UTXO – Invisible Balances in Bitcoin
How much bitcoin do I actually have?
In a traditional bank, this is simple – you log in to online banking and immediately see your account balance, for example €720. This number is stored in the bank’s database and is clearly linked to your name or account. With Bitcoin, however, things work very differently.
In the Bitcoin world, there is no central account and no user name. Nowhere is it written: “John Smith owns 0.075 BTC.” Instead, Bitcoin relies on an elegant system called UTXO – Unspent Transaction Outputs.
At first glance, this term may sound complicated, but with a few examples and analogies, anyone can understand it.
So what exactly is a UTXO?
Every Bitcoin transaction has inputs and outputs. Inputs define where the coins come from. Outputs define where the coins are going – in other words, who receives them.
Any output that has not yet been used in a new transaction is called a UTXO. This is essentially what you have in your wallet.
The balance of your Bitcoin “account” is not a single number, but rather a list of multiple UTXOs that together form your total value. Simply put: UTXOs are individual pieces of bitcoin that you own and can spend in the future.
These “pieces” do not have to be the same size, but together they make up your total balance.
An analogy with cash
Imagine you have a wallet at home with three banknotes:
- 1× €50
- 1× €20
- 1× €10
In total, you have €80, but you do not have a single “€80 banknote” – you have three separate bills. Each of these banknotes is a separate UTXO.
When you pay, you use them together. The shopkeeper receives €80, but also sees which banknotes you handed over. Bitcoin works in a similar way: when you combine multiple UTXOs in a single transaction, you reveal what your balance consists of.
What happens to the change?
If you paid €75, you would receive €5 back as change. In Bitcoin, this is called a change output, and it is sent back to you to a new address.
Let’s say you want to pay a friend 0.08 BTC. Your wallet selects the most suitable UTXO, for example one worth 0.1 BTC.
The transaction then looks like this:
- 0.08 BTC goes to your friend (main output)
- 0.0199 BTC goes back to you as change (change output)
- 0.0001 BTC is paid as a fee to the miner (fee)
This is just like paying with a €10 banknote for goods worth €7.50 and receiving change. Bitcoin transactions behave in the same way.
No need to worry about technical details
Even if this sounds complicated, most of these processes are handled automatically by Bitcoin wallets in the background. The user simply sees their balance – just like in a bank.
UTXO and privacy
One advantage of the UTXO system is privacy. Each transaction works with multiple UTXOs and generates new addresses, making it harder to determine who owns which coins.
However, the more UTXOs you combine in a single transaction, the more you reveal about yourself. When multiple UTXOs are used as inputs in one transaction, a blockchain analyst can logically conclude that they all belong to the same person – even if they originally came from different addresses.
This is why it is recommended not to reuse the same address and to be careful when combining UTXOs from different sources.
Too many small UTXOs
The opposite extreme can also be a problem: having too many small UTXOs. If your wallet contains dozens or hundreds of tiny fragments, a larger payment requires combining all of them into a single transaction.
This significantly increases the transaction size and therefore the fee, because in Bitcoin you pay for data size, not a percentage of the amount.
For this reason, it is recommended to occasionally consolidate UTXOs – that is, merge many small outputs into a single larger one, ideally at a time when network fees are low (for example, on weekends).
It is like exchanging all your loose coins at home for one larger banknote.
Understanding UTXOs at least on a basic level is important if you truly want to understand Bitcoin – especially in the context of privacy, efficiency, and the security of your transactions.