Bitcoin Mempool Explained: Fees, Queues and Blocks
What is the Bitcoin mempool? Learn how unconfirmed transactions wait for a block, how fee rates in sat/vB are set, and why the median fee changes per block.
The Bitcoin mempool (short for memory pool) is the waiting area for valid transactions that have been broadcast but not yet included in a block. Miners choose from it, usually picking the transactions that pay the highest fee per unit of block space.
There is no single mempool
Every full node keeps its own mempool in memory. When a transaction is broadcast, nodes check it against the consensus rules and their own relay policy, store it and pass it on to their peers. Because nodes differ in settings, connections and uptime, their mempools are similar but never identical. When people talk about "the mempool", they usually mean the view of a typical, well-connected node, like the one shown on mempool.space.
Nodes cap how much memory the mempool can use; Bitcoin Core defaults to 300 MB. When that fills up, the node evicts the lowest-paying transactions first and raises the minimum fee it will accept. Transactions that stay unconfirmed for a long time — two weeks by default — also expire from a node's mempool, although they can be rebroadcast.
How fees and fee rates work
Each block is limited to 4 million weight units, which is roughly 1 million virtual bytes (vB). Because space is the scarce resource, what matters is not the total fee but the fee rate, measured in satoshis per virtual byte (sat/vB). A simple transaction of about 140 vB paying 2 sat/vB pays around 280 satoshis in total.
Miners build a block template by ranking transactions roughly by fee rate — taking into account parents and children that must be included together — and filling the block from the top. Anything below the cut-off waits for a later block.
Two tools help when a transaction is stuck:
- Replace-by-fee (RBF): the sender broadcasts a new version of the transaction that pays a higher fee.
- Child-pays-for-parent (CPFP): someone spends an output of the stuck transaction with a high-fee child, making the pair attractive to miners as a package.
Why fees rise and fall
Fees come from an auction for limited space, so they follow demand. Busy periods — sharp price moves, waves of exchange withdrawals, popular token or inscription mints — fill the mempool and push fee rates up. Quiet periods drain it, and rates fall back towards the minimum that nodes relay. At the time of writing (October 2026), median fees are very low, around 1–3 sat/vB, so most transactions confirm within a block or two without much competition.
Block timing matters as well. After a long gap between blocks, more transactions have piled up, so the next block often has a higher median fee. After a quick run of blocks, the backlog thins and the following block's fees can dip. The randomness of block times is one reason fee rates jump around from block to block even on an ordinary day.
The median fee of a block
A useful way to summarise a block is its median fee rate: the fee rate of the transaction in the middle when all of its transactions are sorted. Unlike the average, it is not distorted by a handful of transactions that overpay by mistake. Comparing one block's median fee with the previous block's tells you whether competition for space is heating up or cooling down. FreeBTC's block list shows the median fee of each recent block.
Frequently asked questions
How long can a transaction stay in the mempool?
As long as nodes keep it. With default settings, Bitcoin Core drops an unconfirmed transaction after two weeks, or sooner if the mempool is full and its fee is among the lowest. Being dropped does not cancel it: it can be rebroadcast, and the coins stay unspent until something confirms.
Do miners have to pick the highest fees?
No. Miners may include any valid transactions they like, and pools sometimes add their own or prioritise transactions through private arrangements. In practice most follow fee rate, because that maximises their revenue.
What does an empty mempool mean?
That demand for block space is currently below supply. Blocks may not be full, and paying the minimum relay fee is usually enough to confirm quickly.
One of the three predictions in every FreeBTC round is whether the next block's median fee will be higher than the previous block's. Watching the mempool, and remembering how a long wait builds up a backlog, helps you reason it through. Make your call on the next block.