Mempool
The mempool is the network's waiting room: transactions already broadcast but not yet in a block. A seat in the next block is bought with a fee.
How the mempool works
Every node keeps its own list of pending transactions, there is no single mempool, but the lists of large nodes almost match. When a pool assembles a block, it picks the most profitable transactions from that list: sorting goes by fee per byte, not by time spent in the queue.
Hence the main rule: the fee is a bid at an auction. An empty mempool means even the minimum bid gets through. In a rush the queue grows, and a transaction with a tiny fee hangs for hours or days until it gets pushed out entirely.
For a miner the mempool matters from two sides. As income: fees from a block go to the pool that found it and stack on top of the subsidy, and on busy days that extra grows noticeably. And as a user: pool payouts and UTXO consolidation are best done into an empty mempool, when bytes are cheap.
You can watch the queue in public mempool explorers: they show the queue size, current fee rates and a forecast of which rate makes it into the next blocks.
The short version
| What's inside | broadcast transactions not yet in a block |
| How it sorts | by fee per byte |
| Empty mempool | the time for cheap transfers |
| Full mempool | fees climb, the queue takes days |
| What miners get | block fees on top of the subsidy |
How to use it
Do planned transfers and UTXO consolidation when the queue is empty: the fee difference can run into dozens of times.
Before a pool payout, glance at a mempool explorer: a minute of reading saves real money on the rate.
Don't panic over a stuck transaction: it will either pass when the queue drains or return to the wallet after being pushed out.
Remember the seasonality of income: in busy weeks fees add a visible share to the block reward, and revenue per terahash grows without you lifting a finger.
An example from practice
A miner had been piling up small pool payouts and decided to sweep them into one output. In a busy week that consolidation would have cost as much as a good dinner, on a quiet Sunday it went through for pennies at the minimum rate. Same transaction either way, the only difference was the state of the queue.
Related terms
Where to go next on the site
Coins
- Bitcoin coins
Numbers
Questions and answers
Why has my transaction been stuck for two days
The bid sits below the current market: the queue keeps filling with transactions more generous than yours. Wait for it to drain, bump the fee if your wallet can, or wait for the return.
Is there one mempool for the whole network
No, each node has its own, but large nodes hold nearly the same contents. Explorers show their own node's picture, and that is enough for decisions.
How much do fees matter for a miner's income
In calm weeks a few percent on top of the subsidy, in a frenzy it has been comparable to it. With every halving the weight of fees in income grows.
How the terms connect
Every link in the chain is clickable. Orange marks where you are now.
Looking for an ASIC miner
The catalog holds 212 models. You can compare them by hashrate and by joules per terahash, then plug your own rate into the calculator and see what stays in your pocket.
Page written and checked by Denys Klimchuk. Updated .