Fee Floor

The fee floor is the practical minimum fee rate (in sat/vByte) below which Bitcoin transactions are unlikely to confirm in a reasonable timeframe. It's not a single protocol-level constant; it's an emergent property of the mempool state and miner behavior at any given moment.

Two related concepts that get conflated:

• The relay-policy minimum. Bitcoin Core's default minimum to accept a transaction into its mempool: 1 sat/vB. Below this, the transaction won't be relayed by Core nodes at all. Knots and other implementations can set this higher.
• The market floor. The actual fee rate the cheapest currently-mined transactions are paying. During congestion this can be much higher than the relay minimum; during quiet periods it equals the relay minimum.

How the market floor behaves:

• Idle periods (typical 2024-2026 low-traffic times): the cheapest mined transactions pay 1-2 sat/vB. Nearly anything gets in.
• Moderate congestion: 5-20 sat/vB floor. Lowest-fee transactions sit in mempool for hours.
• Severe congestion (Ordinals mints, exchange exodus, market panic): floor spikes to 50-500+ sat/vB. Transactions below this can wait days or get evicted.

What sets the floor:

• Block space is fixed. Roughly 4MB equivalent weight per block, ~144 blocks/day = bounded throughput.
• Demand varies wildly. Spikes can be 10x baseline within hours.
• Miners pack by fee rate. Highest-paying transactions get in first; lower-paying ones queue.

The fee floor isn't enforced by anyone. It's just what happens when there's more transaction demand than block space. Modern wallets do fee estimation to set rates that comfortably clear the current floor; aggressive users sometimes underpay and rely on RBF to bump up later if needed.

In the long run, the fee floor becomes a more meaningful number as block subsidies decline and miner revenue depends more on fees. The 2140 endgame requires fees alone to fund mining; the floor under those conditions has to be high enough to sustain network security.