Why a Bitcoin Swap Cannot Use Every Input Satoshi

A Bitcoin swap cannot send every input satoshi to its destination because the transaction must also pay a network fee. Bitcoin spends whole UTXOs—unspent transaction outputs, or chunks of bitcoin that can be spent—and the fee comes out of their total value.

Why does a Bitcoin transaction spend whole inputs?

A wallet does not take a few satoshis from each input like coins from a pocket. It selects one or more UTXOs, then spends each selected one in full. If you want to send less than their total value, the transaction usually sends the remainder back to you as change.

For example, if the wallet selects a UTXO worth 10,000 satoshis to make a 6,000-satoshi payment, the rest can return as change, minus the fee. The transaction records both the payment and the change as new outputs. For criteria to weigh when making that route choice, read how to choose a Chainflip native swap.

Where do the leftover satoshis go?

They may become change, pay the miner fee, or both. The fee is based mainly on the transaction’s size in virtual bytes, not just the bitcoin amount being sent. More inputs make a transaction larger, so spending several small UTXOs can cost more than spending one larger UTXO.

A wallet may also leave a very small remainder out of the change output. Tiny outputs can cost more to spend later than they are worth, so wallets may add that value to the fee instead. The minimum size depends on the output type and network policy.

• Input value is the total value of the selected UTXOs.
• Payment value is what the transaction sends to its recipient.
• Change is the remaining input value after the payment and fee.

Why can a swap deliver less than the input amount?

A swap has costs on both sides of the exchange. The Bitcoin transaction that sends funds pays a network fee, and the swap service may account for its own charges or the cost of delivering the other asset. The exact quote depends on the route and the service’s terms.

That means “swap 10,000 satoshis” may describe the amount offered for exchange, not the amount that arrives as the other asset. Check whether the quote treats the Bitcoin network fee as included or separate, and compare the expected output with the amount you are willing to trade. A quoted output can also move before the swap is completed, depending on how the service executes it.

For most users, the practical choice is to check the final receive amount and fee before confirming. If a wallet offers a fee choice, a lower fee can mean a slower confirmation. The input satoshis do not vanish: they are divided between the swap, any change returned to the wallet, and the fees required to complete the transactions.