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. The lowest fee rate a node will accept into its mempool and pass on, set in Bitcoin Core by -minrelaytxfee. Core's default was 1 sat/vB until version 29.1 (September 2025) lowered it to 0.1 sat/vB. Below it, a Core node won't take the transaction on its own, though a child transaction that pays enough can sometimes bring a cheaper parent in with it. Each node operator can change the setting.
• The market floor. The fee rate the cheapest transactions in recent blocks actually paid. During congestion this can be far above the relay minimum; in quiet periods it can fall close to it.

How the market floor behaves:

• Quiet periods: when demand is low, the floor can drop below 1 sat/vB. Block 970,789, mined on October 10, 2026, had fee rates from about 0.27 sat/vB, with a median of about 0.28.
• Busy periods: when many people want block space at once, the floor climbs. Block 840,000, mined on April 20, 2024, was both the fourth halving and the block where the Runes token protocol switched on. Its fee rates started at about 100 sat/vB, with a median of about 200.
• Below the floor: a transaction waits. Bitcoin Core drops it from its mempool after two weeks by default, or sooner if the mempool fills up and it is among the cheapest.

What sets the floor:

• Block space is limited. A block can hold at most 4 million weight units, which is 1 million virtual bytes, and blocks come about every 10 minutes, roughly 144 a day.
• Demand varies. Compare the two blocks above: a median of about 0.28 sat/vB in one, about 200 in the other.
• 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 matters more as block subsidies shrink and miner revenue depends more on fees. The subsidy halves every 210,000 blocks and reaches zero around 2140. After that, fees alone pay miners, as the whitepaper anticipated.