Definition
Mempool is the “memory pool” of unconfirmed transactions that each Bitcoin node keeps in RAM — a waiting room of broadcast-but-not-yet-mined transactions from which miners assemble the next block.
Also known as: memory pool, tx pool, the transaction queue.
What lives in the mempool
When you send Bitcoin, your transaction is gossiped across the peer-to-peer network and lands in the mempool of every node that accepts it under its relay policy. It sits there, unconfirmed, until a miner includes it in a block. Because each node maintains its own mempool independently, there is no single global mempool — only a rough consensus shaped by the relay and policy rules each node runs. Transactions are ordered primarily by fee rate (satoshis per virtual byte), so the network behaves like an open fee market: when blockspace is scarce, fee rates rise as transactions compete for the limited room in the next block template.
How miners turn the mempool into a block
A miner does not hash random data — it hashes a candidate block built from mempool transactions. A Bitcoin full node selects high-fee transactions from its mempool and packages them, via the getblocktemplate RPC, into a template that becomes the basis for work. That template fixes the merkle root committing to every included transaction, plus the coinbase transaction that pays the miner the block subsidy and the sum of all included transaction fees. The node’s mempool and policy settings determine which transactions are chosen, so whoever controls the template controls what the chain confirms.
Who actually does that selection depends on the protocol. Under classic Stratum V1, the pool builds the template from its own mempool and your ASIC just hashes what it is handed. Stratum V2 keeps pool-built templates as the default but adds an optional Job Declaration path that lets a miner pick its own transactions; only BraiinsOS+ ships native Stratum V2 today. The most direct route — running your own node and constructing templates locally with a gateway-style stack — means the mempool feeding your hashrate is genuinely yours. That is one more layer decentralized, building on the work of the pool developers and node maintainers who came before.
Why a home miner cares about the mempool
For anyone running an ASIC in a garage or spare room, the mempool is where a meaningful slice of revenue comes from. As the halving schedule keeps shrinking the subsidy, fees collected from the mempool become a larger share of each block reward. A fat mempool with high fee rates lifts the value of every block; an empty one means thinner margins. If you run solo mining against your own node — the way many Bitaxe and small-ASIC owners do — you also choose which transactions you confirm, a quiet act of transaction-selection sovereignty most pool miners never exercise.
The mempool also affects how you reason about profitability. Tools that quote a live fee market let you estimate the fee portion of a block before you even find one, and that feeds straight into your break-even math. Open and self-hosted tuning stacks — including the closed-beta, GPL-3.0 work D-Central is building toward a public beta in summer 2026 — aim to make running your own node and template alongside your hashrate the normal path rather than the exotic one. Whether you mine on a pool or build templates from your own mempool, understanding what the queue holds is part of mining with eyes open. If you want to put that into practice on real hardware, the Bitaxe hub and the miner catalog are good places to start.
Related terms: Block template, Coinbase transaction, Transaction fees, Merkle root, Stratum protocol, Solo mining
In Simple Terms
The waiting area for unconfirmed transactions. Miners pick transactions from here to include in blocks.
Mempool is the "memory pool" of unconfirmed transactions that each Bitcoin node keeps in RAM — a waiting room of broadcast-but-not-yet-mined transactions from which miners assemble the next block.
Also known as: memory pool, tx pool, the transaction queue.
What lives in the mempool
When you send Bitcoin, your transaction is gossiped across the peer-to-peer network and lands in the mempool of every node that accepts it under its relay policy. It sits there, unconfirmed, until a miner includes it in a block. Because each node maintains its own mempool independently, there is no single global mempool — only a rough consensus shaped by the relay and policy rules each node runs. Transactions are ordered primarily by fee rate (satoshis per virtual byte), so the network behaves like an open fee market: when blockspace is scarce, fee rates rise as transactions compete for the limited room in the next block template.
How miners turn the mempool into a block
A miner does not hash random data — it hashes a candidate block built from mempool transactions. A Bitcoin full node selects high-fee transactions from its mempool and packages them, via the getblocktemplate RPC, into a template that becomes the basis for work. That template fixes the merkle root committing to every included transaction, plus the coinbase transaction that pays the miner the block subsidy and the sum of all included transaction fees. The node's mempool and policy settings determine which transactions are chosen, so whoever controls the template controls what the chain confirms.
Who actually does that selection depends on the protocol. Under classic Stratum V1, the pool builds the template from its own mempool and your ASIC just hashes what it is handed. Stratum V2 keeps pool-built templates as the default but adds an optional Job Declaration path that lets a miner pick its own transactions; only BraiinsOS+ ships native Stratum V2 today. The most direct route — running your own node and constructing templates locally with a gateway-style stack — means the mempool feeding your hashrate is genuinely yours. That is one more layer decentralized, building on the work of the pool developers and node maintainers who came before.
Why a home miner cares about the mempool
For anyone running an ASIC in a garage or spare room, the mempool is where a meaningful slice of revenue comes from. As the halving schedule keeps shrinking the subsidy, fees collected from the mempool become a larger share of each block reward. A fat mempool with high fee rates lifts the value of every block; an empty one means thinner margins. If you run solo mining against your own node — the way many Bitaxe and small-ASIC owners do — you also choose which transactions you confirm, a quiet act of transaction-selection sovereignty most pool miners never exercise.
The mempool also affects how you reason about profitability. Tools that quote a live fee market let you estimate the fee portion of a block before you even find one, and that feeds straight into your break-even math. Open and self-hosted tuning stacks — including the closed-beta, GPL-3.0 work D-Central is building toward a public beta in summer 2026 — aim to make running your own node and template alongside your hashrate the normal path rather than the exotic one. Whether you mine on a pool or build templates from your own mempool, understanding what the queue holds is part of mining with eyes open. If you want to put that into practice on real hardware, the Bitaxe hub and the miner catalog are good places to start.
Related terms: Block template, Coinbase transaction, Transaction fees, Merkle root, Stratum protocol, Solo mining