The rows of blocks and numbers on the mempool.space homepage serve two main purposes: judging how much fee to pay right now, and checking where a transaction is stuck. The yellow blocks on the left are "projected blocks" that show unconfirmed transactions sorted by fee rate. The blue blocks on the right are already confirmed blocks. Enter a transaction ID in the search box to see the specific status of that transaction.
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How to Read the Homepage: Yellow Blocks and Blue Blocks
When you open mempool.space, the first thing you see is two rows of blocks.
The blue blocks on the right are confirmed blocks. Each block represents a Bitcoin block that has already been mined and contains confirmed transactions. Click on one to see information such as the miner, fee range, and transaction count for that block. Blue blocks do not change unless an extremely rare chain reorganization happens.
The yellow blocks on the left are "projected blocks." They are not real blocks. Instead, mempool.space simulates which waiting transactions would be included if a block were mined right now, based on the current mempool and sorted by fee rate from high to low. Each yellow block corresponds to an estimated future block.
Transactions inside yellow blocks are sorted by fee rate, not by amount, and not by time. Transactions with higher fee rates are placed into earlier blocks first. Whether a transaction's fee is enough depends on which yellow block it appears in and what the minimum fee threshold for that block is.
The number and color intensity of the left-side blocks reflect congestion. If there are only one or two yellow blocks, the mempool is quite empty, and even a modest fee can get confirmed quickly. If there are seven or eight or more yellow blocks, the backlog is severe, and low-fee transactions will have to wait a long time.
Fees: What sat/vB Means and How to Use It
Bitcoin transaction fees do not depend on the amount being sent. They depend on the size of the transaction data. The fee rate unit is sat/vB (satoshis per virtual byte). The larger a transaction is in size, the higher the total fee will be at the same fee rate.
mempool.space shows several recommended fee tiers on the homepage or fee panel, usually including:
| Tier | Approximate Meaning |
|---|---|
| fastestFee | Likely to get into the next block |
| halfHourFee | May confirm within about 30 minutes |
| hourFee | Around the 1-hour level |
| economyFee | Low priority, for when you are not in a hurry |
| minimumFee | The lowest fee rate currently accepted by the mempool; below this, nodes may drop the transaction |
These fee rates are dynamic and can change every second. You may see fastestFee at 25 sat/vB now, and ten minutes later it could be 15 or 40. When choosing a fee rate, look at the value at the moment you send.
What I suggest instead: If you are not in a hurry, look at economyFee or hourFee and pick a tier that matches the waiting time you can accept. You do not need to chase fastestFee every time, unless you really need confirmation in the next block, such as when you withdraw from an exchange and need to complete another action quickly, or when you are making a time-sensitive transaction. The cost of chasing a high fee is real money paid, while the benefit is only being confirmed a few dozen minutes earlier.
Do not treat "projected blocks" as a guarantee. Miners are free to choose which transactions to include, and they may have already received your transaction through other channels before mempool.space shows it. Projected blocks reflect the competitive state at a given moment, not a miner's promise.
Transaction Details Page: How to Read the Fields
Enter a transaction ID (txid) in the search box to open the details page for that transaction. For a transaction still waiting for confirmation, these are the main fields to check.
Status. At the top of the page, the transaction will show as Unconfirmed (still waiting in the mempool) or Confirmed (already included in a block). If confirmed, it will show the block height and the number of confirmations.
Fee Rate. This is the actual sat/vB your transaction paid. Compare it with the current projected block thresholds on the homepage to judge why it has not been confirmed yet. If your fee rate is lower than the minimum threshold of the first projected block, it is not in the "next block" range. If it is below minimumFee, it may have already been evicted from nodes' mempools.
Inputs / Outputs. This is the most intuitive part of a Bitcoin transaction page. The left side shows inputs (where the money comes from), and the right side shows outputs (where the money goes).
A normal transfer usually shows two outputs, even if you only entered one receiving address. The other output is a change address generated by your wallet. The one or more UTXOs you spent were larger than the amount you wanted to send, so the leftover amount returns to your own wallet. This is normal and does not mean an extra payment was made.
Total fee. Subtract total outputs from total inputs, and the difference is the fee paid to miners. Check that this number matches what you expected when sending.
RBF and CPFP status. If your transaction is marked with RBF (Replace-by-Fee), the page will show RBF status, meaning you or the sender can replace the transaction with a higher fee rate to speed up confirmation. If this transaction is the "child" of some low-fee transaction, the page will show the CPFP relationship and the effective fee rate after considering parent and child being mined together. Sometimes a transaction's own fee rate is low, but it has a high-fee "child" that raises the overall priority.
An Easily Overlooked Field: The Mempool "Eviction Line"
On the right side or bottom of mempool.space, you can see the current mempool memory usage and the purging threshold.
Each node's mempool capacity is limited, usually about 300 MB. When the mempool is nearly full, nodes start evicting the lowest-fee transactions first, kicking them out of the mempool. An evicted transaction is as if it was never broadcast. It does not automatically "recover" and needs to be broadcast again.
If your transaction's fee rate is below the eviction line, it may disappear from most nodes' mempools before a miner ever includes it. This is a common reason why low-fee transactions "disappear." If a transaction has not confirmed for a long time, first check whether it still appears on mempool.space as "in the mempool," rather than becoming unfindable.
Use the Transaction Page to Answer Three Practical Questions
The transaction details page is not just for looking at "technical parameters." It directly answers the three things people are most often confused about after sending:
Did the transaction broadcast successfully? If you can find the transaction on mempool.space and see its status as Unconfirmed, it has entered the network. If you cannot find it, it may not have been broadcast successfully yet, or its fee rate was too low and it has already been evicted.
Why has it not confirmed yet? Compare your fee rate with the minimum threshold of the first projected block. If it is not enough, it is simply queued further back. If the fee rate is extremely low, it may be on the edge of being evicted.
Can it be sped up? If the transaction is marked RBF, you can use your wallet to send a replacement transaction with a higher fee. If it is not marked RBF, or your wallet does not support it, CPFP is another path, but CPFP requires you to have another spendable UTXO to build a child transaction, and not all wallets support it. These operations are done in your wallet. mempool.space only displays the status and does not send replacements for you.
Never enter your mnemonic phrase, private key, or wallet backup into any block explorer. mempool.space only needs a txid or address to query. It does not need, and should never receive, any credential that can control your funds.
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