What is the mempool?
Bitcoin’s waiting room explained

The waiting room analogy

Imagine a busy restaurant on a Saturday night. Orders come in constantly from every table. The kitchen can only cook so many dishes at once. So there’s a ticket rail — a line of orders waiting to be picked up and prepared. The kitchen works through them as fast as it can. When the restaurant is packed, the rail gets long and some orders wait longer than others.

Bitcoin’s mempool is that ticket rail.

When you send a Bitcoin transaction — or when a customer pays you — that transaction doesn’t go straight onto the blockchain. It gets broadcast to the network and enters the mempool (short for memory pool): a holding area of unconfirmed transactions waiting to be included in the next block.

Miners pick transactions from the mempool, bundle them into a block, and add that block to the blockchain roughly every 10 minutes. Once a transaction is in a confirmed block, it’s settled. Until then, it’s pending.

How miners decide which transactions to pick

Miners aren’t required to include every transaction waiting in the mempool. They get to choose — and they choose rationally. Each transaction includes a fee paid to the miner who includes it. Miners naturally prioritize transactions with higher fees per byte of data, because they earn more for the same block space.

Each Bitcoin block has a maximum size of 1–4 MB (depending on transaction type). When the mempool is empty or quiet, miners include virtually everything, often at very low fees. When the mempool is congested — when more transactions are arriving than blocks can absorb — a fee market emerges. Transactions bidding higher fees jump the queue; lower-fee transactions wait.

What causes the mempool to fill up

The mempool fills when the rate of incoming transactions exceeds Bitcoin’s processing capacity. Bitcoin adds one block roughly every 10 minutes, and each block has limited space. If a surge of transactions arrives — triggered by a price spike, a new token launch, an ordinals inscription wave, or just organic adoption growth — the mempool backs up fast.

Some notable mempool congestion events:

• Late 2017 bull run — fees reached $50+ per transaction as millions of new users tried to buy and move Bitcoin simultaneously. Transactions with low fees sat unconfirmed for days.
• May 2023 — Ordinals/BRC-20 mania — a wave of Bitcoin-native NFT and token inscriptions flooded the mempool, pushing fees to levels not seen since 2017. Over 400,000 transactions were pending at peak.
• April 2024 — Runes launch — a new token protocol launched on the Bitcoin halving block, causing another fee spike. Average fees briefly exceeded $100 per transaction.

These spikes are temporary. The mempool eventually clears as miners work through the backlog and the surge of new transactions subsides. But during a spike, low-fee transactions can wait hours or even days.

Replace-by-Fee (RBF): how to speed up a stuck transaction

If you send a Bitcoin transaction with a fee that turns out to be too low — and it’s sitting unconfirmed — many wallets support a mechanism called Replace-by-Fee (RBF). RBF lets you rebroadcast the same transaction with a higher fee, effectively replacing the stuck version in the mempool. Miners prefer the higher-fee version and will pick it up sooner.

Not all wallets support RBF (it must be enabled when the original transaction is created), but most modern wallets — including Phoenix and Manna — do. If you or a customer ever has a stuck transaction, RBF is the first thing to try.

There’s also Child-Pays-for-Parent (CPFP): if you’ve received an unconfirmed transaction (you’re the recipient, not the sender), you can create a new transaction spending those unconfirmed funds with a high fee. Miners see that picking up the parent transaction is necessary to collect the child’s high fee, so they prioritize both together.

What the mempool means for merchants specifically

Most of the time, the mempool is not your problem. During normal conditions, a Bitcoin payment from a customer will appear as pending within seconds and confirm within one block (~10 minutes). For small retail transactions, most merchants accept the payment once it’s visible as pending — the risk of non-confirmation on a legitimate payment is extremely low.

Where it becomes relevant:

• During fee spikes — if the customer’s wallet set a low fee during a congestion event, their payment might take longer than expected to confirm. It hasn’t disappeared; it’s just queued. You can look it up on mempool.space using the transaction ID.
• For high-value transactions — if someone is paying you a significant amount in Bitcoin, waiting for 3+ confirmations before releasing goods is prudent. During congestion, that might take 30–60 minutes.
• For understanding fee estimates — when you’re sending Bitcoin from your wallet (e.g., to an exchange to convert), check mempool.space first. If fees are spiking, waiting a few hours can save you meaningfully.

Why USDC doesn’t have this problem

USDC on Solana confirms in under a second and costs a fraction of a cent in fees — regardless of network conditions. This is because Solana’s architecture processes thousands of transactions per second, so congestion rarely develops the way it does on Bitcoin’s base layer. For merchants who want payment finality in seconds rather than minutes, and zero exposure to fee spikes, USDC on Solana is the practical answer.

Bitcoin’s Lightning Network addresses the same problem differently — moving payments off-chain entirely so the mempool is irrelevant for everyday transactions. That’s why Lightning is such an important part of Bitcoin’s payment future. OrangeTill has Lightning integration on the roadmap.