How trustworthy is mempool.space for fee estimates?

I usually count on mempool.space to help me decide what fee to use for my transactions, but lately it seems like the fees they're suggesting are higher than what I really need. There were a couple of times I wasn’t in a rush for confirmations and when the site said high priority was 8 sat/vB, medium was 7 sat/vB, and low was 6 sat/vB, I just went with 6 sat/vB, and my transaction got confirmed in about three or four blocks. I didn’t keep track, but I know it was definitely less than an hour. Then, the last time I sent a transaction, I opted for a lower fee again; if low priority was listed as 10 sat/vB and no priority as 9 sat/vB, I sent it at 9, and it also got confirmed pretty quickly. I wonder if anyone else has noticed this with mempool.space or whatever sites you might be using. The thing is, I could send it with a really low fee and then increase it using RBF, but if I'm not in a hurry and just want the transaction confirmed within 48 hours, it's kinda silly to send it at 2 sat/vB if mempool.space says 9 sat/vB is no priority. I’d end up having to increase the fee anyway, and honestly, I don’t see the point in stressing over a few satoshis.

Mempool congestion is not constant. It can drop and it can increase. If the mempool congestion drops, the fee rate and the fee required to get high priority confirmation will be reduced. If the mempool is becoming more congested, the fee rate and the fee required to get high priority confirmation will be increasing. It depends on how mempool is. But right now, the purging is at 5.96 sat/vbute (not constant either). Any transaction with fee rate that is less than 5.96 sat/vbyte will not be successfully broadcasted right now.

For having a good estimation, the best thing you can do is to check and analyze the mempool by yourself. To do so, you can use jochen-hoenicke website. At the time I am writing this post, mempool.space is suggesting the fee rate of 11 sat/vbyte for high priority and 9 sat/vbyte for low priority. If you check jochen-hoenicke website, you will see that you can put your transaction at 1 vMB from the tip with only 7 sat/vbyte. If I want to make a transaction now and I want it to be confirmed very fast, I would set the fee rate to around 10 sat/vbyte. If I can wait for a few hours, 7 sat/vbyte is more than enough.

I prefer to use Johoe's Bitcoin Mempool Site Statistics, which quickly shows how fees developed in the past 24 hours. It shows there was a bump in fees yesterday, and if you would have made a 6.x sat/vbyte transaction at that point, it would have taken 10 hours to confirm. You couldn't predict that bump in fees before it happened. Correct. I like to put it this way: the recommended fee is an estimate based on past transactions and blocks, while your new transaction depends on other transactions made and blocks found in the future. It's an estimate, that's it.

Well, "estimates" are just estimation based from statistics and shouldn't be relied on with 100% accuracy. The longer your projection is (like 48hours), the harder it is to make a close estimate since nodes' mempools states in the future cannot be predicted. As for mempool.space, it seems like they have their own fee rate estimation using their own API  (e.g.: mempool.space/api/v1/fees/recommended) Based from the result of the above, it's not similar to Bitcoin Core's estimatesmartfee or estimaterawfee. For accuracy, there's no way to tell or rather, fee estimation algorithms aren't really meant to be accurate, those are just trying to suggest the "best" fee rate for your target block. Here's the source code of their fees API, BTW: https://github.com/mempool/mempool/blob/master/backend/src/api/fee-api.ts

I prefer to use Jochen-hoenicke.de website when I need to check mempools too. Two reasons why I dislike mempool.space website. It is very bad designed, bad organized in fee rate bands' colors. Their color bands are mixing from hot to cold colors that is annoying already. Moreover, they use hot colors at bottom and cold at top. We know, most expensive fee rates are considered as 'dead' ones. They should use hot colors like red flags. I don't get ideas why mempool.space team designed like that or perhaps they see low fee rates as red flags.In the past, their chart is static, not dynamic. Static means when I hover a mouse on their chart, fee rates won't be updated. It is not dynamic like Jochen-hoenicke.de website. Recent months (around 2021 or 2022), they updated their chart to be a dynamic one but I still dislike it because of color bands. Static chart: https://web.archive.org/web/20210811200553/https://mempool.space/graphs#24hDynamic chart: https://web.archive.org/web/20220823122230/https://mempool.space/graphs/mempool

Yes, I knew that. The point is that I had previously been guided by mempool.space and found it to be approximately reliable. It has been the last few times that I have had the impression that it was giving estimates that were too high. I guess with the time I've been using bitcoin, it's about time I used something more sophisticated, like this website. It's not the first time I've visited it, I guess I've seen it mentioned on the forum, bud you have to do the calculations yourself instead of being guided by the simplest thing, which is that a website gives you the estimated fees for a quick confirmation. I understand that I can take this as a general rule? 1vMB from the top for a confirmation of a few hours and a few more sats for faster confirmations. Yes, that's for sure.

The issue at the moment, which has been the case for a few weeks, is that there is a huge backlog of transactions paying 6 sats/vbyte waiting to be confirmed, with the volume above that spread very thinly. So it is entirely possible that 8 sats/vbyte would put you within 0.1 MvB of the tip of the mempool, 7 sats/vbyte would put you 1-2 MvB from the tip, while 6 sats/vbyte puts you anywhere from 2-30 MvB from the tip. So you pick low priority at 6 sats/vbyte, and then a few blocks are found in quick succession and your transaction is mined. That's not a failure of mempool.space, but rather a quirk of state of the current mempool. Bear in mind of course that transactions don't pay either 6 or 7 sats/vbyte, and there is a continuous range between these two numbers which most fee estimators will not pay attention to. 6.0 sats/vbyte and 6.9 sats/vbyte will be a difference of over 25 MvB from the tip at the moment. Having said that, mempool.space is currently stating the "No Priority" fee is 14 sats/vbyte, which is only 2.5 MvB from the tip. That's not "No Priority" in my book. If I had a "No Priority" transaction, I'd set it to around 6.8 sats/vbyte and expect it to be confirmed sometime in the next 24 hours. So yeah, as others have said, take a look at Jochen Hoenicke's site instead. Not only can you place your transaction exactly as far from the tip as you want, but more importantly you can view the current mempool trends and see if it is filling up or emptying out.

There are currently over 440,000 transactions with the least priority (which pay 1-6 sat/vb), so don't enter a fee rate of that range, as they're probably there for days. Click on the most left block from mempool.space to see yourself what fits your need best. For example, paying something like 7.1 sat/vb can get you confirmed in a couple of hours in the worst case. What? First time hearing a bad experience with mempool.space. They're like the most favorite block explorer. Tells you exactly what you need to know, and the styling rocks. What hot and cold colors? Why should high fees be red flag if there aren't overpaying?

For newbies: https://mempool.space/ For advanced users: https://jochen-hoenicke.de/queue/#BTC,24h,weight I always prefer to recommend mempool.space for news. Advanced users that understand very well how to use jochen-hoenicke.de will prefer jochen-hoenicke.de. It is still one of the best mempool site that newbies can use, although jochen-hoenicke.de can be more accurate. I have used mempool.space times without number with good fee rate estimation.

The fees required for transactions are always changing, if you use a slightly lower fee and your transactions still get  confirmed, It is possible that within a very short time while you were not monitoring the fees, that the fee dropped to that amount of fee you used which made your transaction to be confirmed before it went back up. I personally don't like to wait so long for transactions to be completed, so I always use a medium or high priority fees so I have the assurance of a quick transaction.

I was just looking and I see that in the last block, transactions with a little under 7 sats/vB have been mined, but one transaction has caught my attention, because I understand that there are people who pay very high fees if they send a large amount of money, but don't you think the fees paid by this transaction for the amount they send is very unreasonable? I understand that this is someone who does not know very well what he is doing.

Right. But it's not really difficult to use the website. I just took a screenshot of jochen-hoenicke website. Acording to the image: There are around 143 vMB of transactions paying at least 1 sat/vbyte. There are around 125 vMB transactions paying at least 2 sat/vbyte. .. .. .. There are around 22 vMB of transaction paying at least 6 sat/vbyte. There are only 0.37 VMB of transaction paying at least 7 sat/vbyte. Considering that each block can contain up to 1 vMB of transactions, if I want my transaction to be confirmed fast, I would set the fee rate to at least 7 sat/vbyte. If I set the fee rate to less than 6 sat/vbyte, my transaction would be at least 22 vMB from the tip and considering that more and more transactions will enter the mempool, it will take a very long time until my transaction is confirmed. The image clearly shows that how increasing the fee rate from 6 sat/vbyte to 7 sat/vbyte can increase the chance of getting confirmation.

Look at any block and you will see dozens of such transactions. Here's the most recent block, for example: https://mempool.space/block/000000000000000000001d63a0516748160bb6b01741ab14b41277416fd5f54d mempool.space lists transactions in a block in the order the miner includes them. Since miners preferentially include higher fee paying transactions, then usually all the highest fee transactions are listed first. I count 19 transactions which pay a fee of 100 sats/vbyte or more, when actually 8 sats/vbyte was all which was needed to get in to that block. Click on any other block and you will see the same thing. You can't say that every single one of these transactions is simply because someone doesn't know what they are doing. Some will be, sure, but there will also be people who absolutely must get their transaction in to the next block for whatever reason, centralized exchanges with their poorly optimized algorithms, people who simply don't care about the extra few bucks they are spending in fees, some poorly configured wallets or software, and so on. I for one have created transactions with excess fees when paying a regular fee would leave me with a change output of a few hundred or a few thousand sats. Rather than be left with an output which would cost 50% of itself in fees just to spend, I'll just add it to the fee I'm already paying and donate it to the miners.

I take it as a challenge to waste as few sats as possible. This small output for instance could have added a few days to my favourite VPN, or topped up the balance at some website.

With the privacy implication of linking whatever payment you were making to your favorite VPN or account at some website. Sometimes it's simply not worth it for the sake of 1000 sats. Alternatively, send it to the public donation address for something like Tor or EFF.

Sometimes mempool.space was recommending me to send transaction with 6 sat/vByte and within minutes fee was increased up to 26 sat/vByte, that happens and mempool.space has nothing to do with this. By the way, you can manually check some recent blocks, upcoming ones too and decide yourself what fee will be enough to include your transaction in next few blocks. You have to keep in mind that sometimes miners manually add transactions in their blocks and you shouldn't include added transactions when you calculate fees.

I don't think I emphasised it well before. Not just the fee. I know that in every block there are people who pay high fees well above what would be reasonable to include the transaction in the next block. I mean the fee in relation to the amount you are sending. If someone sends 2 bitcoins I am not surprised that he overpays the fees. But in the transaction I have posted, there is one input and two outputs of about 0.001 bitcoin and he is paying a third in fees. I don't know how someone who is sending 25 euros can be in such a hurry to pay 8 to get into the next block. That's not what bitcoin is about as far as I know. It's not done for privacy, or to avoid an unspendable change or anything else. That's why I say I think he's someone who I think has no idea what he's doing. I did not say that. It has happened to me sometimes that it is the wallet itself that rounds up the fees.

If the change is smaller than the dust limit, that's the only option. The difference could even be more than the dust limit, as adding another output would increase the fee further. I've made mistakes with fees, especially when I try to rush things instead of taking my time to review the transaction. It happens.

Mempool.space is open source and you can run it yourself as self-hosted, so you don't have to depend on any third party. If you don't want to do that than you can check mempool state from several sources and get median mid value. You can check my BTC Mempool Fees Extension/add-on for Firefox/Chrome browser that includes mempool.space with two more source (blockchain.com and bitcoinber.live). Default settings in mempool.space but you can do manual change quickly. PS Note that nothing is 100% precise since all of this is just estimated fees.