What is the Bitcoin mempool?
Definition · Mempool
The pool of valid transactions a node has received that aren’t in a block yet. Each node keeps its own. Miners build the next block from theirs.
A block holds about 1 million virtual bytes, and one arrives roughly every 10 minutes. When more is waiting than fits, miners take the transactions that pay the most per byte, and the rest waits for the next block.
What’s waiting, sorted into the next blocks
Live
- 1+
Next block
1 to 4.3 sat/vB
- 0.29+
In 2 blocks
0.29 to 1 sat/vB
- 0.24+
In 3 blocks
0.24 to 0.29 sat/vB
- 0.23+
In 4 blocks
0.23 to 0.24 sat/vB
- 0.22+
In 5 blocks
0.22 to 0.23 sat/vB
- 0.21+
In 6 blocks
0.21 to 0.22 sat/vB
77,756
transactions waiting
live
41.5 MvB
total size
1 MvB is about one block
≈ 42
blocks to clear it
if nothing new arrived
0.0805 BTC
fees on offer
all waiting transactions
Nothing forces a miner to follow the queue, but taking the best-paying transactions is how they earn the most. Those fees become part of the block reward.
How are Bitcoin fees worked out?
By size, not by amount. Sending 0.001 BTC or 100 BTC costs the same if the transactions are the same size. Your wallet multiplies the size by the fee rate you pick:
What makes a transaction big is mostly its inputs: each earlier payment you spend adds one. Newer address types store the signature data more cheaply, which is why the same payment is smaller from a bc1 address:
| Address type | Starts with | Size | Each extra input |
|---|---|---|---|
| Legacy | 1… | 226 vB | +148 vB |
| SegWit | bc1q… | 141 vB | +68 vB |
| Taproot | bc1p… | 154 vB | +57.5 vB |
Tip: if you get lots of small payments, like mining payouts, each one becomes an input when you spend it. Combining them while fees are low saves money later.
What do 1, 3 and 6 confirmations mean?
A confirmation is a block. Your first comes when a miner includes your transaction, and each block built on top adds one. The deeper it is, the harder it is to undo. Step through it:
Interactive · confirmations
The orange “tx” is your transaction. Green numbers count confirmations: your block plus every block built on it.
1 confirmation: In a block
A miner included it. Reversing it now means out-mining the whole network to replace that block. Fine for small, everyday amounts.
Typical wait: the moment a miner includes it
Chance an attacker with
of the hashrate ever rewrites it
20.5%
From the whitepaper, section 11.
The odds come from the Bitcoin whitepaper. They fall fast because an attacker has to out-mine everyone else for every block they want to rewrite. That’s proof of work doing its job.
Why do Bitcoin fees spike?
Block space is fixed, and demand isn’t. When more people want to send at once, the only thing that moves is the price. It works like an auction:
Ten slots per block, highest bids first
A quiet day
everyone fits, the lowest bid still gets in
A busy day
only the top ten fit, the rest wait
Rushes usually come from a few things:
- Big price moves, when many people move coins to or from exchanges at the same time.
- New token or inscription launches that fill blocks with thousands of small transactions.
- Special blocks. The halving block in April 2024 paid 37.6 BTC in fees as people competed to be in it.
- A run of slow blocks, which lets the queue pile up until the next one arrives.
Spikes tend to pass within hours or days. If your payment isn’t urgent, pick a lower rate and let it wait.
What if my Bitcoin transaction is stuck?
It isn’t lost. It paid less than miners are picking right now, so it waits. You have three options: wait, replace it, or push it through with a child transaction.
Replace by fee (RBF)
The sender re-signs the payment with a higher fee. Needs a wallet with a “bump fee” or “speed up” button.
Child pays for parent (CPFP)
Whoever can spend an output of the stuck payment, sender or receiver, spends it with a fee high enough to pull both in.
Interactive · unstick a transaction
The last box starts at today’s fast rate (2 sat/vB). Your wallet shows the size and the rate you paid.
Replace it (RBF)
Send the same payment again with a bigger fee. The new version replaces the old one.
282 sats
total fee, up from 57 sats (225 more)
Child pays for parent (CPFP)
Spend the stuck coins in a new transaction with a big fee. Miners take both to collect it.
445 sats
fee on a 110 vB child, about 4 sat/vB, so both average 2
Waiting works more often than you’d think, because queues clear when things calm down. If a transaction waits too long (two weeks by default in Bitcoin Core), nodes drop it and the coins stay where they were, ready to send again.
Why is Lightning cheaper than on-chain?
Lightning payments don’t queue for a block. Two parties open a channel with one on-chain transaction, and after that payments move through channels in seconds. Each node along the way charges a tiny routing fee.
| On-chain | Lightning | |
|---|---|---|
| Speed | About 10 minutes per confirmation | Seconds |
| Fee | Size × fee rate, rises when blocks are full | A tiny routing fee |
| Best for | Large amounts, long-term savings | Small and everyday payments |
That’s why Lightning is often the cheapest way to withdraw small amounts from the CloudMineCrypto app. Here are the live fees and limits for each withdrawal network:
| Network | Fee | Minimum | Maximum |
|---|---|---|---|
| Lightning | 0.00000005BTC | 0.00000010BTC | 0.00004000BTC |
| Bitcoin on-chain | 0.00002792BTC | 0.00005584BTC | None |
| BNB Smart Chain (BEP-20) | 0.00001181BTC | 0.00002362BTC | None |
How it all works is in our Lightning Network guide, and the full list of networks, minimums and timing is on the withdrawals page. Withdrawals are requested by you, to your own wallet. Validated requests are processed automatically, and some are reviewed.
Frequently asked questions
What is the Bitcoin mempool?
The set of valid transactions that nodes have seen but that aren’t in a block yet. Every node keeps its own copy. Miners pick from it, usually the highest fee rates first.
What does sat/vB mean?
Satoshis per virtual byte. It’s the price of block space: your fee divided by your transaction’s size. A 141 vB payment at 2 sat/vB pays 282 satoshis, whatever amount it sends.
Why is my Bitcoin transaction stuck?
Its fee rate is below what miners are currently picking. You can wait for a quieter moment, bump the fee with replace by fee (RBF) if your wallet allows it, or spend its output with a high-fee child transaction (CPFP).
How many confirmations do I need?
One is enough for small amounts. Many services treat three as settled for ordinary payments, and six, about an hour, is the long-standing rule of thumb for large ones. The service you’re paying decides.
How long does a Bitcoin transaction take?
A block comes about every 10 minutes on average, so with a fee at or above the fast rate the first confirmation usually lands within 10 to 20 minutes. Lower fees can wait hours when the network is busy.
Why are Lightning payments cheaper?
They don’t go into a block one by one. Payments move through channels that were opened on-chain once, and each routing node charges a tiny fee, often a few satoshis.
Can a stuck transaction disappear?
Yes. Nodes drop transactions that wait too long (Bitcoin Core’s default is two weeks) or that fall below their minimum when the mempool is full. The coins then simply stay where they were and can be sent again.
Sources and further reading
- Transaction fees · Bitcoin Wiki
- Confirmation · Bitcoin Wiki
- Transaction size calculator · Bitcoin Optech
- Replace by fee (RBF) · Bitcoin Optech
- Child pays for parent (CPFP) · Bitcoin Optech
- Bitcoin: A Peer-to-Peer Electronic Cash System (section 11, the odds) · Satoshi Nakamoto, 2008
- Live mempool and fee rates · mempool.space
Live figures on this page come from the CloudMineCrypto API or from the public blockchain data services named next to them, refresh on a schedule, and are labelled where they appear. This guide is educational and not financial advice. CloudMineCrypto is not an investment product; rewards in the app are estimates and aren’t guaranteed.
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