Why organising around the mempool changes everything
The mempool is the set of valid transactions that have been broadcast but not yet included in a block. Every Bitcoin node keeps its own copy, they differ slightly from one another, and it is where your transaction sits during the period when you are most likely to be worried about it.
Traditional explorers treat this as a footnote. A transaction is either confirmed, in which case here is the block, or it is not, in which case the page says "unconfirmed" and offers nothing else. That is precisely the moment when "unconfirmed" is the least useful word available, because the question you actually have is how long, and the answer depends on where you sit relative to everyone else who is also waiting.
mempool.space inverts the priority. The landing page is the queue: projected blocks stacked to the left, confirmed blocks to the right, each projected block labelled with the fee range it will likely contain. You can see the boundary your transaction needs to clear. It is one of those interface decisions that seems obvious afterwards and that nobody had made properly before.
Reading the fee estimates properly
The fee recommendation endpoint returns five bands, and the difference between them matters more than the numbers themselves:
GET https://mempool.space/api/v1/fees/recommended
{"fastestFee":2,"halfHourFee":1,"hourFee":1,
"economyFee":1,"minimumFee":1}Those were real figures when we last checked. During quiet periods the bands collapse together, and paying the "fastest" rate buys you nothing over the economy rate because there is no competition to outbid. During congestion they spread dramatically, and the gap between them is the actual decision.
The band worth understanding is minimumFee. That is the floor below which nodes will not relay your transaction at all — not "slow", but "may never propagate". Wallets that let you set an arbitrary fee will happily let you go below it and then leave you with a transaction that appears to exist on your device and nowhere else.
Fee rate, not fee
Bitcoin miners select by fee per unit of block space, not total fee. A large transaction paying a high total fee can easily be less attractive than a small one paying less. This is why the unit everywhere on mempool.space is sat/vB — satoshis per virtual byte — and why a wallet that shows you only a total in pounds is hiding the number that decides your position in the queue. Our guide on unconfirmed transactions works through what to do when you have got this wrong.
What to do when something is stuck
This is the job mempool.space is best at, and the sequence is worth knowing.
Open the transaction. The page shows your fee rate, the current mempool composition, and an estimate of which projected block you fall into. If your rate sits inside the first projected block, you are minutes away and nothing needs doing. If you are several blocks down, the estimate is a genuine projection based on what is queued ahead of you.
Then check the RBF flag. Replace-by-fee signalling means the transaction can be resubmitted with a higher fee, and most modern wallets set it by default. If it is signalled and your wallet supports fee bumping, that is the clean fix. If it is not, your options narrow to child-pays-for-parent — spending an output of the stuck transaction with a fee high enough to make the pair attractive together — or waiting.
The thing not to do is panic and rebroadcast the same transaction through a different service. It will not help, it cannot help, and the "transaction accelerator" services that advertise into exactly this moment are mostly selling you the thing your wallet already does for free.
From our testing
We tracked a deliberately underpaid transaction across four explorers. Three of them said "unconfirmed" for eleven hours and nothing else. mempool.space showed its position in the queue drifting as higher-fee transactions arrived, which made it obvious within two minutes that waiting was futile and a fee bump was required. That is the entire value proposition in one example.
The API, and why the WebSocket matters
The REST API needs no key, no registration and no account. The base is https://mempool.space/api and the surface is the Esplora-compatible one — the same shape as Blockstream.info, which means code written against one mostly works against the other. That compatibility is a quiet gift to anyone building something that needs a fallback.
The endpoints you will actually use are /address/:address, /tx/:txid, /blocks/tip/height, /v1/fees/recommended and /v1/difficulty-adjustment. The last one is underrated: it returns the progress through the current retarget period, the estimated change, the blocks remaining and the projected date, all in a single call.
The WebSocket is what sets it apart though. Rather than polling for new blocks, you subscribe and get pushed updates as they happen — new blocks, mempool changes, and transaction status for addresses you are tracking. For any interface that needs to feel live, that is the difference between a clean implementation and a polling loop that hammers someone else's server.
Rate limits exist and are not published as numbers. Exceed them and you get HTTP 429; persist and you can be blocked. The project is explicit that heavy users should run their own instance, which is a reasonable position for a free service to take and is made reasonable by the fact that self-hosting is genuinely documented.
Running your own
mempool.space is open source and the full stack is publishable — a Bitcoin Core node, an Electrum-protocol server, the backend indexer and the frontend. The project maintains Docker images and the documentation is honest about what it needs.
What it needs is not trivial. A fully indexed Bitcoin node is comfortably over 700 GB, you want an SSD rather than spinning disk, and the initial sync and index build is measured in days rather than hours. Once it is running the resource cost is modest, but the setup is a weekend rather than an evening.
The reason to do it is privacy, and it is the strongest version of that argument available. An explorer running on your own hardware, queried by your own wallet, tells nobody anything. Every hosted alternative — including this one — necessarily learns which addresses you asked about. If you run a node already, adding mempool.space to it is the highest-value thing you can do with it. Our guide to running your own explorer compares the options and their real costs.
Where it is the wrong tool
It does Bitcoin. That is the entire limitation and it is not going to change, because the focus is what makes it good. If your question involves Ethereum, Solana, Tron or anything else, you want a different page — start from our directory or the relevant chain hub.
It is also not a research tool. If you want to count transactions matching a condition across a date range, Blockchair answers that from a URL and mempool.space cannot answer it at all. Different question, different instrument.
And for a genuinely casual lookup — someone sent you a transaction ID and you want to confirm it exists — the density works against it. Blockstream.info is plainer and faster to parse if that is all you need.
One happy note: the codebase has been adopted elsewhere. litecoinspace.org is the same software pointed at Litecoin, including MWEB handling, which means the good ideas here have outlived their original chain.