Bitcoin Forum

In the years I have used Bitcoin Core wallet, the 'fee estimation is not currently possible' warning only turned up occasionally. Recently, it happens frequently and takes longer to go away and calculate a fee. What could be causing this?

Is your Bitcoin Core fully synchronized? This error pops up if your client doesn't have enough transactions in the mempool to make any estimation (ie. still being synchronized), or if you've got sufficient transactions but Bitcoin Core hasn't seen them being confirmed yet to build a reasonable estimate for their priority system.

This problem should go away if you're fully synchronized, and you've let the client run for a short period of time.

Fee estimation in Bitcoin Core gets problems many times but were fixed and issues you have now can come from your side.

You should not rely on the wallet estimation for fee when you are broadcasting your transaction.

You can use it as one source and double check with some websites for mempool information includes recommended fee rates.

https://jochen-hoenicke.de/queue/#BTC%20(default%20mempool),24h,weight
https://mempool.space/
https://mempool.jhoenicke.de/#BTC,24h,weigh

Or register this bot on Telegram @BitcoinFeesAlert_bot, The bot announcement (https://bitcointalk.org/index.php?topic=5445635.0)

Whenever this method
https://github.com/bitcoin/bitcoin/blob/8426e018bf22b5e7e1ffa4b42e06813c470b1cc6/src/policy/fees.cpp#L867-L943
returns zero (CFeeRate(0)) the UI will show the error message.
https://github.com/bitcoin/bitcoin/blob/8426e018bf22b5e7e1ffa4b42e06813c470b1cc6/src/qt/sendcoinsdialog.cpp#L871
One of the reasons is not being synced.

I can only say it doesn't happen to me. My Bitcoin Core is always running, although I occasionally put my laptop in sleep mode.
I prefer to set fees manually though: Johoe's Statistics (https://jochen-hoenicke.de/queue/#BTC,24h,weight) or mempool.space (https://mempool.space/) give a better indication of current fees than Bitcoin Core itself.

But if there are many unconfirmed transaction on mempool, I noticed it could take decent amount of time before Bitcoin Core obtain most of those.

It's at most 300 MB by default, that shouldn't take longer than catching up on the latest blocks if you run Bitcoin Core once a day.

Based on my past experience which doesn't accept incoming connection, 300MB mempool isn't the problem. The problem is rate of received unconfirmed transaction could be slow (less than 20 TX/s), which is problematic when there are so many unconfirmed transaction. For example, https://mempool.space/ (https://mempool.space/) currently state there are about 177K TX with 759MB mempool (about 70K with 300MB mempool). I haven't checked whether it's also happen with newest version of Bitcoin or if you accept incoming connection.

It depends on how well-connected, or how loose your peer's mempool policies are. Johoe and Mempool.space are very well-connected and are likely to have transactions that would otherwise be purged from the reference nodes with default settings.

Other than that, Bitcoin Core uses inv messages to notify their peers of the blocks and the transactions that they have. For which, each node will subsequently request for the transactions that they're not aware of using getdata. I'm not exactly sure what is the exact limit, I don't recall exactly seeing a limit for that. Mempool.space and others are displaying the new transactions that are being relayed, as opposed to ones that are offline for too long.

Regardless, by how Bitcoin Core estimates fees, you don't actually have to know the entire mempool to get a good estimate. This is further compounded by the fact that you're only looking at the higher quantiles, which is pretty much guaranteed by the minfees limit set by mempool.

Since it's specific to your use-case, that can't be answered for certain without providing these info:

• Have you updated your Bitcoin Core recently?
• Have you set any option in your bitcoin.conf or started using command line args since then?
• How often do you use Bitcoin Core or how long do you keep Bitcoin Core running on average?
• Regarding the question above, is it the same as your usual usage before the fee estimation being to initiate slower?