Why do transactions get stuck?
Usually because the fee rate offered is below what blocks are clearing, so newer, better-paying transactions keep moving ahead of it.
- Demand jumps. A rate that cleared an hour ago can fall behind. The largest backlog in mempool.space's one-year record, about 58.2 million vbytes on February 3, 2026, built up while blocks were slow after a US winter storm took miners offline.
- An unconfirmed parent. A transaction spending coins from another unconfirmed transaction cannot confirm before it.
- Dropped, not stuck. By default a Bitcoin Core node evicts the lowest fee rates once its mempool uses more than 300 MB of memory, and forgets anything still waiting after 336 hours. The coins never left the sender's wallet.
- Ethereum nonce order. Each account's transactions must confirm in sequence, so one underpriced transaction holds up every later one, and transactions left behind a missing nonce drop out of Geth's default pool after three hours.
Payments sent from an exchange are a special case: only the exchange can change the fee, as custodial vs non-custodial wallets explains.
How do you speed up a stuck bitcoin transaction?
Replace it with a better-paying version, called replace-by-fee (RBF), or spend one of its outputs in a new transaction that pays for both, called child-pays-for-parent (CPFP).
Since Bitcoin Core 28.0, released in October 2024, nodes accept a replacement for any unconfirmed transaction by default, whether or not it signaled that it could be replaced. Under the version 31.0 rules, a replacement for a transaction standing alone needs a higher fee and a higher fee rate than the original, and it can keep the same recipient and amount. CPFP suits whoever controls an output of the stuck payment, sender or recipient, and its strength is the pair's combined rate:
Worked example: Lifting a 141-vbyte payment to 1 sat/vB
- Stuck payment: 141 vbytes paying 15 satoshis, about 0.11 sat/vB, near the back of the queue on November 2, 2026.
- Replace-by-fee: re-sign it to pay 141 × 1 = 141 satoshis, about 11 cents at mempool.space's $78,234 bitcoin price that morning.
- Child-pays-for-parent: a 110-vbyte child must pay 1 × (141 + 110) − 15 = 236 satoshis, so the pair costs 251 satoshis in all, about 20 cents.
- Replacement is cheaper because it adds no bytes; a child is the route for someone who holds an output but not the keys that signed the original.
Wallets usually present these as a speed-up or bump-fee option. The cents scale with the bitcoin price, and the rate needed scales with the queue, so the same rescue cost far more during the April 2024 fee spike described in what is Bitcoin.
How does Ethereum's mempool differ?
It is organized by account and nonce: each account's transactions confirm strictly in order, and a replacement must reuse the nonce and outbid the original.
In Geth's default settings, a replacement must raise both the tip and the maximum fee by at least 10%, or by 100% for the blob transactions that layer 2 networks use to post data. The default pool holds 5,120 ready transactions across all accounts and 1,024 waiting on a missing nonce. Pending transactions are public, so bots can trade ahead of them; some wallets therefore route transactions privately to block builders, and Flashbots says its Protect service sends them to a private mempool hidden from front-running and sandwich bots.
How is a mempool different from a block, a txpool and a private mempool?
The mempool is the public waiting stage; a block is the confirmed result, and the other terms are variations on who can see the queue and what it is called.
| Term | What it is | How it differs from a public mempool |
|---|---|---|
| Block | The confirmed batch a miner or validator publishes | Transactions leave the mempool once a block includes them |
| Transaction pool (txpool) | Ethereum clients' name for the same queue | Same idea, organized by account and nonce |
| Private mempool | A channel sending transactions straight to builders or miners | Invisible on public explorers until confirmed |
| Fee estimate | A suggested rate from a wallet or explorer | A guess drawn from the queue and recent blocks, not a price |
| Pending balance | A wallet's display of coins still on the way | Shows a transfer the network has not confirmed yet |
Not every chain has one. Solana's developer documentation says it has no mempool: nodes forward each transaction straight to the current and next scheduled leaders, and one not processed before its recent blockhash expires simply fails. On proof-of-work chains, the miners drawing from the queue are measured by hash rate; the race itself is described in proof of work.