Discard Threshold

The discard threshold is the smallest change output Bitcoin Core's wallet will create. When the change left over from a payment would come in under it, the wallet leaves the change output out, and the leftover goes to the miner as part of the fee.

The reason is cost. Change is only worth having if you can spend it later, and spending it means paying for one more input. If that future fee would eat the whole amount, the output is worth nothing to you, so the wallet doesn't make it.

How Bitcoin Core works it out:

• It picks a discard fee rate. The default is 10 sat/vB (the -discardfee setting). If the node's fee estimate for its longest target is lower, the wallet uses that instead, and it never goes below the dust relay rate of 3 sat/vB.
• At that rate, the threshold is the larger of two amounts: the fee to spend the change output later plus one satoshi, and the output's dust limit.
• Change below the threshold is added to the fee. Change at or above it gets its own output.

The setting arrived in Bitcoin Core 0.15.0 (2017), in the same change that redefined dust as an output that would cost its own value in fees to create and spend at the dust relay rate.

Don't confuse it with the mempool's minimum fee. That one decides which unconfirmed transactions a node keeps:

• Bitcoin Core caps its mempool at 300 MB of memory by default.
• When the mempool is full, the node evicts the lowest-feerate transactions and raises its minimum fee to just above the feerate of what it evicted. That floor then decays, halving every 12 hours, or faster once the mempool is under half full.
• In calm periods the floor is the relay minimum, which Bitcoin Core 29.1 (September 2025) cut from 1 to 0.1 sat/vB. The release notes say the dust rate and the wallet's fee rates stayed the same.
• mempool.space shows this floor on its dashboard as "Minimum fee", relabeled "Purging" when it rises above the relay minimum.