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The Bitcoin Mempool – A Beginner’s Explanation

By: Ofir Beigel | Last updated: 1/9/21

If you’ve been into Bitcoin long enough, you may have heard the term “Mempool” being thrown around. In this post I’ll explain exactly what the Mempool is and why it’s important.

Bitcoin Mempool Summary

The Mempool is a “waiting area” for Bitcoin transactions that each full node maintains for itself. After a transaction is verified by a node, it waits inside the Mempool until it’s picked up by a Bitcoin miner and inserted into a block.

That’s the Bitcoin mempool in a nutshell. If you want a more detailed explanation about the Mempool keep on reading, here’s what I’ll cover:

  1. Mempool Explained
  2. Mempool Management
  3. The Importance of the Mempool
  4. Frequently Asked Questions
  5. Conclusion

1. Mempool Explained

Before we begin, here’s a short and important video about how Bitcoin transactions get confirmed:

When a Bitcoin transaction is transmitted to the network, it first gets verified by all of the Bitcoin nodes available (i.e. computers participating in the Bitcoin network).

After it successfully passes verification by a node, it sits inside that node’s “Unconfirmed Transactions” area called the “Mempool” (short for Memory Pool). The transaction patiently waits until a miner picks it up and includes it in the next block.

The Mempool is basically the node’s holding area for all the pending transactions.

Each node has a different capacity for storing unconfirmed transactions. As a result, each node has its own version of the pending transactions. This explains the variety of Mempool sizes & transaction counts found in different sources.


2. Mempool Management

You may be wondering: how do nodes keep from crashing due to overload of pending transactions in the Mempool?

The answer is simple. Once the Mempool reaches full capacity, the nodes start prioritizing transactions by setting up a minimal transaction fee threshold.

Transactions with a fee-rate lower than the threshold are immediately removed from the Mempool and only new transactions with a large enough fee are allowed access to the Mempool.


3. The Importance of the Mempool

The Mempool is part of BIP 35 (Bitcoin Improvement Proposal No.35). The idea was that outside nodes would be able to access other nodes’ Mempool. This is useful for several cases:

  1. SPV clients (also known as lite wallets), wishing to know about transactions before they were confirmed and entered into a block.
  2. Miners who want to check for lucrative fees or download the current “transaction waiting list” in order to start confirming transactions.
  3. Remote network diagnostics.

The Mempool is the “waiting room” of the Bitcoin network. The faster transactions are cleared from it and added into blocks on the Blockchain, the better experience users will get.

In other words, if new transactions arrive at a higher rate than they are cleared from the mempool into blocks, a “traffic jam” will occur and transactions can take a long time to get approved (depending on their size and attached fee).

When a node receives the latest mined block from the miner, it removes all the transactions contained in this block from its mempool. This results in a sharp drop in the Mempool size.

If you want to see the current status on the Mempool you can take a look at this graph

Mempool

If, for example, the Mempool size is around 3MB then most transactions will have to wait at least one or two blocks until they get confirmed. This is because each block is 1MB in size, so it can take up to 3 confirmations to clear the whole mempool (assuming no new transactions are coming in).

Keep in mind that some of the Mempool transactions are low priority transactions that their sender knows they will take a long time to be confirmed – for example “dust transactions” (sending really small amounts of Bitcoin).


4. Frequently Asked Questions

How Long Does it Take to Confirm a Bitcoin Transaction?

On average a new block of Bitcoin transactions is mined every 10 minutes. This is an average, meaning it could take 1 minute or 1 hour as well. Each block holds a few thousand transactions.

Depending on the fee you’ve attached to your transaction you’ll be able to estimate how long it will take your transaction to get confirmed (again, on average).

You can use this page to see how much of a fee you need to attach to get confirmed within a certain amount of time.

What Should I Do If My Transaction is Stuck in the Mempool?

As a rule of thumb, if you wait long enough (usually around 48 hours) your transaction will drop from all of the Bitcoin mempools and the funds will be returned to your wallet.

However, here are some other things you can do to expedite transaction confirmation:

  1. Use Replace By Fee (RBF) – Some wallets (e.g. Electrum) will allow you to replace an existing transaction with a new one with a higher fee.
  2. Use transaction accelerators – Some mining pools supply a service of accelerating specific transactions for a fee or on a first come first serve basis.

5. Conclusion

The Mempool is a very important part of the Bitcoin network. It allows us to understand how crowded the network is and if there are “transaction traffic jams” which result in slower confirmation times and higher fees.

Got more questions about the Mempool? Leave the in the comment section below!

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21 comments on “Bitcoin Mempool”

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  1. I make a transaction since February 13 and it not yet confirmed till now please what can I do with that to make it confirm because it almost 3 month now and not yet confirmed please what can I do

  2. Using Bitcoin core I choose a fee that supposedly would result in the transaction being confirmed in 4 hours or less. I don’t how Bitcoin Core calculates this as it was way off and only put a 22.112 sat/B fee. Looking at mempool observer sites this is way too low for 4 hours. In fact, it’s been about 32 hours and the transaction is still stuck in the Mempool. I can’t increase the fee as I get a “transaction does not have a change output message.” I wish I could just cancel the transaction and do a new one. The barriers to BTC for non-tech people who don’t want to just leave their coins on a exchange are still pretty high I’d say.

  3. The 48 hours rule is not correct. I had a transaction with a very low fee that was confirmed after more than 3 weeks. It wasn’t returned.

  4. Hi Steven,

    Thanks for your response and validating what I thought. The reason I posited the question is there are a couple “analysts” out there like Marius Kramer on Quora who believe that BTC will dump because fees get to high because of MEMPool restraints. In my 40+ years of investing experience it seemed to me that these so called analysts were wrong given the underlying fundamentals of why BTC is rising – namely adoption. The fees are the cost of doing business and the price of admission. The fes for BTC in general even when they are high are far less than that of say buying Gold or Silver and that has little effect on the price power of the metals. As for the congestion like you said I believe that the developers and exchanges will figure a solution as necessity is the mother of invention. Again thanks for your response,

  5. Hi Roger,

    You’re welcome, I’m glad you found it helpful.

    I haven’t heard of that analyst but unless he was early in advising people to invest in Bitcoin – say in 2012 or 2013 before it went mainstream – then I’d question his judgement on this subject.

    Bitcoin fees can get relatively high but even then they remain flat rather than percentage fees. So while a $5 or even $50 fee may be prohibitive for small transactions, it’s peanuts for moving $5 million or even $5 billion. Such sums do move over Bitcoin, internationally, rapidly, unstoppably, and with relative privacy. Apart from other cryptos, there’s really no other payment service which can compete with that, at any fee point.

    If the low value transactions, which take up as much room on the blockchain as high value ones more or less, can be offloaded to other layers – at least if required by constraints – then we could see the main chain used more as a settlement layer for major value transfers and things like Lightning handling day-to-day payments for groceries and the like. I believe this is the direction in which things are likely to evolve… but who knows, it’s really hard to predict the future of this tech!

  6. I think you are correct Roger, originally the block size was 1mb and it was said that in the future when the tech scales it would not be bad to increase block size once we reach ‘the wall’. Unfortunately the network has calcified and unable to reach consensus. Keep in mind alternatives such as Monero use a flexible block size to clear up times of congestion, much the same way as BTC adjusts it’s difficulty. If this was implemented in BTC, it would be able to adjust automatically as needed.

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