Understanding the Bitcoin mempool

[I_Anime]

Greeting Nigeria LB , some folk here understand how the bitcoin mempool works while some don’t especially those that are new to bitcoin . But am going try my best to break it down in a simple way .

What’s Bitcoin Mempool?

The mempool which is the short form of (memory pool), is where unconfirmed transactions wait before being mined , is like a waiting space for unconfirmed transactions for miners to select those unconfirmed transactions base on their fee rate  (measured in satoshis per byte ) , and those with higher fees are usually confirmed first or prioritized.

How it works : when a transaction is being sent , it’s verified by nodes and added to their mempools (the nodes stores and shares the unconfirmed transactions with other nodes, help them reach miners who can confirm the transactions) . Once confirmed those transactions are removed from mempools .

Fees and Congestion: Each Bitcoin node has a limit to how many transactions it can keep in its mempool at a time, most Bitcoin nodes set their mempool limit to about 300mb by default. So when the memory is crowded ( space is filled up ) fees rises as users compete for block space , which often lead to fee congestion.

To access the mempool you can use this link: https://mempool.space/ , or you can go to your browser search bar and search for “mempool” . Please drop your contribution to this post cause we are all here to learn from each other .

[Charles-Tim]

To access the mempool you can use this link: https://mempool.space/ , or you can go to your browser search bar and search for “mempool” . Please drop your contribution to this post cause we are all here to learn from each other .

Each full nodes have its own mempool, but that site that you mentioned is just one of the sites that you can also know the fee priority needed that can make your transaction get confirmed earlier.

There are many of the sites, but we are just use to one on this forum. List of Bitcoin Mempool Viewers for Reference

The thread was updated last in July 2021, I do not know if all the sites are still making good estimation

It I worth knowing that https://mempool.space does work more than a mempool.

For fee priorities lower than 1 sat/vbyte, I am using https://jochen-hoenicke.de/queue/#BTC,24h,weight

[pooya87]

Fees and Congestion: Each Bitcoin node has a limit to how many transactions it can keep in its mempool at a time, most Bitcoin nodes set their mempool limit to about 300mb by default. So when the memory is crowded ( space is filled up ) fees rises as users compete for block space , which often lead to fee congestion.

It is worth mentioning that technically the memory doesn't need to be "crowded" or exceed the 300 MB limit for the fees to go up. As you already said people compete for the block space not the memory space and keep in mind that the block size cannot exceed 4 MB. So when the total size of transactions in the mempool exceed what can be included in the next block, some transactions won't be able to be mined. Therefore the competition begins and fee can go up as people compete to get into the next block. This happens way below the 300 MB limit.

When we go above that limit by a little bit, the fee rise is small too so it is not noticeable most of the times. But when we go higher (not necessarily reaching the 300 MB limit though) the fee would go higher as well.
Look at the chart below from the past week. The largest the mempool got was around 43 MB and the highest tip of these fee spikes is at 6 sat/vb.
https://jochen-hoenicke.de/queue/#BTC,1w,weight

[I_Anime]

Fees and Congestion: Each Bitcoin node has a limit to how many transactions it can keep in its mempool at a time, most Bitcoin nodes set their mempool limit to about 300mb by default. So when the memory is crowded ( space is filled up ) fees rises as users compete for block space , which often lead to fee congestion.

It is worth mentioning that technically the memory doesn't need to be "crowded" or exceed the 300 MB limit for the fees to go up. As you already said people compete for the block space not the memory space and keep in mind that the block size cannot exceed 4 MB. So when the total size of transactions in the mempool exceed what can be included in the next block, some transactions won't be able to be mined. Therefore the competition begins and fee can go up as people compete to get into the next block. This happens way below the 300 MB limit.

When we go above that limit by a little bit, the fee rise is small too so it is not noticeable most of the times. But when we go higher (not necessarily reaching the 300 MB limit though) the fee would go higher as well.
Look at the chart below from the past week. The largest the mempool got was around 43 MB and the highest tip of these fee spikes is at 6 sat/vb.
https://jochen-hoenicke.de/queue/#BTC,1w,weight

Oh now I get , is the block that get filled up not the memory pool , so congestion happen because the blocks have limited space ( around 4MB of transaction data in bitcoin ). The block contains the confirmed transaction and whenever a block is filled (after reaching 4MB ) no more transactions can be added to it , and the remaining transactions that didn’t make it into that filled block stay in the mempool , still unconfirmed waiting for the next block .

Wow thanks for pointing this out for me , this why i always say we learn everyday . Now and others can also learn from this too . And thanks for the provided links Charles-Tim

[Mia Chloe]

~snip

Well there are a couple of mistakes in your post and I'll identify them in summary. First, there is nothing like a global mempool. Each Bitcoin node has its own mempool, which contains transactions that that specific node has validated and not yet seen confirmed in a block. Also because of this miners don’t pick transactions from everyone’s mempool. Instead a miner selects transactions from their own node’s mempool (or a node they’re connected to).

Also, it's best you measure in sat/vB and not sat/byte because of the segwit upgrade. 300 MB is just the default setting in Bitcoin Core node runners can change this value or use different software with different limits. which is part of why some use knots.

[SmartGold01]

When we go above that limit by a little bit, the fee rise is small too so it is not noticeable most of the times. But when we go higher (not necessarily reaching the 300 MB limit though) the fee would go higher as well.
Look at the chart below from the past week. The largest the mempool got was around 43 MB and the highest tip of these fee spikes is at 6 sat/vb.
https://jochen-hoenicke.de/queue/#BTC,1w,weight

So this was the reason why the fee was high few days ago and when I was making transaction I checked on the mempool to know where the fee is and when I check it was higher than expected but for me to make use of my funds to what I want to do, I had to just perform transaction increases it to 6.3sat/vbyte for my transaction to quickly so that I can continue what I am doing, although after some while I had to check again and I noticed that has drop to around 1-4 as higher priority.

Thank you the clear explanation.

[Doan9269]

How it works : when a transaction is being sent , it’s verified by nodes and added to their mempools (the nodes stores and shares the unconfirmed transactions with other nodes, help them reach miners who can confirm the transactions) . Once confirmed those transactions are removed from mempools .

This aspect also talks about the unique characteristics of the bitcoin nodes, they vallidate transaction, they don't just accept a transaction except been confirmed  to be in consensus with the blockchain protocols, which anyone that doesn't gets rejected, we may want  to read further about bitcoin nodes https://river.com/learn/what-is-a-bitcoin-node/

which often lead to fee congestion.

This lead to mempool being congested and not the fee, when there is high volume of transactions, the mempool gets congested and those with higher fee rate get priority over the others with lower transactions feel.

[Ambatman]

Technically the Mempool has no limit and could be considered unlimited usually limited by nodes RAM or decision.
Most times those with lower fees limit like 300Mb push out lower fees to accommodate higher ones
If I'm not mistaken we crossed above a Gigabyte during the ordinals and like frenzy
Blockchair has around 5GB mempool limit and when these limits are met
Lower fees are been pushed.

[Charles-Tim]

Oh now I get , is the block that get filled up not the memory pool , so congestion happen because the blocks have limited space ( around 4MB of transaction data in bitcoin ). The block contains the confirmed transaction and whenever a block is filled (after reaching 4MB ) no more transactions can be added to it , and the remaining transactions that didn’t make it into that filled block stay in the mempool , still unconfirmed waiting for the next block .

The memory pool can also be filled up. If that happened, low fee transactions will be purged out of the mempool in a way that you can be able to spend the coin again on your wallet in another transaction. Although, if the coin remain not spent in another transaction, do not be surprised if the coin later become confirmed because not all nodes that set it as 300MB which is the default.

What was explained above is that as there are more new unconfirmed transactions in the mempool getting higher than those new confirmed transaction, the fee rate for high priority will also get higher even if the mempool is not yet full.

[Agbamoni]

So this was the reason why the fee was high few days ago and when I was making transaction I checked on the mempool to know where the fee is and when I check it was higher than expected but for me to make use of my funds to what I want to do, I had to just perform transaction increases it to 6.3sat/vbyte for my transaction to quickly so that I can continue what I am doing, although after some while I had to check again and I noticed that has drop to around 1-4 as higher priority.

For more understanding. Miners prioritize blocks with the highest fee rate down to the lowest.
So if you are making a transaction, you can always choose to pay more sat/vB than other users at that moment. Immediately your transaction will stay at the top of the mempool to get mined faster.

Some wallet  (RBF-enabled wallet only)actually allows you to alter your transaction fee by rebroadcasting it with a fee that is higher than the previous.

[WhoYouCantKill]

Lovely explanation — clear and simple to follow. Break down of a technical topic in a way starter can really understand.

The mempool truly is like the waiting room, and understanding it may help people see why fees rise or confirmation take longer time. A lot of users overlook it, yet it is one of the best equipment for network activity judgement and monitoring transactions wisely. You've done well simplifying it.

[Sonia_123]

Fees and Congestion: Each Bitcoin node has a limit to how many transactions it can keep in its mempool at a time, most Bitcoin nodes set their mempool limit to about 300mb by default. So when the memory is crowded ( space is filled up ) fees rises as users compete for block space , which often lead to fee congestion.

It is worth mentioning that technically the memory doesn't need to be "crowded" or exceed the 300 MB limit for the fees to go up. As you already said people compete for the block space not the memory space and keep in mind that the block size cannot exceed 4 MB. So when the total size of transactions in the mempool exceed what can be included in the next block, some transactions won't be able to be mined. Therefore the competition begins and fee can go up as people compete to get into the next block. This happens way below the 300 MB limit.

When we go above that limit by a little bit, the fee rise is small too so it is not noticeable most of the times. But when we go higher (not necessarily reaching the 300 MB limit though) the fee would go higher as well.
Look at the chart below from the past week. The largest the mempool got was around 43 MB and the highest tip of these fee spikes is at 6 sat/vb.
https://jochen-hoenicke.de/queue/#BTC,1w,weight

This same congestion made me had a huge loss, because I was told that my transaction fees was low and can't be done because I really need the fund I increased it for two times before I could remember that it might have been congestion that is causing the delay in transaction, I quickly cancelled it but even in the cancellation process there was still congestion and I was confused and don't know what to do, the next thing I saw huge debit and I lost almost all I had .

[Hustle2025]

The mempool can be seen as the waiting room for Bitcoin transaction, until it is confirmed. The mempool stand's for "memory pool". It is the temporary holding area for unconfirmed Bitcoin transactions.

When Bitcoin is sent from one wallet to another, the transaction doesn't go straight into the Blockchain, rather it goes to the mempool where it wait's for confirmation by miner's.

How it works:
1.  Initiation of transaction; sending Bitcoin from one wallet to another.
2.  Broadcast of transaction; the senders wallet sends the transaction to the Bitcoin network.
3.  Transaction entering the mempool; Every nodes adds the transaction to its own mempool while waiting for miner's to confirm it.
4.  Selection of transaction by miner's; miner's choose transaction from the mempool to include in the next block.
5.  Transaction confirmation; Immediately the miner include the transaction in a block, it moves from the mempool to the Blockchain where it stays permanently.

[EL MOHA]

This same congestion made me had a huge loss, because I was told that my transaction fees was low and can't be done because I really need the fund I increased it for two times before I could remember that it might have been congestion that is causing the delay in transaction, I quickly cancelled it but even in the cancellation process there was still congestion and I was confused and don't know what to do, the next thing I saw huge debit and I lost almost all I had .

I don’t understand most of the things you said here you said after two RBF which is increase in transaction fee the transaction still didn’t get picked which simply signifies that you weren’t using high priority fees then or many other transactions were matching the fee or paying higher than what you increased yours to. As for cancellation, you can never cancel a transaction already broadcasted the only thing you can do is send such transactions through RBF or CPFP back to your own wallet.

So for now example the one you saw in the electrum wallet which has cancellation doesn’t actually cancels transactions but sends the transaction to the same address or wallet it’s coming from and in this you still need to pay for the transaction fee meant for it and the transaction fee has to be of highest priority because other RBFed transactions might not be removed by some nodes and still stands a chance to be spent but once the last transaction is confirmed it invalidates all the other transactions

[Cryptomultiplier]

Lovely explanation — clear and simple to follow. Break down of a technical topic in a way starter can really understand.

The mempool truly is like the waiting room, and understanding it may help people see why fees rise or confirmation take longer time. A lot of users overlook it, yet it is one of the best equipment for network activity judgement and monitoring transactions wisely. You've done well simplifying it.

New users to the Bitcoin network or crypto currency world will have to understand that when sending complex transactions probably involving one with 10 inputs, the total fees must be higher than a simple transaction with single input.
Also, both transactions including complex and simple, may offer competitive fee rate at some point, but the complex transactions would of course take up more space in the mempool.

So the mempool basically is all about data and fee rate, not about the amount either large or small, to be transacted with.