What is a UTXO?

Imagine having two banknotes in your wallet, rather than a number written in a bank account. That’s roughly how Bitcoin organizes available funds: as separate transaction outputs. UTXO stands for Unspent Transaction Output.

Each such output has a specific value and conditions that must be met for it to be spent. In an ordinary payment, this requires the appropriate digital signature. Your wallet finds the outputs you can spend and shows their sum as your balance.

Say you have a UTXO worth 0.1 BTC and want to send 0.03 BTC. The original UTXO is consumed in full as an input to the new transaction. If the fee is 0.0001 BTC, two new outputs are created: 0.03 BTC for the recipient and 0.0699 BTC as change to an address your wallet controls. The fee is the difference between the total inputs and the total outputs.

Why it matters

This model lets the network’s nodes check whether the funds a transaction uses are still available. A UTXO that has already been spent can’t be used again in a second valid transaction on the same chain. That’s how preventing double spending is supported.

For you, the practical difference shows up mainly in fees. Two wallets with the same total balance may need different fees for a payment. If one has to combine many small UTXOs, the transaction usually takes up more space. The fee isn’t calculated as a percentage of the amount, but is tied to the size of the transaction and the fee rate you choose.

What to watch out for

Lots of small incoming payments can create lots of small UTXOs, which become expensive to use when fees rise. Also, combining them in a single transaction can make it easier for third parties to link your different addresses. We think it’s useful for you to know this detail, even if you leave UTXO selection to your wallet.