Waste Metric

The waste metric is the score Bitcoin Core's wallet uses to choose between the input sets its coin selection algorithms propose. The wallet runs several algorithms on each payment and keeps the candidate that scores lowest. The score, computed by RecalculateWaste in src/wallet/coinselection.cpp, has two parts. The first is the inputs: the fee they cost at the transaction's fee rate minus the fee they would cost at a long-term fee rate, 10 sat/vB by default (the -consolidatefeerate setting). Below 10 sat/vB that difference is negative, so every extra coin the wallet sweeps up lowers the score; above it, every extra coin raises it. The second part is the change. If the candidate makes a change output, add what that output costs to create now and to spend later, the future spend priced at the wallet's discard fee rate, the rate below which it would rather give the change to the miner than create it. If it makes none, add the few satoshis of excess that go to the miner instead. Lowest wins, and a tie goes to the candidate that spends more inputs.

PR #22009 by Andrew Chow introduced the score at this level in version 23.0 (2022); it had been part of Branch and Bound's own search since 0.17.0. The pull request's description gives the purpose of the long-term rate: it is "the feerate that we switch between consolidating and optimizing for fees". Mark Erhardt, who designed that search, describes waste as a "measure of the fees for the input set at the current feerate compared to spending the same inputs at the hypothetical long-term feerate".

It is a fee score, and its authors say so: the pull request that added CoinGrinder in 2024 says the wallet still lacks "additional metrics to score different input set candidates, e.g. on basis of their privacy and wallet health impact". Which addresses get linked by a selection is not in the number.

Go deeper in Which Coins Does Your Wallet Spend? - the four algorithms the score judges, a worked example at 1 and 100 sat/vB, and what the score cannot see.