What Is UTXO?
A UTXO, or unspent transaction output, is a chunk of bitcoin that has been received but not yet spent. Your wallet balance is the sum of these outputs, and every payment consumes some UTXOs and creates new ones.
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A UTXO (unspent transaction output) is a chunk of cryptocurrency that an address has received and not yet spent. The Bitcoin balance your wallet shows isn’t a single account entry; it’s the sum of all the UTXOs your keys can spend. Every payment consumes some of those outputs in full and creates new ones in their place.
How do UTXOs work?
Think of a wallet full of banknotes. You might have $75 in total, made up of a $50, a $20 and a $5 bill. You can’t tear a bill in half to pay; you hand over enough and get change back.
Bitcoin transactions work the same way. A transaction’s inputs are UTXOs you received earlier, and its outputs are new UTXOs: one for the recipient and usually one returning change to you. Once a UTXO is spent it is gone for good and can’t be spent again. Every node keeps a list of all unspent outputs, known as the UTXO set, which lets it quickly catch any attempt at double spending.
Each UTXO is protected by a kind of lock. The output carries a short script stating the condition for spending it, which in most cases is a valid signature from the matching private key. Your wallet balance is simply the set of outputs your keys can unlock.
How is change created? An example
Say you hold a single UTXO worth 0.01 BTC and want to send 0.003 BTC, paying a 0.00001 BTC fee. The transaction uses the entire 0.01 BTC output as its input and creates two new outputs:
- 0.003 BTC to the recipient,
- 0.00699 BTC back to a change address in your wallet.
The remaining 0.00001 BTC, the difference between inputs and outputs, goes to the miner as the fee. If a block explorer shows an input larger than what you meant to send and an unfamiliar second output, don’t panic; that second output is almost always your wallet’s change address.
UTXO model vs. account model
Ethereum takes a different approach, the account model. Each address has a balance, and transactions simply debit and credit it, much like a bank account. The UTXO model makes it easier to validate transactions in parallel and keeps the history of each coin simple to trace. The account model is more convenient for complex smart contracts. Besides Bitcoin, Litecoin, Bitcoin Cash and Dogecoin use UTXOs, while Cardano uses an extended version known as eUTXO.
How do UTXOs affect fees and privacy?
Bitcoin fees depend on how much data a transaction takes up, not on the amount sent, and every input adds data. Spending many small UTXOs in one transaction costs more than sending the same amount from a single large one. That’s why some users consolidate small UTXOs into one output when network fees are low. Outputs worth less than the fee needed to spend them are called “dust.”
UTXOs matter for privacy too. Outputs spent together in the same transaction are generally assumed to belong to the same owner. Wallets with a “coin control” feature let you choose exactly which UTXOs to spend, which helps you avoid linking coins from different sources.
Frequently asked questions
Why don’t I see my UTXOs in my wallet?
Most wallets add them up and show a single balance for simplicity. A detailed view, if available, is usually tucked into advanced settings.
Is having lots of UTXOs a problem?
It doesn’t put your funds at risk, but it can make future transactions more expensive because each input adds to the fee.