What Is the Bitcoin Mempool? Why Transactions Wait

Key takeaways

• The mempool is a node’s waiting room for valid transactions that are not yet in a block. Every node keeps its own, so there is no single global mempool.
• Miners fill each block with the transactions that pay the most per unit of block space first. Space is limited, so users compete, and that competition is the fee market.
• A pending transaction usually means its fee rate is too low for current demand. The coins have not moved, and nothing is lost.
• Replace-by-fee lets a sender raise the fee on a pending transaction. Child-pays-for-parent lets someone pull a stuck transaction along by spending its output with a high fee.
• Nodes drop transactions that wait too long, about two weeks by default in Bitcoin Core, and they can evict low-fee ones when their mempool fills.

What is the Bitcoin mempool?

The mempool, short for memory pool, is the set of valid transactions that a node has received and is holding while they wait to be included in a block. It is a waiting room, not part of the blockchain.

When you send a payment, your wallet hands the transaction to a node. The node checks that the signatures are valid, the coins are unspent and the fee meets its minimum. If it passes, the node stores it and passes it to its peers, who repeat the checks. Within seconds a valid transaction sits in the mempools of many nodes. When a miner includes it in a block, nodes remove it because it is now confirmed. How Does Bitcoin Work? covers blocks and confirmations.

Each node decides for itself what to hold, so there is no single mempool. Nodes receive transactions at slightly different times, and operators set their own limits for minimum fees and mempool size. The blockchain is the record every node agrees on, while each mempool is one node’s working copy of what might come next. What Is a Bitcoin Node? explains what nodes do.

How do miners choose which transactions to include?

Miners generally fill a block with the transactions that pay the highest fee per unit of space, working down from the top until the block is full. Space, not the amount of bitcoin sent, is the scarce thing, so the fee rate matters more than the total fee. The usual unit is satoshis per virtual byte, and How Many Satoshis Are in a Bitcoin? explains satoshis.

Picture a shuttle that leaves about every ten minutes with a fixed number of seats. Passengers offer a tip per seat, and the driver boards the best tips first. A passenger with a lot of luggage needs more seats and so pays more in total at the same tip per seat. Likewise a transaction with many inputs takes more room, as What Is a UTXO? shows.

If transactions A and B are the same size and A offers a fee rate four times higher, A goes in the next block and B waits unless there is room left. In most pools the operator’s node makes this selection, as Bitcoin Mining Pools Explained covers.

Why is there a fee market?

There is a fee market because a block holds only a limited amount of data and arrives about every ten minutes, so users compete for space and its price moves with demand. When few transactions are waiting, each block clears the mempool and a low fee rate is enough. When many are waiting, a backlog builds and the rate needed for the next block rises.

The ten minutes is only an average. One block can arrive within seconds and the next after half an hour.

Fees are also part of what miners earn. The subsidy is cut at every halving, so fees carry more weight over time, as What Is the Bitcoin Halving? explains. Bitcoin Fees Explained covers how network fees are set.

Why is my Bitcoin transaction pending?

A transaction is pending because it has been broadcast but no miner has put it in a block yet, most often because its fee rate is lower than what is needed right now. “Pending” or “unconfirmed” in a wallet describes exactly this state.

The usual causes are:

• The fee rate is below what current demand requires.
• A slow, random gap between blocks.
• The transaction spends an output from an earlier transaction that is itself unconfirmed, so it has to wait for that one.
• The wallet has not broadcast it, or the site you are checking has not seen it yet.

Your coins have not moved while the transaction waits. Until it confirms, the payment is not final, which is why receivers traditionally wait for confirmations. How to Send and Receive Bitcoin Safely describes what that looks like in practice.

How do wallets estimate fees from the mempool?

Wallets look at what is waiting in the mempool and at how quickly transactions at different fee rates were confirmed in recent blocks, then suggest rates for faster or slower confirmation. The result is an estimate for the next few blocks, not a promise.

Estimates go wrong when demand changes after you send, because a mempool that looked empty can fill within minutes. Many wallets let you pick a speed or enter your own rate, as Bitcoin Wallets Explained describes. An exchange sets the fee on its own withdrawals.

What is replace-by-fee?

Replace-by-fee, or RBF, lets a sender swap a pending transaction for a new version that spends the same coins and pays a higher fee. Nodes drop the old version and pass the new one along. Only one version can ever confirm, because both spend the same coins.

Many wallets offer this as a “bump fee” or “speed up” button. The recipient and amount stay the same, and the extra fee usually comes out of your change. Whether a transaction can be replaced depends on how the wallet built it and on node settings. A receiver may see an unconfirmed payment vanish and be replaced, which is another reason it is not final.

What is child-pays-for-parent?

Child-pays-for-parent, or CPFP, speeds up a stuck transaction by spending one of its outputs in a new transaction with a generous fee. The new “child” cannot confirm before the “parent” it spends, so miners who want the child’s fee include both and judge the pair by its combined fee rate.

	Replace-by-fee	Child-pays-for-parent
Who can use it	The sender	The sender, through change, or the recipient, through the output they received
What it does	Replaces the pending transaction with a higher-fee version	Adds a second transaction that pulls the first one into a block
What happens to the original	It is dropped	It stays and confirms together with the child
What it needs	A wallet that supports replacement	A wallet that can spend unconfirmed coins

What happens if a transaction waits too long?

Nodes drop transactions that wait too long. The default in Bitcoin Core is about two weeks, and each operator can change it. A node can also evict transactions sooner: if its mempool grows past the size limit its operator set, it removes the lowest-fee ones first and temporarily raises the minimum fee rate it accepts.

A dropped transaction does not lose your coins. They were never spent on the chain, and your keys still control them. Some nodes may have kept the transaction longer, and a wallet can rebroadcast it, so it can still confirm later. Paying again with different coins before you know the first one is gone could mean paying twice.

What should you do about a stuck transaction?

Find out where it stands, then decide whether to wait or to raise the fee.

1. Copy the transaction ID from your wallet and look it up on a block explorer or mempool viewer, which shows whether it is waiting and what fee rate it offered compared with current demand.
2. If there is no hurry, wait. When demand falls, lower fee rates start getting into blocks.
3. If your wallet has a fee bump option, use it. A recipient with a suitable wallet can use CPFP on the output they are about to receive.
4. For an exchange withdrawal, contact the exchange’s support, because you do not control the fee.
5. Ignore any site or message that offers to “unstick” a transaction or asks for your seed phrase. No fix needs it, as What Is a Seed Phrase? explains.

If a delay has legal or financial consequences, ask a qualified professional about your options. New to Bitcoin? The Start Here path covers the basics in order.

Where can you look at the mempool?

Block explorers and mempool visualizers show a live view of waiting transactions, usually drawn as upcoming blocks stacked by fee rate. They show how many are waiting, which fee rates are likely to confirm soon and where a particular transaction sits. Each site runs its own nodes, so numbers differ slightly and change by the minute, which is why this article gives none. A node you run yourself shows your own mempool.

Where to go next

• Bitcoin Fees Explained: Exchange Fees vs Network Fees: what sets the fee rate and how to keep costs down.
• How to Send and Receive Bitcoin Safely: the steps and checks for a safe payment.
• What Is a Bitcoin Node? What Running One Means: who keeps a mempool and checks the rules.
• What Is a UTXO? How Bitcoin Tracks Ownership: why transaction size depends on inputs.