Discard Threshold

The discard threshold is the dynamic minimum fee rate a transaction must pay to stay in a Bitcoin node's mempool when memory is under pressure. Below the threshold, the node drops the transaction; at the threshold or above, it stays.

How it works in Bitcoin Core:

• Each node enforces a maxmempool size (default 300 MB).
• When the mempool exceeds that size, the node evicts the lowest-feerate transactions first.
• The feerate of the evicted transactions becomes a floor: any incoming transaction below that feerate is rejected outright.
• The floor decays slowly over time as new low-fee transactions become acceptable, but during fee spikes it can rise dramatically.

Why this is operationally important:

• During fee-rate escalation (Ordinals era, post-halving periods, market spikes), the discard threshold can jump from ~1 sat/vB to 50+ sat/vB within hours.
• A transaction broadcast at "normal" fees during a calm period can become unrelayable if the mempool fills before it confirms.
• Bumping with RBF is the standard recovery: replace the stuck transaction with a higher-feerate version.
• Tools like mempool.space's "next-block fee" estimate effectively show the current discard threshold so wallets can set fees that will actually stick.

The discard threshold is also a defense against mempool spam: an attacker who wants to flood the network with low-fee garbage finds that the floor rises as their attack continues, naturally pricing out the spam. The mempool is a fee market, and the discard threshold is the price-clearing mechanism.