Round-Amount Heuristic

The round-amount heuristic is a change-detection shortcut used in blockchain analysis. People tend to send payments in tidy figures, 0.05 BTC, a flat dollar amount, or a round number of satoshis, while the change a wallet returns is whatever is left over after fees and is therefore an arbitrary, jagged value with many decimal places. When a transaction has one round output and one ragged output, an analyst guesses the round one is the real payment and the ragged one is change going back to the sender.

Why it works, and when it fails

Correctly tagging the change output lets a surveillance firm attribute that new address to the sender's wallet, extending their cluster. The heuristic is probabilistic, not certain: it breaks down when both outputs are round, when a payment is itself an odd amount (such as a fiat-denominated invoice converted to BTC), or when a wallet deliberately randomizes amounts. But on the average transaction it is a cheap and surprisingly effective signal.

Defeating the heuristic

Privacy-aware wallets undercut round-amount analysis by avoiding obviously round sends, by matching output script types so neither output stands out, and by using protocols like PayJoin that alter the apparent amounts entirely. Sending fiat-pegged or randomized values makes it ambiguous which output is the payment. As with all chain-analysis heuristics, the defender's goal is not to make detection impossible but to make the analyst's guess unreliable.

This heuristic is one of several signals discussed in our Change Address entry, and it contributes to the broader process of Output Linking.