The easiest way in is physical cash. A note in your pocket has a fixed denomination; you cannot tear off a third of it. To pay for something you hand over one or more notes and take change back. UTXOs behave the same way: each one holds a set amount and can only be spent in full.
So a transaction is a list of inputs — the outputs being consumed — and a list of outputs it creates. If your inputs total more than you are paying, the surplus goes to a change output your own wallet controls, which is why the sums moving <a href="/glossary/on-chain/">on-chain</a> often look far larger than the payment itself. Whatever you do not assign to an output becomes the fee; it is never written out as a separate field.
This is also why fees depend on your coin history. A transaction's size in bytes grows with the number of inputs, so a wallet funded by many small deposits pays more to send a given amount than one holding a single large output. Very small outputs can become uneconomic to spend at all, and consolidating them into one output while the fee market is calm is routine housekeeping. Our <a href="/toolkit/how-to-estimate-a-network-fee/">fee estimation walkthrough</a> shows how input count feeds into the total.
Two further points. Every output is public, so combining coins from different sources in one transaction links them together — a genuine privacy consideration, and the reason consolidation is not free of downsides. And the model is not universal: account-based chains simply store a balance per address and subtract from it, which is easier to reason about but loses the clean, auditable lineage a UTXO set gives you. You can trace any output's history through a <a href="/glossary/block-explorer/">block explorer</a>.
Key takeaways
- Your balance is a wallet-side sum of spendable outputs, not a number stored anywhere on the chain.
- Fees track transaction size, and transaction size grows with the number of inputs you have to spend.
- Spending two unrelated outputs together publicly links them, so consolidation carries a privacy cost alongside its fee benefit.
UTXO — frequently asked questions
Why does my wallet appear to send more than I asked for?
It does not, in the end — it is spending whole outputs and returning the remainder to itself. If you pay a small amount using a large unspent output, the transaction shows the full amount leaving and most of it arriving at a change address you control. Explorers sometimes present that as one enormous transfer, which looks alarming until you know what change outputs are.
What is dust, and can I get rid of it?
Dust is an output so small that spending it would cost more in fees than it is worth. It is not lost, it simply sits there until fees are low enough to make moving it sensible. Some wallets let you select specific outputs so you can sweep several tiny ones into a single larger one during a quiet period; others hide them from the interface by default.
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