What is Mempool?

The mempool is the waiting room: transactions broadcast to the network but not yet in a block, visible to everyone, ordered by fee when a validator builds. It is where your payment sits between signing and settling, and its depth is why fees spike at rush hour.

For payments the mempool explains the wait: a transaction that pays the prevailing fee rides the next block; one that skimps waits until traffic clears or gets replaced. Checkouts quote the current fee precisely so the wait matches expectations.

How Mempool works

The mempool is the waiting room: transactions that have been broadcast and validated but not yet included in a block. Nodes hold them locally, fees ordering the queue that block producers draw from. When your transaction sits at pending, it lives in mempools across the network; when a producer includes it, it leaves the queue and starts confirming.

Fee markets live here. During congestion, the transactions paying more per unit of computation get included first, and the rest wait or get dropped back to the sender. Watching the mempool is how fee estimators know what the going rate is.

The payments-relevant nuance is that mempools are public on most chains: pending transactions are visible before they confirm, which is how front-running and sandwich attacks are attempted. Chains and L2s with encrypted or sequencer-ordered flows remove that visibility, trading a little transparency for protection.

Mempool in practice

A transfer stuck during a busy mainnet session is sitting in the mempool underbid. Bump the fee and it re-enters the queue at a competitive rate, getting included in the next block or two. Wallets surface this as a replace-by-fee option; the replacement supersedes the stuck original rather than double-spending it, so the merchant sees exactly one settlement either way.

Mempool on U.CASH

U.CASH quotes network fees with the transaction and watches the chain against your confirmation policy, so mempool delays surface as status, not surprises.

For merchants, the mempool is the explanation behind a pending payment: the customer signed, the network is sequencing, settlement waits for inclusion. Confirmation policies on each asset page state how many blocks the platform treats as settled. For customer-facing messaging, pending means in the queue, not in doubt: the signature is done, the funds are committed, and the network is sequencing. Setting expectations with that framing turns the wait from an anxiety into a status. The payment record moves from pending to confirmed on its own, with the hash as the tracking number.