Definition
The mempool minimum fee is the feerate floor a transaction must meet to be accepted into a particular node's memory pool. Reported as mempoolminfee by the getmempoolinfo RPC, it is defined as the maximum of the static minimum relay feerate and the mempool's own dynamic minimum. Under light load that floor sits at the relay minimum (historically 1 sat/vByte). As the mempool approaches its size cap, the floor rises, so the same transaction that would be accepted on an idle node can be rejected by a congested one.
How the dynamic floor works
Each node bounds its mempool memory with -maxmempool, default 300 MB. When an incoming transaction would push the pool over that limit, the node evicts the lowest-feerate package and raises its effective minimum to that evicted feerate plus an increment. This is why, during fee spikes, low-fee transactions silently fail to relay: they never clear the elevated floor, and they may be dropped from nodes that already accepted them once the pool tightens further.
Why it matters for users
Wallets read mempoolminfee to avoid broadcasting a transaction that peers will simply ignore. Understanding the floor explains stuck low-fee transactions and motivates fee-bumping tools like Replace-by-Fee and Child-Pays-for-Parent. Because each node sets its own value based on its own maxmempool and traffic, the floor can differ slightly across the network.
For related concepts, see mempool persistence.
In Simple Terms
The mempool minimum fee is the feerate floor a transaction must meet to be accepted into a particular node’s memory pool. Reported as mempoolminfee by…