Confirmations explained. Why BTC (Solana even) deposits sometimes just stall

[zhephoi]

Why your deposit isn't "stuck," it's waiting on confirmations
Came across a decent explainer breaking down what actually happens between broadcasting a tx and it showing up as credited. Summarizing the useful parts here:

What a confirmation actually is
Your tx sits in the mempool until a miner bundles it into a block — that's confirmation #1. Every block stacked on top after that is another confirmation. Platforms wait for a threshold before crediting funds because rolling back a tx gets exponentially harder the deeper it's buried.

Confirmation times by chain
BTC: ~10 min/block, most platforms now require 1-3 confirmations (down from the old 6-conf standard) → ~10-60 min depending on threshold
ETH: 12 sec blocks, but real economic finality (rolling back would cost >1/3 of all staked ETH) takes 12-15 min under Casper FFG
SOL: optimistic confirmation ~400ms, full finality ~13 sec

Why BTC specifically can stall completely, not just slow down
Fee-per-byte. Miners order the mempool by what pays most per unit of block space. Underpay during high traffic and the tx doesn't fail or expire, it just sits there until fees drop or you RBF/CPFP it.

What the platform can and can't control
Once broadcast, the platform can't pull funds from the mempool or speed up block production. It only controls the confirmation threshold it sets and how fast it processes its end once those confirmations land.

Just found the breakdown useful. Full writeup with an FAQ section here if useful: https://blog.chips.gg/post/why-your-crypto-deposit-isn-t-instant-and-when-it-will-be

[UchihaSarada]

What a confirmation actually is
Your tx sits in the mempool until a miner bundles it into a block — that's confirmation #1. Every block stacked on top after that is another confirmation. Platforms wait for a threshold before crediting funds because rolling back a tx gets exponentially harder the deeper it's buried.

There is probability of reversibility of transactions, and a more confirmation one transaction has, the less risk of reversibility as well as a more expensive cost for attackers to do that.

The chapter The blockchain in Mastering Bitcoin book has very detailed explanation about blockchain and confirmations, their roles for transactions on a blockchain.
https://github.com/bitcoinbook/bitcoinbook/blob/develop/ch11_blockchain.adoc

One way to think about the blockchain is like layers in a geological formation, or glacier core sample. The surface layers might change with the seasons, or even be blown away before they have time to settle. But once you go a few inches deep, geological layers become more and more stable. By the time you look a few hundred feet down, you are looking at a snapshot of the past that has remained undisturbed for millions of years. In the blockchain, the most recent few blocks might be revised if there is a chain reorganization due to a fork. The top six blocks are like a few inches of topsoil. But once you go more deeply into the blockchain, beyond six blocks, blocks are less and less likely to change. After 100 blocks back there is so much stability that the coinbase transaction—​the transaction containing the reward in bitcoin for creating a new block—​can be spent. While the protocol always allows a chain to be undone by a longer chain and while the possibility of any block being reversed always exists, the probability of such an event decreases as time passes until it becomes infinitesimal.

BTC: ~10 min/block, most platforms now require 1-3 confirmations (down from the old 6-conf standard) → ~10-60 min depending on threshold

Bitcoin is a strongest and most secure blockchain so if you want to have best security for your transactions and funds, choose Bitcoin blockchain.
https://howmanyconfs.com/

How many Bitcoin confirmations is enough?
https://blog.lopp.net/how-many-bitcoin-confirmations-is-enough/

Bitcoin confirmations risk calculator
https://jlopp.github.io/bitcoin-confirmation-risk-calculator/

[zhephoi]

Good point, and that's really the piece I skipped over.

The more confirmations a transaction has, the harder and pricier it gets for anyone to try to undo it, that's the actual reason platforms wait, not just habit or caution for its own sake. I explained what happens but not why it matters, so thanks for filling that in. Appreciate you pointing it out.

[Charles-Tim]

Underpay during high traffic and the tx doesn't fail or expire, it just sits there until fees drop or you RBF/CPFP it.

This is not true. If the purging gets to the fee rate of the transaction, the transaction will be dropped or purged out from the mempool in a way that you can double spent the coin.

Another one is when the transaction stayed too long in the mempool like in the mempool of Bitcoin Core nodes which is 14 days by default, the transaction will be dropped and you can double spend the coin.

[Upgrade00]

The more confirmations a transaction has, the harder and pricier it gets for anyone to try to undo it, that's the actual reason platforms wait, not just habit or caution for its own sake. I explained what happens but not why it matters, so thanks for filling that in. Appreciate you pointing it out.

People tend to pay too much attention to this point that it should get. I would guess that 1 confirmation is safe for about 99% of transactions. Services have their requirements because they could receive deposits in millions that could motivate someone to double spend.  A regular bitcoin user does not need to wait for a certain number of confirmations.

Another one is when the transaction stayed too long in the mempool like in the mempool of Bitcoin Core nodes which is 14 days by default, the transaction will be dropped and you can double spend the coin.

This should be just spending the coin which never left the wallet, not double spending.

[dzungmobile]

Another one is when the transaction stayed too long in the mempool like in the mempool of Bitcoin Core nodes which is 14 days by default, the transaction will be dropped and you can double spend the coin.

14 days is a most common setting among Bitcoin nodes and mempools but there are nodes that customize settings and can keep Bitcoin pending transactions in their mempools longer than 14 days.

14 days is from this parameter for 336 hours.

Code:

mempoolexpiry=336

It's visible with this Bitcoin Core Config Generator
https://jlopp.github.io/bitcoin-core-config-generator/

Code:

Mempool Expiration
Do not keep transactions in the mempool longer than <n> hours.
n = 336

[Charles-Tim]

Another one is when the transaction stayed too long in the mempool like in the mempool of Bitcoin Core nodes which is 14 days by default, the transaction will be dropped and you can double spend the coin.

14 days is a most common setting among Bitcoin nodes and mempools but there are nodes that customize settings and can keep Bitcoin pending transactions in their mempools longer than 14 days.

Yes. That is how it is also on Bitcoin Core. That is the reason I included by default on my post, because if you want your node to retain unconfirmed transactions longer, you will have to manually set it, but it is 14 days by default.

People tend to pay too much attention to this point that it should get. I would guess that 1 confirmation is safe for about 99% of transactions. Services have their requirements because they could receive deposits in millions that could motivate someone to double spend.  A regular bitcoin user does not need to wait for a certain number of confirmations.

I also think so. Even if there is a chain reorg, most people do not know what chain reorg is or know if the transaction can be on the mempool again.

Although, it is good to act safer.

Once most people see the transaction on their wallet, they accept it and all the wallets that I have used show the transaction has been received after 1 confirmation.  Although, it is different on Electrum because unconfirmed transaction will show on the balance, but while still show unconfirmed on the history.

[Mitchell]

All about "stuck" transactions and what you can do to fix them by achow101 is a much better explanation about how transactions work, including confirmations. That topic is linked in the stickies. Not sure this topic adds much...

[Dex_master25]

Another one is when the transaction stayed too long in the mempool like in the mempool of Bitcoin Core nodes which is 14 days by default, the transaction will be dropped and you can double spend the coin.

14 days is a most common setting among Bitcoin nodes and mempools but there are nodes that customize settings and can keep Bitcoin pending transactions in their mempools longer than 14 days.

Yes. That is how it is also on Bitcoin Core. That is the reason I included by default on my post, because if you want your node to retain unconfirmed transactions longer, you will have to manually set it, but it is 14 days by default.

People tend to pay too much attention to this point that it should get. I would guess that 1 confirmation is safe for about 99% of transactions. Services have their requirements because they could receive deposits in millions that could motivate someone to double spend.  A regular bitcoin user does not need to wait for a certain number of confirmations.

I also think so. Even if there is a chain reorg, most people do not know what chain reorg is or know if the transaction can be on the mempool again.

Although, it is good to act safer.

Once most people see the transaction on their wallet, they accept it and all the wallets that I have used show the transaction has been received after 1 confirmation.  Although, it is different on Electrum because unconfirmed transaction will show on the balance, but while still show unconfirmed on the history.

So basically different networks have or allow different confirmations ceiling depending on the particular kind of network or coin one is using, because that's what I can understand from the post and I think newbies may not understand what chain reorg means unless after personal research but without a real experience on how it works.