Why Is My Transaction Not Going Through? Understanding the Bitcoin Mempool

On-chain Bitcoin transactions typically take a few minutes to confirm. However, sometimes your transaction can “get stuck” and take a little longer to settle. In that case, your transaction is in the Bitcoin mempool, waiting to get added to a block by a miner.   

In this guide, you will learn what the mempool is, what role it plays in Bitcoin transactions, and how you can speed up your Bitcoin transaction in case it remains unconfirmed in the mempool longer than you’d like. 

How Does a Bitcoin Transaction Work? 

Before we dive into what the mempool is, let’s first take a look at the inner workings of a Bitcoin transaction.

All on-chain Bitcoin transactions are recorded on the Bitcoin blockchain. The following are the four key stages that a Bitcoin transaction goes through from initiation to completion:

Transaction Creation

Before you can send BTC from your Bitcoin wallet, a transaction has to be created on the wallet. A valid transaction is made up of various elements, including

• The sender’s address
• The recipient’s address
• Amount of BTC to be sent
• A transaction fee that you are willing to pay miners

Digital Signing

To prove ownership of the bitcoin you intend to send, you must sign the transaction cryptographically using your private key. This key is also called a digital signature. It’s important to keep your private keys a secret because anyone who has access to them has access to the funds in your wallet.

Broadcasting and Approval

After a transaction is signed successfully, it’s broadcasted to the Bitcoin network, where a node picks it up. The node verifies the transaction’s validity and adds it to its mempool. The node then relays this transaction to its peers in the network, who also add the transaction to their mempools. 

From the mempool, a Bitcoin miner picks up the transaction and adds it to the next block. The miner then broadcasts the new block to the rest of the nodes in the network, which accept and approve the transaction.

Transaction Finalization

Once a new block is created on Bitcoin, every node in the network adds the new block to its copy of the blockchain. The transactions in the new block are marked as confirmed and removed from all other mempools across the network. However, it’s standard protocol to wait for at least six block confirmations before a transaction is considered final. This precaution prevents any possibility of the transaction being reversed by a user or double-spent in the event of a temporary fork in the blockchain.

The on-chain Bitcoin transaction process guarantees that transactions are authentic, secure, and immutable.

What Is the Bitcoin Mempool? 

Bitcoin has millions of daily users. Therefore, there needs to be a way for all these transactions to be organized. That’s where the Bitcoin mempool comes in.

Transactions made on the Bitcoin network aren’t added to the blockchain immediately. Instead, they are collected and stored in a temporary waiting area called the mempool. 

Every full node in the Bitcoin network has its mempool, where it stores a list of transactions it has checked and considers authentic. In a nutshell, all Bitcoin transactions first exist as ‘unconfirmed’ transactions in the mempool before they are permanently recorded on the Bitcoin blockchain.

The Bitcoin Mempool feature was added to the Bitcoin network after the BIP 35 (Bitcoin Improvement Proposal 35) by Jeff Garzik was approved. 

How Does the Bitcoin Mempool Work?  

Adding a transaction to a mempool begins when a Bitcoin node receives the transaction from another node in the network. However, before a transaction may be added to a Bitcoin’s mempool, it must pass a few tests to ensure it is syntactically correct and authentic.

First, the node will check the syntax of the transaction to ensure that the input and output values are not empty. It will also establish that the transaction size doesn’t surpass the maximum block size, among other tests. If the transaction clears all verification checks, it’s added to the mempool and broadcasted to other connected nodes where the same process is repeated.

Note that when the size of a mempool nears its storage limit, transactions with fees lower than a predefined threshold are eliminated from the mempool. This precaution helps to protect the mempool from crashing. When a mempool crashes, it has to start over with zero transactions.

After a miner validates a new block, all transactions recorded on the approved block are removed from the mempool of each node across the Bitcoin network. This creates additional space in each node’s mempool for new transactions and also synchronizes the nodes before a new block is created on the blockchain.

Why Is My Bitcoin Transaction Stuck? 

The mempool size always changes based on the rate at which new transactions are being created. 

When there is high demand in the network, the mempool may become congested due to many unconfirmed transactions. This typically results in longer confirmation times and higher transaction fees, causing your Bitcoin transaction to take longer than expected. 

Transaction fees usually spike because network users are forced to compete to have their transactions included in a block by paying higher transaction fees. 

Therefore, your Bitcoin transaction has not yet been confirmed because the transaction fee you are paying is not high enough for miners to prioritize it. Usually, this means your transaction will stay in the mempool for a while until it’s added to a block by a miner. 

If the fee remains too low to be added to any block, you will eventually receive the BTC back that you tried to send, enabling you to try again with a transaction higher fee.  

How To Accelerate an Unconfirmed Bitcoin Transaction

If your Bitcoin transaction is sitting in the mempool too long, there are a few ways you can do to accelerate your Bitcoin transaction. To utilize these options, you will need to use a wallet like Sparrow or Electrum as they are not yet supported in Xverse.

Replace By Fee

The first thing you can do is to increase your transaction's priority by paying higher fees. Transactions with higher fees are likely to be processed faster. This feature is known as Replace By Fee (RBF). 

When choosing RBF to speed up your transaction, check the status of the Bitcoin mempool to ensure that your new fee is above average.

Child Pays For Parent (CPFP)

Alternatively, you could use the Child Pays For Parent (CPFP) approach. CPFP is a technique used to speed up the confirmation of a transaction stuck in the Bitcoin mempool due to very low fees.

CPFP involves creating a new transaction (the child) with a higher mining fee, forcing the approval of a previous transaction (the parent) stuck in the mempool. The purpose of a CPFP is to incentivize miners to approve both transactions quickly.

Note that CPFP transactions are different from RBF transactions. In CPFP, the previous transaction is not invalidated. Instead, both transactions are confirmed at the same time. The secondary transaction only serves as a motivation for miners to process both transactions and receive the associated fees.

Bitcoin Transaction Accelerators

Arguably, the easiest way to speed up your unconfirmed Bitcoin transaction is to use Bitcoin transaction accelerators. Examples of free transaction accelerators include BitAccelerate and ViaBTC’s Transaction Accelerator. 

To use them, all you need to do is submit your Bitcoin Transaction ID to increase the chances of your transaction settling quicker. 

To successfully navigate the Bitcoin ecosystem, you need a Bitcoin Wallet that enables you to securely buy, store, send, and receive BTC. Download Xverse now to use the most advanced Bitcoin wallet to manage your digital currency. 

FAQs

What is mempool depth?

Mempool depth (also known as mempool weight) refers to the sum of all the transactions in a mempool based on storage space, denoted in bytes or megabytes. A larger mempool size indicates congestion in the Bitcoin network. Mempools have a default limit of 300MB, but this may be adjusted by node operators.

How to check if your transaction is in the mempool? 

To check if your Bitcoin transaction is in the mempool, copy your transaction ID from your wallet transaction history. Next, visit a blockchain explorer website like blockchain.com or bitcoinexplorer.org and paste your transaction ID in the search bar. 

The explorer will display information about your transaction, including its current status. If your transaction is in the mempool, it will be shown as an unconfirmed or pending. 

What do you do with a stuck Bitcoin transaction?

You have several options if your transaction is stuck in the Bitcoin mempool. First, you can wait it out. Given sufficient time (usually 24 to 48 hours), your transaction will be sent back to your wallet if a miner had not yet added it to any new blocks.

To accelerate your transaction, you can pay higher fees using the Replace By Fee (RBF) or  use the Child Pays For Parent (CPFP) method, which allows you to create a new transaction with higher fees that incentivize miners to approve both transactions. Alternatively, you can use Bitcoin Transaction Accelerators, typically offered by mining pools to speed up your transaction for free. 

How to avoid Bitcoin transactions getting stuck in the mempool by using the Lighting Network? 

If you are making small Bitcoin payments, you may want to consider off-chain payments using the Bitcoin Lighting Network. 

The Lightning Network (LN) is a Bitcoin layer-2 solution that uses peer-to-peer payment channels to enable high-speed, low-cost Bitcon transactions. LN transactions don’t land in the mempool and need miners to add them to a block before transactions can go through. As a result, transactions on the Lightning Network take a few seconds to complete and only cost a few cents. Xverse is planning to add Lightning Network support soon.