Mempool Minimum Fee

Overview

Meaning and scope

The mempool minimum fee is the lowest effective fee rate a node currently accepts or retains for unconfirmed transactions under local policy. This value differs from a protocol-mandated minimum relay fee and from the fee likely needed for prompt block inclusion. Nodes can report different thresholds because their mempool contents, limits, peers, and configuration vary.

Mechanism and operation

A node’s mempool minimum fee can rise when its configured memory capacity is full. Lower-fee transactions are evicted, and new transactions below the rolling threshold are rejected until demand falls and the threshold decays. Wallets should combine node policy data with recent block and mempool fee distributions when estimating fees. A transaction above one node’s minimum may still propagate poorly or wait indefinitely. An estimate should identify the fee unit, network, block context, and transaction parameters used to calculate it. Calculations should use integer base units and preserve the original fields submitted by the signer, because rounded display values are insufficient for reconciliation or incident review.

Practical payment example

During congestion, a node raises its effective mempool minimum fee and drops low-fee transactions. A payment service interprets that status as a propagation risk, not proof that funds returned to the sender.

Operational checks

Operational handling of Mempool Minimum Fee should account for congestion, replacement rules, rejected fee settings, and software-version differences. Integrations should store the canonical transaction identifier, inclusion block, execution result, consumed resources, and effective fee reported by the final receipt. A pending quote can change before inclusion, and a transaction that fails at execution may still consume a network fee.

Distinctions and limits

Operators should monitor threshold changes and communicate specific rejection reasons rather than presenting every low-fee failure as consensus invalidity. Wallet defaults, quoted caps, and minimum protocol charges are different values, so interfaces should label each field and avoid presenting a maximum authorization as the amount ultimately paid.

Related reading

Related definitions: mempool, fee.