UTXO Model (Unspent Transaction Output)
Bitcoin manages balances not as account totals but as sums of unspent transaction outputs (UTXOs). The coin model's principles and its privacy and security advantages.
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UTXO (Unspent Transaction Output) is the fundamental way Bitcoin manages balances. Rather than recording an "account balance" like a bank, Bitcoin calculates your balance as the sum of individual outputs that have not yet been spent.
0.5 BTC"] U2["UTXO B<br/>0.3 BTC"] end subgraph Transaction TX["Transaction"] end subgraph Outputs O1["Recipient<br/>0.7 BTC"] O2["Change<br/>0.0999 BTC"] end U1 --> TX U2 --> TX TX --> O1 TX --> O2 FEE["Fee: 0.0001 BTC"] TX -.-> FEE style TX fill:#f7931a,stroke:#f7931a,color:#000 style FEE fill:none,stroke:none,color:#8b949e`} />
The Coin Model
Think of a cash transaction. If your wallet holds two ¥10,000 notes and one ¥5,000 note, your balance is not a single number "¥25,000" - it is the sum of three separate bills. Bitcoin works the same way.
When a Bitcoin transaction occurs:
- Inputs: One or more previously received UTXOs are "consumed"
- Outputs: New UTXOs are created - the amount sent to the recipient and the change returned to the sender
- Consumed UTXOs are permanently destroyed, and new UTXOs are born
For example, if someone with a 0.7 BTC UTXO sends 0.3 BTC:
- The 0.7 BTC UTXO is consumed
- A 0.3 BTC UTXO is created for the recipient
- Approximately 0.3999 BTC UTXO returns to the sender as change
- The remaining 0.0001 BTC goes to the miner as a fee
Why Not an Account Model?
Ethereum uses a bank-like account balance model. Bitcoin chose the seemingly more complex UTXO model for good reasons:
- Parallel verification: Each UTXO is independent, allowing transactions to be verified simultaneously. Account models create sequential dependencies
- Double-spend prevention: A UTXO exists in only two states - "spent" or "unspent." Partial spending is impossible, making double-spend detection simple
- Privacy: Generating a new address (new UTXO) for each transaction makes tracking more difficult
Practical UTXO Management
The number and size of your UTXOs directly affect fees:
- Dust UTXOs: If you accumulate many tiny UTXOs, the fee to spend them may exceed their value
- UTXO Consolidation: A strategy of merging multiple small UTXOs into one during low-fee periods
- Coin Control: Manually selecting which UTXOs to use as inputs, optimizing for privacy and fees
Desktop wallets like Sparrow Wallet provide features to inspect and manage individual UTXOs.
Related Concepts
- Bitcoin Wallet Guide - The tools for managing UTXOs in practice
- Multisig - Strengthening UTXO security with multiple signatures
- SegWit - Improving UTXO processing efficiency by separating signature data
- Lightning Network - Off-chain payment channels built on UTXOs